Marine life

Viruses are small infectious agents that do not have their own metabolism and can replicate only inside the living cells of other organisms.[103] Viruses can infect all types of life forms, from animals and plants to microorganisms, including bacteria and archaea.[104] The linear size of the average virus is about one one-hundredth that of the average bacterium. Most viruses cannot be seen with an optical microscope so electron microscopes are used instead.[105]

Viruses are found wherever there is life and have probably existed since living cells first evolved.[106] The origin of viruses is unclear because they do not form fossils, so molecular techniques have been used to compare the DNA or RNA of viruses and are a useful means of investigating how they arise.[107]

Viruses are now recognised as ancient and as having origins that pre-date the divergence of life into the three domains.[108] But the origins of viruses in the evolutionary history of life are unclear: some may have evolved from plasmids—pieces of DNA that can move between cells—while others may have evolved from bacteria. In evolution, viruses are an important means of horizontal gene transfer, which increases genetic diversity.[109]

Bacteriophages (phages)

Multiple phages attached to a bacterial cell wall at 200,000x magnification

Diagram of a typical tailed phage

These are cyanophages, viruses that infect cyanobacteria (scale bars indicate 100 nm)

Opinions differ on whether viruses are a form of life or organic structures that interact with living organisms.[110] They are considered by some to be a life form, because they carry genetic material, reproduce by creating multiple copies of themselves through self-assembly, and evolve through natural selection. However they lack key characteristics such as a cellular structure generally considered necessary to count as life. Because they possess some but not all such qualities, viruses have been described as replicators[110] and as "organisms at the edge of life".[111]

In terms of individual counts, tailed phage are the most abundant biological entities in the sea.

Bacteriophages, often just called phages, are viruses that parasite bacteria and archaea. Marine phages parasite marine bacteria and archaea, such as cyanobacteria.[112] They are a common and diverse group of viruses and are the most abundant biological entity in marine environments, because their hosts, bacteria, are typically the numerically dominant cellular life in the sea. Generally there are about 1 million to 10 million viruses in each mL of seawater, or about ten times more double-stranded DNA viruses than there are cellular organisms,[113][114] although estimates of viral abundance in seawater can vary over a wide range.[115][116] Tailed bacteriophages appear to dominate marine ecosystems in number and diversity of organisms.[112] Bacteriophages belonging to the families Corticoviridae,[117] Inoviridae[118] and Microviridae[119] are also known to infect diverse marine bacteria.

Microorganisms make up about 70% of the marine biomass.[7] It is estimated viruses kill 20% of this biomass each day and that there are 15 times as many viruses in the oceans as there are bacteria and archaea. Viruses are the main agents responsible for the rapid destruction of harmful algal blooms,[114] which often kill other marine life.[120] The number of viruses in the oceans decreases further offshore and deeper into the water, where there are fewer host organisms.[91]

There are also archaeal viruses which replicate within archaea: these are double-stranded DNA viruses with unusual and sometimes unique shapes.[121][122] These viruses have been studied in most detail in the thermophilic archaea, particularly the orders Sulfolobales and Thermoproteales.[123]

Viruses are an important natural means of transferring genes between different species, which increases genetic diversity and drives evolution.[109] It is thought that viruses played a central role in the early evolution, before the diversification of bacteria, archaea and eukaryotes, at the time of the last universal common ancestor of life on Earth.[124] Viruses are still one of the largest reservoirs of unexplored genetic diversity on Earth.[91]

Vibrio vulnificus, a virulent bacterium found in estuaries and along coastal areas

Pelagibacter ubique, the most abundant bacteria in the ocean, plays a major role in the global carbon cycle.

Bacteria constitute a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a number of shapes, ranging from spheres to rods and spirals. Bacteria were among the first life forms to appear on Earth, and are present in most of its habitats. Bacteria inhabit soil, water, acidic hot springs, radioactive waste,[125] and the deep portions of Earth's crust. Bacteria also live in symbiotic and parasitic relationships with plants and animals.

Once regarded as plants constituting the class Schizomycetes, bacteria are now classified as prokaryotes. Unlike cells of animals and other eukaryotes, bacterial cells do not contain a nucleus and rarely harbour membrane-bound organelles. Although the term bacteria traditionally included all prokaryotes, the scientific classification changed after the discovery in the 1990s that prokaryotes consist of two very different groups of organisms that evolved from an ancient common ancestor. These evolutionary domains are called Bacteria and Archaea.[126]

The ancestors of modern bacteria were unicellular microorganisms that were the first forms of life to appear on Earth, about 4 billion years ago. For about 3 billion years, most organisms were microscopic, and bacteria and archaea were the dominant forms of life.[64][127] Although bacterial fossils exist, such as stromatolites, their lack of distinctive morphology prevents them from being used to examine the history of bacterial evolution, or to date the time of origin of a particular bacterial species. However, gene sequences can be used to reconstruct the bacterial phylogeny, and these studies indicate that bacteria diverged first from the archaeal/eukaryotic lineage.[128] Bacteria were also involved in the second great evolutionary divergence, that of the archaea and eukaryotes. Here, eukaryotes resulted from the entering of ancient bacteria into endosymbiotic associations with the ancestors of eukaryotic cells, which were themselves possibly related to the Archaea.[66][65] This involved the engulfment by proto-eukaryotic cells of alphaproteobacterial symbionts to form either mitochondria or hydrogenosomes, which are still found in all known Eukarya. Later on, some eukaryotes that already contained mitochondria also engulfed cyanobacterial-like organisms. This led to the formation of chloroplasts in algae and plants. There are also some algae that originated from even later endosymbiotic events. Here, eukaryotes engulfed a eukaryotic algae that developed into a "second-generation" plastid.[129][130] This is known as secondary endosymbiosis.

• 
  The marine Thiomargarita namibiensis, the largest known bacterium
• 
  Cyanobacteria blooms can contain lethal cyanotoxins.
• 
  The chloroplasts of glaucophytes have a peptidoglycan layer, evidence suggesting their endosymbiotic origin from cyanobacteria.[131]
• 
  Bacteria can be beneficial. This Pompeii worm, an extremophile found only at hydrothermal vents, has a protective cover of bacteria.

The largest known bacterium, the marine Thiomargarita namibiensis, can be visible to the naked eye and sometimes attains 0.75 mm (750 μm).[132][133]

Archaea were initially viewed as extremophiles living in harsh environments, such as the yellow archaea pictured here in a hot spring, but they have since been found in a much broader range of habitats.[134]

The archaea (Greek for ancient[135]) constitute a domain and kingdom of single-celled microorganisms. These microbes are prokaryotes, meaning they have no cell nucleus or any other membrane-bound organelles in their cells.

Archaea were initially classified as bacteria, but this classification is outdated.[136] Archaeal cells have unique properties separating them from the other two domains of life, Bacteria and Eukaryota. The Archaea are further divided into multiple recognized phyla. Classification is difficult because the majority have not been isolated in the laboratory and have only been detected by analysis of their nucleic acids in samples from their environment.

Archaea and bacteria are generally similar in size and shape, although a few archaea have very strange shapes, such as the flat and square-shaped cells of Haloquadratum walsbyi.[137] Despite this morphological similarity to bacteria, archaea possess genes and several metabolic pathways that are more closely related to those of eukaryotes, notably the enzymes involved in transcription and translation. Other aspects of archaeal biochemistry are unique, such as their reliance on ether lipids in their cell membranes, such as archaeols. Archaea use more energy sources than eukaryotes: these range from organic compounds, such as sugars, to ammonia, metal ions or even hydrogen gas. Salt-tolerant archaea (the Haloarchaea) use sunlight as an energy source, and other species of archaea fix carbon; however, unlike plants and cyanobacteria, no known species of archaea does both. Archaea reproduce asexually by binary fission, fragmentation, or budding; unlike bacteria and eukaryotes, no known species forms spores.

Archaea are particularly numerous in the oceans, and the archaea in plankton may be one of the most abundant groups of organisms on the planet. Archaea are a major part of Earth's life and may play roles in both the carbon cycle and the nitrogen cycle.

• 
  Halobacteria, found in water near saturated with salt, are now recognised as archaea.
• 
  Flat, square-shaped cells of the archaea Haloquadratum walsbyi
• 
  Methanosarcina barkeri, a marine archaea that produces methane
• 
  Thermophiles, such as Pyrolobus fumarii, survive well over 100 °C.
• 
  Drawing of another marine thermophile, Pyrococcus furiosus

Protists are eukaryotes that cannot be classified as plants, fungi or animals. They are usually single-celled and microscopic. Life originated as single-celled prokaryotes (bacteria and archaea) and later evolved into more complex eukaryotes. Eukaryotes are the more developed life forms known as plants, animals, fungi and protists. The term protist came into use historically as a term of convenience for eukaryotes that cannot be strictly classified as plants, animals or fungi. They are not a part of modern cladistics, because they are paraphyletic (lacking a common ancestor). Protists can be broadly divided into four groups depending on whether their nutrition is plant-like, animal-like, fungus-like,[138] or a mixture of these.[139]

Protists according to how they get food
Type of protist		Description	Example		Other examples
Plant-like	Algae (see below)	Autotrophic protists that make their own food without needing to consume other organisms, usually by using photosynthesis		Red algae, Cyanidium sp.	Green algae, brown algae, diatoms and some dinoflagellates. Plant-like protists are important components of phytoplankton discussed below.
Animal-like	Protozoans	Heterotrophic protists that get their food consuming other organisms		Radiolarian protist as drawn by Haeckel	Foraminiferans, and some marine amoebae, ciliates and flagellates.
Fungus-like	Slime moulds and slime nets	Saprotrophic protists that get their food from the remains of organisms that have broken down and decayed		Marine slime nets form labyrinthine networks of tubes in which amoeba without pseudopods can travel	Marine lichen
Mixotropes	Various	Mixotrophic and osmotrophic protists that get their food from a combination of the above		Euglena mutabilis, a photosynthetic flagellate	Many marine mixotropes are found among protists, including among ciliates, Rhizaria and dinoflagellates [140]

micrograph

cell schematic

Choanoflagellates, unicellular "collared" flagellate protists, are thought to be the closest living relatives of the animals.[141]

---

Getting to know our single-celled ancestors - MicroCosmos

Protists are highly diverse organisms currently organised into 18 phyla, but are not easy to classify.[142][143] Studies have shown high protist diversity exists in oceans, deep sea-vents and river sediments, suggesting a large number of eukaryotic microbial communities have yet to be discovered.[144][145] There has been little research on mixotrophic protists, but recent studies in marine environments found mixotrophic protests contribute a significant part of the protist biomass.[140]

• Single-celled and microscopic protists
• 
  Diatoms are a major algae group generating about 20% of world oxygen production.[146]
• 
  Diatoms have glass like cell walls made of silica and called frustules.[147]
• 
  Fossil diatom frustule from 32 to 40 mya
• 
  Radiolarian
• 
  Single-celled alga, Gephyrocapsa oceanica
• 
  Two dinoflagellates
• 
  Zooxanthellae is a photosynthetic algae that lives inside hosts like coral.
• 
  A single-celled ciliate with green zoochlorellae living inside endosymbiotically.
• 
  Euglenoid
• 
  This ciliate is digesting cyanobacteria. The cytostome or mouth is at the bottom right.

In contrast to the cells of prokaryotes, the cells of eukaryotes are highly organised. Plants, animals and fungi are usually multi-celled and are typically macroscopic. Most protists are single-celled and microscopic. But there are exceptions. Some single-celled marine protists are macroscopic. Some marine slime molds have unique life cycles that involve switching between unicellular, colonial, and multicellular forms.[148] Other marine protist are neither single-celled nor microscopic, such as seaweed.

• Macroscopic protists (see also unicellular macroalgae →)
• 
  The single-celled giant amoeba has up to 1000 nuclei and reaches lengths of 5 mm (0.20 in).
• 
  Gromia sphaerica is a large spherical testate amoeba which makes mud trails. Its diameter is up to 3.8 cm (1.5 in).[149]
• 
  Spiculosiphon oceana, a unicellular foraminiferan with an appearance and lifestyle that mimics a sponge, grows to 5 cm long.
• 
  The xenophyophore, another single-celled foraminiferan, lives in abyssal zones. It has a giant shell up to 20 cm (7.9 in) across.[150]
• 
  Giant kelp, a brown algae, is not a true plant, yet it is multicellular and can grow to 50m.

Protists have been described as a taxonomic grab bag where anything that doesn't fit into one of the main biological kingdoms can be placed.[151] Some modern authors prefer to exclude multicellular organisms from the traditional definition of a protist, restricting protists to unicellular organisms.[152][153] This more constrained definition excludes seaweeds and slime molds.[154]

External video
Copepods: The Diatom-Devouring King of Plankton - Journey to the Microcosmos

As juveniles, animals develop from microscopic stages, which can include spores, eggs and larvae. At least one microscopic animal group, the parasitic cnidarian Myxozoa, is unicellular in its adult form, and includes marine species. Other adult marine microanimals are multicellular. Microscopic adult arthropods are more commonly found inland in freshwater, but there are marine species as well. Microscopic adult marine crustaceans include some copepods, cladocera and tardigrades (water bears). Some marine nematodes and rotifers are also too small to be recognised with the naked eye, as are many loricifera, including the recently discovered anaerobic species that spend their lives in an anoxic environment.[155][156] Copepods contribute more to the secondary productivity and carbon sink of the world oceans than any other group of organisms. While mites are not normally thought of as marine organisms, most species of the family Halacaridae live in the sea.[157]

• Marine microanimals
• 
  Over 10,000 marine species are copepods, small, often microscopic crustaceans
• 
  Darkfield photo of a gastrotrich, a worm-like animal living between sediment particles
• 
  Armoured Pliciloricus enigmaticus, about 0.2 mm long, live in spaces between marine gravel.
• 
  Drawing of a tardigrade (water bear) on a grain of sand
• 
  Rotifers, usually 0.1–0.5 mm long, may look like protists but have many cells and belongs to the Animalia.

Kimberella, an early mollusc important for understanding the Cambrian explosion. Invertebrates are grouped into different phyla (body plans).

Invertebrates are grouped into different phyla. Informally phyla can be thought of as a way of grouping organisms according to their body plan.[196][197]^: 33 A body plan refers to a blueprint which describes the shape or morphology of an organism, such as its symmetry, segmentation and the disposition of its appendages. The idea of body plans originated with vertebrates, which were grouped into one phylum. But the vertebrate body plan is only one of many, and invertebrates consist of many phyla or body plans. The history of the discovery of body plans can be seen as a movement from a worldview centred on vertebrates, to seeing the vertebrates as one body plan among many. Among the pioneering zoologists, Linnaeus identified two body plans outside the vertebrates; Cuvier identified three; and Haeckel had four, as well as the Protista with eight more, for a total of twelve. For comparison, the number of phyla recognised by modern zoologists has risen to 35.[197]

Taxonomic biodiversity of accepted marine species, according to WoRMS, 18 October 2019.[198][199]

Opabinia, an extinct stem group arthropod appeared in the Middle Cambrian.[200]^{: 124–136}

Historically body plans were thought of as having evolved rapidly during the Cambrian explosion,[201] but a more nuanced understanding of animal evolution suggests a gradual development of body plans throughout the early Palaeozoic and beyond.[202] More generally a phylum can be defined in two ways: as described above, as a group of organisms with a certain degree of morphological or developmental similarity (the phenetic definition), or a group of organisms with a certain degree of evolutionary relatedness (the phylogenetic definition).[202]

In the 1970s there was already a debate about whether the emergence of the modern phyla was "explosive" or gradual but hidden by the shortage of Precambrian animal fossils.[195] A re-analysis of fossils from the Burgess Shale lagerstätte increased interest in the issue when it revealed animals, such as Opabinia, which did not fit into any known phylum. At the time these were interpreted as evidence that the modern phyla had evolved very rapidly in the Cambrian explosion and that the Burgess Shale's "weird wonders" showed that the Early Cambrian was a uniquely experimental period of animal evolution.[203] Later discoveries of similar animals and the development of new theoretical approaches led to the conclusion that many of the "weird wonders" were evolutionary "aunts" or "cousins" of modern groups[204]—for example that Opabinia was a member of the lobopods, a group which includes the ancestors of the arthropods, and that it may have been closely related to the modern tardigrades.[205] Nevertheless, there is still much debate about whether the Cambrian explosion was really explosive and, if so, how and why it happened and why it appears unique in the history of animals.[206]

The deepest-branching animals — the earliest animals that appeared during evolution — are marine non-vertebrate organisms. The earliest animal phyla are the Porifera, Ctenophora, Placozoa and Cnidaria. No member of these clades exhibit body plans with bilateral symmetry.

There has been much controversy over which invertebrate phyla, sponges or comb jellies, is the most basal.[207] Currently, sponges are more widely considered to be the most basal.[208][209]

Sponges are perhaps the most basal animals. They have no nervous, digestive or circulatory system.

Sponges are animals of the phylum Porifera (from Modern Latin for bearing pores[210]). They are multicellular organisms that have bodies full of pores and channels allowing water to circulate through them, consisting of jelly-like mesohyl sandwiched between two thin layers of cells. They have unspecialized cells that can transform into other types and that often migrate between the main cell layers and the mesohyl in the process. Sponges do not have nervous, digestive or circulatory systems. Instead, most rely on maintaining a constant water flow through their bodies to obtain food and oxygen and to remove wastes.

Sponges are similar to other animals in that they are multicellular, heterotrophic, lack cell walls and produce sperm cells. Unlike other animals, they lack true tissues and organs, and have no body symmetry. The shapes of their bodies are adapted for maximal efficiency of water flow through the central cavity, where it deposits nutrients, and leaves through a hole called the osculum. Many sponges have internal skeletons of spongin and/or spicules of calcium carbonate or silicon dioxide. All sponges are sessile aquatic animals. Although there are freshwater species, the great majority are marine (salt water) species, ranging from tidal zones to depths exceeding 8,800 m (5.5 mi). Some sponges live to great ages; there is evidence of the deep-sea glass sponge Monorhaphis chuni living about 11,000 years.[211][212]

While most of the approximately 5,000–10,000 known species feed on bacteria and other food particles in the water, some host photosynthesizing micro-organisms as endosymbionts and these alliances often produce more food and oxygen than they consume. A few species of sponge that live in food-poor environments have become carnivores that prey mainly on small crustaceans.[213]

• 
  Sponge biodiversity. There are four sponge species in this photo.
• 
  Branching vase sponge
• 
  Venus' flower basket at a depth of 2572 meters
• 
  Barrel sponge
• 
  The long-living Monorhaphis chuni

Linnaeus mistakenly identified sponges as plants in the order Algae.[214] For a long time thereafter sponges were assigned to a separate subkingdom, Parazoa (meaning beside the animals).[215] They are now classified as a paraphyletic phylum from which the higher animals have evolved.[216]

Together with sponges, brilliantly bioluminescent ctenophores (comb jellies) are the most basal animals.

Ctenophores (from Greek for carrying a comb), commonly known as comb jellies, are a phylum that live worldwide in marine waters. They are the largest non-colonial animals to swim with the help of cilia (hairs or combs).[217] Coastal species need to be tough enough to withstand waves and swirling sediment, but some oceanic species are so fragile and transparent that it is very difficult to capture them intact for study.[218] In the past ctenophores were thought to have only a modest presence in the ocean, but it is now known they are often significant and even dominant parts of the planktonic biomass.[219]^: 269

The phylum has about 150 known species with a wide range of body forms. Sizes range from a few millimeters to 1.5 m (4 ft 11 in). Cydippids are egg-shaped with their cilia arranged in eight radial comb rows, and deploy retractable tentacles for capturing prey. The benthic platyctenids are generally combless and flat. The coastal beroids have gaping mouths and lack tentacles. Most adult ctenophores prey on microscopic larvae and rotifers and small crustaceans but beroids prey on other ctenophores.

• 
  Light diffracting along the comb rows of a cydippid, left tentacle deployed, right retracted
• 
  Deep-sea ctenophore trailing tentacles studded with tentilla (sub-tentacles)

• 
  Egg-shaped cydippid ctenophore
• 
  Group of small benthic creeping comb jellies streaming tentacles and living symbiotically on a starfish.
• 
  Lobata sp. with paired thick lobes
• 
  The sea walnut has a transient anus which forms only when it needs to defecate.[220]

The beroid ctenophore, mouth gaping, preys on other ctenophores.

Early writers combined ctenophores with cnidarians. Ctenophores resemble cnidarians in relying on water flow through the body cavity for both digestion and respiration, as well as in having a decentralized nerve net rather than a brain. Also like cnidarians, the bodies of ctenophores consist of a mass of jelly, with one layer of cells on the outside and another lining the internal cavity. In ctenophores, however, these layers are two cells deep, while those in cnidarians are only a single cell deep. While cnidarians exhibit radial symmetry, ctenophores have two anal canals which exhibit biradial symmetry (half-turn rotational symmetry).[221] The position of the ctenophores in the evolutionary family tree of animals has long been debated, and the majority view at present, based on molecular phylogenetics, is that cnidarians and bilaterians are more closely related to each other than either is to ctenophores.[219]^: 222

External video
Iridescent red ctenophore — EVNautilus

Placozoa (from Greek for flat animals) have the simplest structure of all animals. They are a basal form of free-living (non-parasitic) multicellular organism[222] that do not yet have a common name.[223] They live in marine environments and form a phylum containing sofar only three described species, of which the first, the classical Trichoplax adhaerens, was discovered in 1883.[224] Two more species have been discovered since 2017,[225][226] and genetic methods indicate this phylum has a further 100 to 200 undescribed species.[227]

Placozoa have the simplest structure of all animals.

Crawling motility and food uptake by T. adhaerens

Trichoplax is a small, flattened, animal about one mm across and usually about 25 µm thick. Like the amoebae they superficially resemble, they continually change their external shape. In addition, spherical phases occasionally form which may facilitate movement. Trichoplax lacks tissues and organs. There is no manifest body symmetry, so it is not possible to distinguish anterior from posterior or left from right. It is made up of a few thousand cells of six types in three distinct layers.[228] The outer layer of simple epithelial cells bear cilia which the animal uses to help it creep along the seafloor.[229] Trichoplax feed by engulfing and absorbing food particles – mainly microbes and organic detritus – with their underside.

Cnidarians, like this starlet sea anemone, are the simplest animals to organise cells into tissue. Yet they have the same genes that form the vertebrate (including human) head.

Cnidarians (from Greek for nettle) are distinguished by the presence of stinging cells, specialized cells that they use mainly for capturing prey. Cnidarians include corals, sea anemones, jellyfish and hydrozoans. They form a phylum containing over 10,000[230] species of animals found exclusively in aquatic (mainly marine) environments. Their bodies consist of mesoglea, a non-living jelly-like substance, sandwiched between two layers of epithelium that are mostly one cell thick. They have two basic body forms: swimming medusae and sessile polyps, both of which are radially symmetrical with mouths surrounded by tentacles that bear cnidocytes. Both forms have a single orifice and body cavity that are used for digestion and respiration.

Fossil cnidarians have been found in rocks formed about 580 million years ago. Fossils of cnidarians that do not build mineralized structures are rare. Scientists currently think cnidarians, ctenophores and bilaterians are more closely related to calcareous sponges than these are to other sponges, and that anthozoans are the evolutionary "aunts" or "sisters" of other cnidarians, and the most closely related to bilaterians.

Cnidarians are the simplest animals in which the cells are organised into tissues.[231] The starlet sea anemone is used as a model organism in research.[232] It is easy to care for in the laboratory and a protocol has been developed which can yield large numbers of embryos on a daily basis.[233] There is a remarkable degree of similarity in the gene sequence conservation and complexity between the sea anemone and vertebrates.[233] In particular, genes concerned in the formation of the head in vertebrates are also present in the anemone.[234][235]

• 
  Sea anemones are common in tidepools.
• 
  Their tentacles sting and paralyse small fish.
• 
  Close up of polyps on the surface of a coral, waving their tentacles.
• 
  If an island sinks below the sea, coral growth can keep up with rising water and form an atoll.
• 
  The mantle of the red paper lantern jellyfish crumples and expands like a paper lantern.[236]

• 
  The Portuguese man o' war is a colonial siphonophore
• 
  Marrus orthocanna another colonial siphonophore, assembled from two types of zooids.
• 
  Porpita porpita consists of a colony of hydroids[237]
• 
  Lion's mane jellyfish, largest known jellyfish[238]
• 
  Turritopsis dohrnii achieves biological immortality by transferring its cells back to childhood.[239][240]
• 
  The sea wasp is the most lethal jellyfish in the world.[241]

Protostomes (from Greek for first mouth) are a superphylum of animals. It is a sister clade of the deuterostomes (from Greek for second mouth), with which it forms the Nephrozoa clade. Protostomes are distinguished from deuterostomes by the way their embryos develop. In protostomes the first opening that develops becomes the mouth, while in deuterostomes it becomes the anus.[248][249]

Many marine worms are related only distantly, so they form a number of different phyla. The worm shown is an arrow worm, found worldwide as a predatory component of plankton.

Worms (Old English for serpents) form a number of phyla. Different groups of marine worms are related only distantly, so they are found in several different phyla such as the Annelida (segmented worms), Chaetognatha (arrow worms), Phoronida (horseshoe worms), and Hemichordata. All worms, apart from the Hemichordata, are protostomes. The Hemichordata are deuterostomes and are discussed in their own section below.

The typical body plan of a worm involves long cylindrical tube-like bodies and no limbs. Marine worms vary in size from microscopic to over 1 metre (3.3 ft) in length for some marine polychaete worms (bristle worms)[250] and up to 58 metres (190 ft) for the marine nemertean worm (bootlace worm).[251] Some marine worms occupy a small variety of parasitic niches, living inside the bodies of other animals, while others live more freely in the marine environment or by burrowing underground. Many of these worms have specialized tentacles used for exchanging oxygen and carbon dioxide and also may be used for reproduction. Some marine worms are tube worms, such as the giant tube worm which lives in waters near underwater volcanoes and can withstand temperatures up to 90 degrees Celsius. Platyhelminthes (flatworms) form another worm phylum which includes a class of parasitic tapeworms. The marine tapeworm Polygonoporus giganticus, found in the gut of sperm whales, can grow to over 30 m (100 ft).[252][253]

Nematodes (roundworms) constitute a further worm phylum with tubular digestive systems and an opening at both ends.[254][255] Over 25,000 nematode species have been described,[256][257] of which more than half are parasitic. It has been estimated another million remain undescribed.[258] They are ubiquitous in marine, freshwater and terrestrial environments, where they often outnumber other animals in both individual and species counts. They are found in every part of the earth's lithosphere, from the top of mountains to the bottom of oceanic trenches.[259] By count they represent 90% of all animals on the ocean floor.[260] Their numerical dominance, often exceeding a million individuals per square meter and accounting for about 80% of all individual animals on earth, their diversity of life cycles, and their presence at various trophic levels point at an important role in many ecosystems.[261]

• 
  Giant tube worms cluster around hydrothermal vents.
• 
  Nematodes are ubiquitous pseudocoelomates which can parasite marine plants and animals.
• 
  Bloodworms are typically found on the bottom of shallow marine waters.

Bigfin reef squid displaying vivid iridescence at night. Cephalopods are the most neurologically advanced invertebrates.[262]

Blue dragon, a pelagic sea slug

Bolinus brandaris, a sea snail from which the Phoenicians extracted
royal Tyrian purple dye
colour code: #66023C _____[263]

Hypothetical ancestral mollusc

Molluscs (Latin for soft) form a phylum with about 85,000 extant recognized species.[264] They are the largest marine phylum in terms of species count, containing about 23% of all the named marine organisms.[265] Molluscs have more varied forms than other invertebrate phyla. They are highly diverse, not just in size and in anatomical structure, but also in behaviour and in habitat.

Drawing of a giant clam (NOAA)

The mollusc phylum is divided into 9 or 10 taxonomic classes. These classes include gastropods, bivalves and cephalopods, as well as other lesser-known but distinctive classes. Gastropods with protective shells are referred to as snails, whereas gastropods without protective shells are referred to as slugs. Gastropods are by far the most numerous molluscs in terms of species.[266] Bivalves include clams, oysters, cockles, mussels, scallops, and numerous other families. There are about 8,000 marine bivalves species (including brackish water and estuarine species). A deep sea ocean quahog clam has been reported as having lived 507 years[267] making it the longest recorded life of all animals apart from colonial animals, or near-colonial animals like sponges.[211]

• Gastropods and bivalves
• 
  Marine gastropods are sea snails or sea slugs. This nudibranch is a sea slug.
• 
  The sea snail Syrinx aruanus has a shell up to 91 cm long, the largest of any living gastropod.
• 
  Molluscs usually have eyes. Bordering the edge of the mantle of a scallop, a bivalve mollusc, can be over 100 simple eyes.
• 
  Common mussel, another bivalve

Cephalopods include octopus, squid and cuttlefish. About 800 living species of marine cephalopods have been identified,[268] and an estimated 11,000 extinct taxa have been described.[269] They are found in all oceans, but there are no fully freshwater cephalopods.[270]

• Cephalopods
• 
  The nautilus is a living fossil little changed since it evolved 500 million years ago as one of the first cephalopods.[271][272][273]
• 
  Reconstruction of an ammonite, a highly successful early cephalopod that appeared 400 mya.
• 
  Cephalopods, like this cuttlefish, use their mantle cavity for jet propulsion.
• 
  Colossal squid, the largest of all invertebrates[274]

Molluscs have such diverse shapes that many textbooks base their descriptions of molluscan anatomy on a generalized or hypothetical ancestral mollusc. This generalized mollusc is unsegmented and bilaterally symmetrical with an underside consisting of a single muscular foot. Beyond that it has three further key features. Firstly, it has a muscular cloak called a mantle covering its viscera and containing a significant cavity used for breathing and excretion. A shell secreted by the mantle covers the upper surface. Secondly (apart from bivalves) it has a rasping tongue called a radula used for feeding. Thirdly, it has a nervous system including a complex digestive system using microscopic, muscle-powered hairs called cilia to exude mucus. The generalized mollusc has two paired nerve cords (three in bivalves). The brain, in species that have one, encircles the esophagus. Most molluscs have eyes and all have sensors detecting chemicals, vibrations, and touch.[275][276]

Good evidence exists for the appearance of marine gastropods, cephalopods and bivalves in the Cambrian period 538.8 to 485.4 million years ago.

Head

___________

Thorax

___________

Abdomen

___________

Segments and tagmata of an arthropod[275]^: 518–52 The thorax bears the main locomotory appendages. The head and thorax are fused in some arthropods, such as crabs and lobsters.

First known air-breathing animal to colonise land, the millipede Pneumodesmus newmani,[277] lived in the Early Devonian.[278]

Arthropods (Greek for jointed feet) have an exoskeleton (external skeleton), a segmented body, and jointed appendages (paired appendages). They form a phylum which includes insects, arachnids, myriapods, and crustaceans. Arthropods are characterized by their jointed limbs and cuticle made of chitin, often mineralised with calcium carbonate. The arthropod body plan consists of segments, each with a pair of appendages. The rigid cuticle inhibits growth, so arthropods replace it periodically by moulting. Their versatility has enabled them to become the most species-rich members of all ecological guilds in most environments.

The evolutionary ancestry of arthropods dates back to the Cambrian period and is generally regarded as monophyletic. However, basal relationships of arthropods with extinct phyla such as lobopodians have recently been debated.[279][280]

Some palaeontologists think Lobopodia represents a basal grade which lead to an arthropod body plan.[281]

Tardigrades (water bears) are a phylum of eight-legged, segmented microanimals able to survive in extreme conditions.

• Arthropod fossils and living fossils
• 
  Fossil trilobite. Trilobites first appeared about 521 Ma. They were highly successful and were found everywhere in the ocean for 270 Ma.[282]
• 
  The Anomalocaris ("abnormal shrimp") was one of the first apex predators and first appeared about 515 Ma.
• 
  The largest known arthropod, the sea scorpion Jaekelopterus rhenaniae, has been found in estuarine strata from about 390 Ma. It was up to 2.5 m (8.2 ft) long.[283][284]
• 
  Horseshoe crabs are living fossils, essentially unchanged for 450 Ma.

Extant marine arthropods range in size from the microscopic crustacean Stygotantulus to the Japanese spider crab. Arthropods' primary internal cavity is a hemocoel, which accommodates their internal organs, and through which their haemolymph - analogue of blood - circulates; they have open circulatory systems. Like their exteriors, the internal organs of arthropods are generally built of repeated segments. Their nervous system is "ladder-like", with paired ventral nerve cords running through all segments and forming paired ganglia in each segment. Their heads are formed by fusion of varying numbers of segments, and their brains are formed by fusion of the ganglia of these segments and encircle the esophagus. The respiratory and excretory systems of arthropods vary, depending as much on their environment as on the subphylum to which they belong.

• Modern crustaceans
• 
  Many crustaceans are very small, like this tiny amphipod, and make up a significant part of the ocean's zooplankton.
• 
  The Japanese spider crab has the longest leg span of any arthropod, reaching 5.5 metres (18 ft) from claw to claw.[285]
• 
  The Tasmanian giant crab is long-lived and slow-growing, making it vulnerable to overfishing.[286]
• 
  Mantis shrimp have the most advanced eyes in the animal kingdom,[287] and smash prey by swinging their club-like raptorial claws.[288]

Arthropod vision relies on various combinations of compound eyes and pigment-pit ocelli: in most species the ocelli can only detect the direction from which light is coming, and the compound eyes are the main source of information. Arthropods also have a wide range of chemical and mechanical sensors, mostly based on modifications of the many setae (bristles) that project through their cuticles. Arthropod methods of reproduction are diverse: terrestrial species use some form of internal fertilization while marine species lay eggs using either internal or external fertilization. Arthropod hatchlings vary from miniature adults to grubs that lack jointed limbs and eventually undergo a total metamorphosis to produce the adult form.

In deuterostomes the first opening that develops in the growing embryo becomes the anus, while in protostomes it becomes the mouth. Deuterostomes form a superphylum of animals and are the sister clade of the protostomes.[248][249] It is once considered that the earliest known deuterostomes are Saccorhytus fossils from about 540 million years ago.[289] However, another study considered that Saccorhytus is more likely to be a ecdysozoan.[290]

Adult echinoderms have fivefold symmetry but as larvae have bilateral symmetry. This is why they are in the Bilateria.

Echinoderms (Greek for spiny skin) is a phylum which contains only marine invertebrates. The phylum contains about 7000 living species,[291] making it the second-largest grouping of deuterostomes, after the chordates.

Adult echinoderms are recognizable by their radial symmetry (usually five-point) and include starfish, sea urchins, sand dollars, and sea cucumbers, as well as the sea lilies. Echinoderms are found at every ocean depth, from the intertidal zone to the abyssal zone. They are unique among animals in having bilateral symmetry at the larval stage, but fivefold symmetry (pentamerism, a special type of radial symmetry) as adults.[292]

Echinoderms are important both biologically and geologically. Biologically, there are few other groupings so abundant in the biotic desert of the deep sea, as well as shallower oceans. Most echinoderms are able to regenerate tissue, organs, limbs, and reproduce asexually; in some cases, they can undergo complete regeneration from a single limb. Geologically, the value of echinoderms is in their ossified skeletons, which are major contributors to many limestone formations, and can provide valuable clues as to the geological environment. They were the most used species in regenerative research in the 19th and 20th centuries.

• 
  Echinoderm literally means "spiny skin", as this water melon sea urchin illustrates.
• 
  The ochre sea star was the first keystone predator to be studied. They limit mussels which can overwhelm intertidal communities.[293]
• 
  Colorful sea lilies in shallow waters

• 
  Sea cucumbers filter feed on plankton and suspended solids.
• 
  The sea pig, a deep water sea cucumber, is the only echinoderm that uses legged locomotion.
• 
  A benthopelagic and bioluminescent swimming sea cucumber, 3200 metres deep

It is held by some scientists that the radiation of echinoderms was responsible for the Mesozoic Marine Revolution. Aside from the hard-to-classify Arkarua (a Precambrian animal with echinoderm-like pentamerous radial symmetry), the first definitive members of the phylum appeared near the start of the Cambrian.

Gill (pharyngeal) slits

The acorn worm is associated with the development of gill slits.

Gill slits in an acorn worm (left) and tunicate (right)

Gill slits have been described as "the foremost morphological innovation of early deuterostomes".[294][295] In aquatic organisms, gill slits allow water that enters the mouth during feeding to exit. Some invertebrate chordates also use the slits to filter food from the water.[296]

Hemichordates form a sister phylum to the echinoderms. They are solitary worm-shaped organisms rarely seen by humans because of their lifestyle. They include two main groups, the acorn worms and the Pterobranchia. Pterobranchia form a class containing about 30 species of small worm-shaped animals that live in secreted tubes on the ocean floor. Acorn worms form a class containing about 111 species that generally live in U-shaped burrows on the seabed, from the shoreline to a depth of 3000 metres. The worms lie there with the proboscis sticking out of one opening in the burrow, subsisting as deposit feeders or suspension feeders. It is supposed the ancestors of acorn worms used to live in tubes like their relatives, the Pterobranchia, but eventually started to live a safer and more sheltered existence in sediment burrows.[297] Some of these worms may grow to be very long; one particular species may reach a length of 2.5 metres (8 ft 2 in), although most acorn worms are much smaller.

Acorn worms are more highly specialised and advanced than other worm-like organisms. They have a circulatory system with a heart that also functions as a kidney. Acorn worms have gill-like structures they use for breathing, similar to the gills of fish. Therefore, acorn worms are sometimes said to be a link between classical invertebrates and vertebrates. Acorn worms continually form new gill slits as they grow in size, and some older individuals have more than a hundred on each side. Each slit consists of a branchial chamber opening to the pharynx through a U-shaped cleft. Cilia push water through the slits, maintaining a constant flow, just as in fish.[298] Some acorn worms also have a postanal tail which may be homologous to the post-anal tail of vertebrates.

The three-section body plan of the acorn worm is no longer present in the vertebrates, except in the anatomy of the frontal neural tube, later developed into a brain divided into three parts. This means some of the original anatomy of the early chordate ancestors is still present in vertebrates even if it is not always visible. One theory is the three-part body originated from an early common ancestor of the deuterostomes, and maybe even from a common bilateral ancestor of both deuterostomes and protostomes. Studies have shown the gene expression in the embryo share three of the same signaling centers that shape the brains of all vertebrates, but instead of taking part in the formation of their neural system,[299] they are controlling the development of the different body regions.[300]

The lancelet, like all cephalochordates, has a head. Adult lancelets retain the four key features of chordates: a notochord, a dorsal hollow nerve cord, pharyngeal slits, and a post-anal tail. Water from the mouth enters the pharyngeal slits, which filter out food particles. The filtered water then collects in the atrium and exits through the atriopore.[301]

The chordate phylum has three subphyla, one of which is the vertebrates (see below). The other two subphyla are marine invertebrates: the tunicates (salps and sea squirts) and the cephalochordates (such as lancelets). Invertebrate chordates are close relatives to vertebrates. In particular, there has been discussion about how closely some extinct marine species, such as Pikaiidae, Palaeospondylus, Zhongxiniscus and Vetulicolia, might relate ancestrally to vertebrates.

• Invertebrate chordates are close relatives of vertebrates
• 
  The lancelet, a small translucent fish-like cephalochordate, is one of the closest living invertebrate relative of the vertebrates.[302][303]
• 
  Tunicates, like these fluorescent-colored sea squirts, may provide clues to vertebrate and therefore human ancestry.[304]
• 
  Pyrosomes are free-floating bioluminescent tunicates made up of hundreds of individuals.
• 
  Salp chain

In chordates, the four above labelled common features appear at some point during development.[296]

The larval stage of the tunicate possesses all of the features characteristic of chordates: a notochord, a dorsal hollow nerve cord, pharyngeal slits, and a post-anal tail.[296]

In the adult stage of the tunicate the notochord, nerve cord, and tail disappear.[296]

Fish typically breathe by extracting oxygen from water through gills and have a skin protected by scales and mucous. They use fins to propel and stabilise themselves in the water, and usually have a two-chambered heart and eyes well adapted to seeing underwater, as well as other sensory systems. Over 33,000 species of fish have been described as of 2017,[305] of which about 20,000 are marine fish.[306]

The Tully monster, a strange looking extinct animal with eyes like a hammerhead protruding from its back, may be an early jawless fish.

Early fish had no jaws. Most went extinct when they were outcompeted by jawed fish (below), but two groups survived: hagfish and lampreys. Hagfish form a class of about 20 species of eel-shaped, slime-producing marine fish. They are the only known living animals that have a skull but no vertebral column. Lampreys form a superclass containing 38 known extant species of jawless fish.[307] The adult lamprey is characterized by a toothed, funnel-like sucking mouth. Although they are well known for boring into the flesh of other fish to suck their blood,[308] only 18 species of lampreys are actually parasitic.[309] Together hagfish and lampreys are the sister group to vertebrates. Living hagfish remain similar to hagfish from around 300 million years ago.[310] The lampreys are a very ancient lineage of vertebrates, though their exact relationship to hagfishes and jawed vertebrates is still a matter of dispute.[311] Molecular analysis since 1992 has suggested that hagfish are most closely related to lampreys,[312] and so also are vertebrates in a monophyletic sense. Others consider them a sister group of vertebrates in the common taxon of craniata.[313]

The Tully monster is an extinct genus of soft-bodied bilaterians that lived in tropical estuaries about 300 million years ago. Since 2016 there has been controversy over whether this animal was a vertebrate or an invertebrate.[314][315] In 2020 researchers found "strong evidence" that the Tully monster was a vertebrate, and was a jawless fish in the lineage of the lamprey.[316][317]

• 
  Hagfish are the only known living animals with a skull but no vertebral column.
• 
  Lampreys are often parasitic and have a toothed, funnel-like sucking mouth.
• 
  The extinct Pteraspidomorphi, ancestral to jawed vertebrates

Pteraspidomorphi is an extinct class of early jawless fish ancestral to jawed vertebrates. The few characteristics they share with the latter are now considered as primitive for all vertebrates.

Around the start of the Devonian, fish started appearing with a deep remodelling of the vertebrate skull that resulted in a jaw.[318] All vertebrate jaws, including the human jaw, have evolved from these early fish jaws. The appearance of the early vertebrate jaw has been described as "perhaps the most profound and radical evolutionary step in vertebrate history".[319][320] Jaws make it possible to capture, hold, and chew prey. Fish without jaws had more difficulty surviving than fish with jaws, and most jawless fish became extinct during the Triassic period.

Jawed fish fall into two main groups: fish with bony internal skeletons and fish with cartilaginous internal skeletons. Cartilaginous fish, such as sharks and rays, have jaws and skeletons made of cartilage rather than bone. Megalodon is an extinct species of shark that lived about 28 to 1.5 Ma. It looked much like a stocky version of the great white shark, but was much larger with fossil lengths reaching 20.3 metres (67 ft).[321] Found in all oceans[322] it was one of the largest and most powerful predators in vertebrate history,[321] and probably had a profound impact on marine life.[323] The Greenland shark has the longest known lifespan of all vertebrates, about 400 years.[324] Some sharks such as the great white are partially warm blooded and give live birth. The manta ray, largest ray in the world, has been targeted by fisheries and is now vulnerable.[325]

• Cartilaginous fishes
• 
  Cartilaginous fishes may have evolved from spiny sharks.
• 
  Stingray
• 
  Manta ray, the largest ray
• 
  Sawfish, rays with long rostrums resembling a saw. All species are now endangered.[326]

• 
  The extinct megalodon resembled a giant great white shark.
• 
  The Greenland shark lives longer than any other vertebrate.
• 
  The largest extant fish, the whale shark, is now a vulnerable species.

Guiyu oneiros, the earliest-known bony fish lived during the Late Silurian 419 million years ago.

Lobe fins are bedded into the body by bony stalks. They evolved into the legs of the first tetrapod land vertebrates.

Ray fins have spines (rays) which can be erected to stiffen the fin for better control of swimming performance.

Bony fish have jaws and skeletons made of bone rather than cartilage. Bony fish also have hard, bony plates called operculum which help them respire and protect their gills, and they often possess a swim bladder which they use for better control of their buoyancy. Bony fish can be further divided into those with lobe fins and those with ray fins. The approximate dates in the phylogenetic tree are from Near et al., 2012[327] and Zhu et al., 2009.[328]

Lobe fins have the form of fleshy lobes supported by bony stalks which extend from the body.[329] Guiyu oneiros, the earliest-known bony fish, lived during the Late Silurian 419 million years ago. It has the combination of both ray-finned and lobe-finned features, although analysis of the totality of its features place it closer to lobe-finned fish.[328] Lobe fins evolved into the legs of the first tetrapod land vertebrates, so by extension an early ancestor of humans was a lobe-finned fish. Apart from the coelacanths and the lungfishes, lobe-finned fishes are now extinct.

The remaining bony fish have ray fins. These are made of webs of skin supported by bony or horny spines (rays) which can be erected to control the fin stiffness.

• The main distinguishing feature of the chondrosteans (sturgeon, paddlefish, bichir and reedfish) is the cartilaginous nature of their skeletons. The ancestors of the chondrosteans are thought to be bony fish, but the characteristic of an ossified skeleton was lost in later evolutionary development, resulting in a lightening of the frame.[330]
• Neopterygians (from Greek for new fins) appeared sometime in the Late Permian, before dinosaurs. They were a very successful group of fish, because they could move more rapidly than their ancestors. Their scales and skeletons began to lighten during their evolution, and their jaws became more powerful and efficient.[331]

Teleosts have homocercal tails.

About 96% of all modern fish species are teleosts,[332] of which about 14,000 are marine species.[333] Teleosts can be distinguished from other bony fish by their possession of a homocercal tail, a tail where the upper half mirrors the lower half.[334] Another difference lies in their jaw bones – teleosts have modifications in the jaw musculature which make it possible for them to protrude their jaws. This enables them to grab prey and draw it into their mouth.[334] In general, teleosts tend to be quicker and more flexible than more basal bony fishes. Their skeletal structure has evolved towards greater lightness. While teleost bones are well calcified, they are constructed from a scaffolding of struts, rather than the dense cancellous bones of holostean fish.[335]

Teleosts are found in almost all marine habitats.[336] They have enormous diversity, and range in size from adult gobies 8mm long [337] to ocean sunfish weighting over 2,000 kg.[338] The following images show something of the diversity in the shape and colour of modern marine teleosts...

• 
  Sailfish
• 
  Mahi-mahi
• 
  Eel
• 
  Sea-
  horse

• 
  Ocean sunfish
• 
  Anglerfish
• 
  Pufferfish
• 
  Clown triggerfish
• 
  Mandarin dragonet

Nearly half of all extant vertebrate species are teleosts.[339]

Tiktaalik, an extinct lobe-finned fish, developed limb-like fins that could take it onto land.

A tetrapod (Greek for four feet) is a vertebrate with limbs (feet). Tetrapods evolved from ancient lobe-finned fishes about 400 million years ago during the Devonian Period when their earliest ancestors emerged from the sea and adapted to living on land.[340] This change from a body plan for breathing and navigating in gravity-neutral water to a body plan with mechanisms enabling the animal to breath in air without dehydrating and move on land is one of the most profound evolutionary changes known.[341][342] Tetrapods can be divided into four classes: amphibians, reptiles, birds and mammals.

Marine tetrapods are tetrapods that returned from land back to the sea again. The first returns to the ocean may have occurred as early as the Carboniferous Period[343] whereas other returns occurred as recently as the Cenozoic, as in cetaceans, pinnipeds,[344] and several modern amphibians.[345] Amphibians (from Greek for both kinds of life) live part of their life in water and part on land. They mostly require fresh water to reproduce. A few inhabit brackish water, but there are no true marine amphibians.[346] There have been reports, however, of amphibians invading marine waters, such as a Black Sea invasion by the natural hybrid Pelophylax esculentus reported in 2010.[347]

Reptiles (Late Latin for creeping or crawling) do not have an aquatic larval stage, and in this way are unlike amphibians. Most reptiles are oviparous, although several species of squamates are viviparous, as were some extinct aquatic clades[348] — the fetus develops within the mother, contained in a placenta rather than an eggshell. As amniotes, reptile eggs are surrounded by membranes for protection and transport, which adapt them to reproduction on dry land. Many of the viviparous species feed their fetuses through various forms of placenta analogous to those of mammals, with some providing initial care for their hatchlings.

Some reptiles are more closely related to birds than other reptiles, and many scientists prefer to make Reptilia a monophyletic group which includes the birds.[349][350][351][352] Extant non-avian reptiles which inhabit or frequent the sea include sea turtles, sea snakes, terrapins, the marine iguana, and the saltwater crocodile. Currently, of the approximately 12,000 extant reptile species and sub-species, only about 100 of are classed as marine reptiles.[353]

Except for some sea snakes, most extant marine reptiles are oviparous and need to return to land to lay their eggs. Apart from sea turtles, the species usually spend most of their lives on or near land rather than in the ocean. Sea snakes generally prefer shallow waters nearby land, around islands, especially waters that are somewhat sheltered, as well as near estuaries.[354][355] Unlike land snakes, sea snakes have evolved flattened tails which help them swim.[356]

• 
  Marine iguana
• 
  Leatherback sea turtle
• 
  Saltwater crocodile
• 
  Marine snakes have flattened tails.
• 
  The ancient Ichthyosaurus communis independently evolved flippers similar to dolphins.

Some extinct marine reptiles, such as ichthyosaurs, evolved to be viviparous and had no requirement to return to land. Ichthyosaurs resembled dolphins. They first appeared about 245 million years ago and disappeared about 90 million years ago. The terrestrial ancestor of the ichthyosaur had no features already on its back or tail that might have helped along the evolutionary process. Yet the ichthyosaur developed a dorsal and tail fin which improved its ability to swim.[357] The biologist Stephen Jay Gould said the ichthyosaur was his favourite example of convergent evolution.[358] The earliest marine reptiles arose in the Permian. During the Mesozoic many groups of reptiles became adapted to life in the seas, including ichthyosaurs, plesiosaurs, mosasaurs, nothosaurs, placodonts, sea turtles, thalattosaurs and thalattosuchians. Marine reptiles were less numerous after mass extinction at the end of the Cretaceous.

Waterbird food web in Chesapeake Bay

Marine birds are adapted to life within the marine environment. They are often called seabirds. While marine birds vary greatly in lifestyle, behaviour and physiology, they often exhibit striking convergent evolution, as the same environmental problems and feeding niches have resulted in similar adaptations. Examples include albatross, penguins, gannets, and auks.

In general, marine birds live longer, breed later and have fewer young than terrestrial birds do, but they invest a great deal of time in their young. Most species nest in colonies, which can vary in size from a few dozen birds to millions. Many species are famous for undertaking long annual migrations, crossing the equator or circumnavigating the Earth in some cases. They feed both at the ocean's surface and below it, and even feed on each other. Marine birds can be highly pelagic, coastal, or in some cases spend a part of the year away from the sea entirely. Some marine birds plummet from heights, plunging through the water leaving vapour-like trails, similar to that of fighter planes.[359] Gannets plunge into the water at up to 100 kilometres per hour (60 mph). They have air sacs under their skin in their face and chest which act like bubble-wrap, cushioning the impact with the water.

• 
  European herring gull attack herring schools from above.
• 
  Gentoo penguin swimming underwater
• 
  Gannets "divebomb" at high speed.
• 
  Albatrosses range over huge areas of ocean and regularly circle the globe.

The first marine birds evolved in the Cretaceous period, and modern marine bird families emerged in the Paleogene.

Sea otter, a classic keystone species which controls sea urchin numbers

Mammals (from Latin for breast) are characterised by the presence of mammary glands which in females produce milk for feeding (nursing) their young. There are about 130 living and recently extinct marine mammal species such as seals, dolphins, whales, manatees, sea otters and polar bears.[360] They do not represent a distinct taxon or systematic grouping, but are instead unified by their reliance on the marine environment for feeding. Both cetaceans and sirenians are fully aquatic and therefore are obligate water dwellers. Seals and sea-lions are semiaquatic; they spend the majority of their time in the water, but need to return to land for important activities such as mating, breeding and molting. In contrast, both otters and the polar bear are much less adapted to aquatic living. Their diet varies considerably as well: some may eat zooplankton; others may eat fish, squid, shellfish, and sea-grass; and a few may eat other mammals.

In a process of convergent evolution, marine mammals, especially cetaceans such as dolphins and whales, redeveloped their body plan to parallel the streamlined fusiform body plan of pelagic fish. Front legs became flippers and back legs disappeared, a dorsal fin reappeared and the tail morphed into a powerful horizontal fluke. This body plan is an adaptation to being an active predator in a high drag environment. A parallel convergence occurred with the now extinct marine reptile ichthyosaur.[361]

• 
  Endangered blue whale, the largest living animal[362]
• 
  The bottlenose dolphin has the highest encephalization of any animal after humans[363]
• 
  Beluga whale
• 
  Dugong grazing on seagrass
• 
  Walrus
• 
  Polar bear

Cyanobacteria

Cyanobacteria from a microbial mat. Cyanobacteria were the first organisms to release oxygen via photosynthesis.

The cyanobacterium genus Prochlorococcus is a major contributor to atmospheric oxygen.

Cyanobacteria were the first organisms to evolve an ability to turn sunlight into chemical energy. They form a phylum (division) of bacteria which range from unicellular to filamentous and include colonial species. They are found almost everywhere on earth: in damp soil, in both freshwater and marine environments, and even on Antarctic rocks.[366] In particular, some species occur as drifting cells floating in the ocean, and as such were amongst the first of the phytoplankton.

The first primary producers that used photosynthesis were oceanic cyanobacteria about 2.3 billion years ago.[367][368] The release of molecular oxygen by cyanobacteria as a by-product of photosynthesis induced global changes in the Earth's environment. Because oxygen was toxic to most life on Earth at the time, this led to the near-extinction of oxygen-intolerant organisms, a dramatic change which redirected the evolution of the major animal and plant species.[369]

The tiny marine cyanobacterium Prochlorococcus, discovered in 1986, forms today part of the base of the ocean food chain and accounts for much of the photosynthesis of the open ocean[370] and an estimated 20% of the oxygen in the Earth's atmosphere.[371] It is possibly the most plentiful genus on Earth: a single millilitre of surface seawater may contain 100,000 cells or more.[372]

Originally, biologists classified cyanobacteria as algae, and referred to it as "blue-green algae". The more recent view is that cyanobacteria are bacteria, and hence are not even in the same Kingdom as algae. Most authorities today exclude all prokaryotes, and hence cyanobacteria from the definition of algae.[373][374]

Diatoms

          Centric

        Pennate

Diatoms have a silica shell (frustule) with radial (centric) or bilateral (pennate) symmetry.

Dinoflagellates

        Armoured

        Unarmoured

Traditionally dinoflagellates have been presented as armoured or unarmoured.

Algae is an informal term for a widespread and diverse group of photosynthetic protists which are not necessarily closely related and are thus polyphyletic. Marine algae can be divided into six groups:

• green algae, an informal group containing about 8,000 recognised species.[375] Many species live most of their lives as single cells or are filamentous, while others form colonies made up from long chains of cells, or are highly differentiated macroscopic seaweeds.
• red algae, a (disputed) phylum containing about 7,000 recognised species,[376] mostly multicellular and including many notable seaweeds.[376][377]
• brown algae, a class containing about 2,000 recognised species,[378] mostly multicellular and including many seaweeds, including kelp
• diatoms, a (disputed) phylum containing about 100,000 recognised species of mainly unicellular algae. Diatoms generate about 20 percent of the oxygen produced on the planet each year,[146] take in over 6.7 billion metric tons of silicon each year from the waters in which they live,[379] and contribute nearly half of the organic material found in the oceans. The shells (frustules) of dead diatoms can reach as much as half a mile deep on the ocean floor.[380]
• dinoflagellates, a phylum of unicellular flagellates with about 2,000 marine species.[381] Many dinoflagellates are known to be photosynthetic, but a large fraction of these are in fact mixotrophic, combining photosynthesis with ingestion of prey (phagotrophy).[382] Some species are endosymbionts of marine animals and play an important part in the biology of coral reefs. Others predate other protozoa, and a few forms are parasitic.
• euglenophytes, a phylum of unicellular flagellates with only a few marine members

Unlike higher plants, algae lack roots, stems, or leaves. They can be classified by size as microalgae or macroalgae.

Microalgae are the microscopic types of algae, not visible to the naked eye. They are mostly unicellular species which exist as individuals or in chains or groups, though some are multicellular. Microalgae are important components of the marine protists (discussed above), as well as the phytoplankton (discussed below). They are very diverse. It has been estimated there are 200,000-800,000 species of which about 50,000 species have been described.[383] Depending on the species, their sizes range from a few micrometers (µm) to a few hundred micrometers. They are specially adapted to an environment dominated by viscous forces.

• 
  Chlamydomonas globosa, a unicellular green alga with two flagella just visible at bottom left
• 
  Chlorella vulgaris, a common green microalgae, in endosymbiosis with a ciliate[384]
• 
  Centric diatom
• 
  Dinoflagellates

Macroalgae are the larger, multicellular and more visible types of algae, commonly called seaweeds. Seaweeds usually grow in shallow coastal waters where they are anchored to the seafloor by a holdfast. Seaweed that becomes adrift can wash up on beaches. Kelp is a large brown seaweed that forms large underwater forests covering about 25% of the world coastlines.[385] They are among the most productive and dynamic ecosystems on Earth.[386] Some Sargassum seaweeds are planktonic (free-floating). Like microalgae, macroalgae (seaweeds) are technically marine protists since they are not true plants.

• 
  A seaweed is a macroscopic form of
  red or brown or green algae.
• 
  Sargassum seaweed is a planktonic brown alga with air bladders that help it float.
• 
  Sargassum fish are camouflaged to live among drifting Sargassum seaweed.

Kelp forests are among the most productive ecosystems on the planet.

• Unicellular macroalgae (see also macroscopic protists ←)
• 
  The unicellular bubble algae lives in tidal zones. It can have a 4 cm diameter.[387]
• 
  The unicellular mermaid's wineglass are mushroom-shaped algae that grow up to 10 cm high.
• 
  Killer algae are single-celled organisms, but look like ferns and grow stalks up to 80 cm long.[388]

Unicellular organisms are usually microscopic, less than one tenth of a millimeter long. There are exceptions. Mermaid's wineglass, a genus of subtropical green algae, is single-celled but remarkably large and complex in form with a single large nucleus, making it a model organism for studying cell biology.[389] Another single celled algae, Caulerpa taxifolia, has the appearance of a vascular plant including "leaves" arranged neatly up stalks like a fern. Selective breeding in aquariums to produce hardier strains resulted in an accidental release into the Mediterranean where it has become an invasive species known colloquially as killer algae.[390]

Evolution of mangroves and seagrasses

Back in the Silurian, some phytoplankton evolved into red, brown and green algae. These algae then invaded the land and started evolving into the land plants we know today. Later, in the Cretaceous, some of these land plants returned to the sea as marine plants, such as mangroves and seagrasses.[391]

Marine plants can be found in intertidal zones and shallow waters, such as seagrasses like eelgrass and turtle grass, Thalassia. These plants have adapted to the high salinity of the ocean environment. Plant life can also flourish in the brackish waters of estuaries, where mangroves or cordgrass or beach grass beach grass might grow.

• 
  Mangroves
• 
  Seagrass meadow
• 
  Sea dragons camouflaged to look like floating seaweed live in kelp forests and seagrass meadows.[392]

The total world area of mangrove forests was estimated in 2010 as 134,257 square kilometres (51,837 sq mi) (based on satellite data).[393][394] The total world area of seagrass meadows is more difficult to determine, but was conservatively estimated in 2003 as 177,000 square kilometres (68,000 sq mi).[395]

Mangroves and seagrasses provide important nursery habitats for marine life, acting as hiding and foraging places for larval and juvenile forms of larger fish and invertebrates.[396]

Phytoplankton are the plant-like components of the plankton community ("phyto" comes from the Greek for plant). They are autotrophic (self-feeding), meaning they generate their own food and do not need to consume other organisms.

Phytoplankton consist mainly of microscopic photosynthetic eukaryotes which inhabit the upper sunlit layer in all oceans. They need sunlight so they can photosynthesize. Most phytoplankton are single-celled algae, but other phytoplankton are bacteria and some are protists.[398] Phytoplankton groups include cyanobacteria (above), diatoms, various other types of algae (red, green, brown, and yellow-green), dinoflagellates, euglenoids, coccolithophorids, cryptomonads, chrysophytes, chlorophytes, prasinophytes, and silicoflagellates. They form the base of the primary production that drives the ocean food web, and account for half of the current global primary production, more than the terrestrial forests.[399]

Coccolithophores

...have plates called coccoliths

...extinct fossil

Coccolithophores build calcite skeletons important to the marine carbon cycle.[400]

• Phytoplankton
• 
  Phytoplankton are the foundation of the ocean food chain.
• 
  Phytoplankton come in many shapes and sizes.
• 
  Diatoms are one of the most common types of phytoplankton.
• 
  Colonial phytoplankton

• 
  The cyanobacterium Prochlorococcus accounts for much of the ocean's primary production.
• 
  Green cyanobacteria scum washed up on a rock in California
• 
  Gyrodinium, one of the few naked dinoflagellates which lack armour
• 
  Zoochlorellae (green) living inside the ciliate Stichotricha secunda

There are over 100,000 species of diatoms which account for 50% of the ocean's primary production.

• 
  Coccolithophores named after the BBC documentary series
  The Blue Planet
• 
  The coccolithophore Emiliania huxleyi
• 
  Algae bloom of Emiliania huxleyi off the southern coast of England
• 
  Guinardia delicatula, a diatom responsible for algal blooms in the North Sea and the English Channel[401]

Radiolarians

          Drawings by Haeckel 1904 (click for details)

Zooplankton are the animal component of the planktonic community ("zoo" comes from the Greek for animal). They are heterotrophic (other-feeding), meaning they cannot produce their own food and must consume instead other plants or animals as food. In particular, this means they eat phytoplankton.

Foraminiferans

...can have more than one nucleus

...and defensive spines

Foraminiferans are important unicellular zooplankton protists, with calcium shells.

Turing and radiolarian morphology

Shell of a spherical radiolarian

Shell micrographs

Computer simulations of Turing patterns on a sphere
closely replicate some radiolarian shell patterns.[402]

Zooplankton are generally larger than phytoplankton, mostly still microscopic but some can be seen with the naked eye. Many protozoans (single-celled protists that prey on other microscopic life) are zooplankton, including zooflagellates, foraminiferans, radiolarians and some dinoflagellates. Other dinoflagellates are mixotrophic and could also be classified as phytoplankton; the distinction between plants and animals often breaks down in very small organisms. Other zooplankton include pelagic cnidarians, ctenophores, molluscs, arthropods and tunicates, as well as planktonic arrow worms and bristle worms.

Radiolarians are unicellular protists with elaborate silica shells

Microzooplankton: major grazers of the plankton

• 
  Radiolarians come in many shapes.
• 
  Group of planktic foraminiferans
• 
  Copepods eat phytoplankton. This one is carrying eggs.
• 
  The dinoflagellate, Protoperidinium extrudes a large feeding veil to capture prey.

Larger zooplankton can be predatory on smaller zooplankton.

Macrozooplankton

• 
  Moon jellyfish
• 
  Venus girdle, a ctenophore
• 
  Arrow worm
• 
  Tomopteris, a planktonic segmented worm with unusual yellow bioluminescence[403]
• 
  Marine amphipod
• 
  Krill
• 
  Pelagic sea cucumber

External video
Venus Girdle - Youtube

Many marine animals begin life as zooplankton in the form of eggs or larvae, before they develop into adults. These are meroplanktic, that is, they are planktonic for only part of their life.

• Larvae and juveniles
• 
  Salmon larva hatching from its egg
• 
  Ocean sunfish larva
• 
  Juvenile planktonic squid
• 
  Larva stage of a spiny lobster

A surf wave at night sparkles with blue light due to the presence of a bioluminescent dinoflagellate, such as Lingulodinium polyedrum

A suggested explanation for glowing seas[404]

Dinoflagellates are often mixotrophic or live in symbiosis with other organisms.

• Mixoplankton
• 
  Tintinnid ciliate Favella
• 
  Euglena mutabilis, a photosynthetic flagellate
• 
  Noctiluca scintillans, a bioluminescence dinoflagellate

Some dinoflagellates are bioluminescent. At night, ocean water can light up internally and sparkle with blue light because of these dinoflagellates.[405][406] Bioluminescent dinoflagellates possess scintillons, individual cytoplasmic bodies which contain dinoflagellate luciferase, the main enzyme involved in the luminescence. The luminescence, sometimes called the phosphorescence of the sea, occurs as brief (0.1 sec) blue flashes or sparks when individual scintillons are stimulated, usually by mechanical disturbances from, for example, a boat or a swimmer or surf.[407]

Pelagic food web

Compared to terrestrial environments, marine environments have biomass pyramids which are inverted at the base. In particular, the biomass of consumers (copepods, krill, shrimp, forage fish) is larger than the biomass of primary producers. This happens because the ocean's primary producers are tiny phytoplankton which tend to be r-strategists that grow and reproduce rapidly, so a small mass can have a fast rate of primary production. In contrast, terrestrial primary producers, such as mature forests, are often K-strategists that grow and reproduce slowly, so a much larger mass is needed to achieve the same rate of primary production.

Because of this inversion, it is the zooplankton that make up most of the marine animal biomass. As primary consumers, they are the crucial link between the primary producers (mainly phytoplankton) and the rest of the marine food web (secondary consumers).[408]

If phytoplankton dies before it is eaten, it descends through the euphotic zone as part of the marine snow and settles into the depths of sea. In this way, phytoplankton sequester about 2 billion tons of carbon dioxide into the ocean each year, causing the ocean to become a sink of carbon dioxide holding about 90% of all sequestered carbon.[409]

In 2010 researchers found whales carry nutrients from the depths of the ocean back to the surface using a process they called the whale pump.[410] Whales feed at deeper levels in the ocean where krill is found, but return regularly to the surface to breathe. There whales defecate a liquid rich in nitrogen and iron. Instead of sinking, the liquid stays at the surface where phytoplankton consume it. In the Gulf of Maine the whale pump provides more nitrogen than the rivers.[411]

Marine biogeochemical cycles

The dominant feature of the planet viewed from space is water – oceans of liquid water flood most of the surface while water vapour swirls in atmospheric clouds and the poles are capped with ice.

Taken as a whole, the oceans form a single marine system where water – the "universal solvent" [412] – dissolves nutrients and substances containing elements such as oxygen, carbon, nitrogen and phosphorus. These substances are endlessly cycled and recycled, chemically combined and then broken down again, dissolved and then precipitated or evaporated, imported from and exported back to the land and the atmosphere and the ocean floor. Powered both by the biological activity of marine organisms and by the natural actions of the sun and tides and movements within the Earth's crust, these are the marine biogeochemical cycles.[413][414]

• 
  Marine carbon cycle[415]
• 
  Oxygen cycle

• 
  Marine nitrogen cycle
• 
  Marine phosphorus cycle

Thickness of marine sediments

Sediments at the bottom of the ocean have two main origins, terrigenous and biogenous. Terrigenous sediments account for about 45% of the total marine sediment, and originate in the erosion of rocks on land, transported by rivers and land runoff, windborne dust, volcanoes, or grinding by glaciers.

Biogenous sediments account for the other 55% of the total sediment, and originate in the skeletal remains of marine protists (single-celled plankton and benthos organisms). Much smaller amounts of precipitated minerals and meteoric dust can also be present. Ooze, in the context of a marine sediment, does not refer to the consistency of the sediment but to its biological origin. The term ooze was originally used by John Murray, the "father of modern oceanography", who proposed the term radiolarian ooze for the silica deposits of radiolarian shells brought to the surface during the Challenger Expedition.[416] A biogenic ooze is a pelagic sediment containing at least 30 percent from the skeletal remains of marine organisms.

Main types of biogenic ooze
type	mineral forms	protist responsible		name of skeleton	description
Siliceous ooze	SiO2 quartz glass opal chert	diatoms		frustule	Individual diatoms range in size from 0.002 to 0.2 mm.[417]
		radiolarians		skeleton	Radiolarians are protozoa with diameters typically between 0.1 and 0.2 mm that produce intricate mineral skeletons, usually made of silica
Calcareous ooze	CaCO3 calcite aragonite limestone chalk	foraminiferans		test	There are about 10,000 living species of foraminiferans,[418] usually under 1 mm in size.
		coccolithophores		coccolith	Coccolithophores are spherical cells usually less than 0.1 mm across, enclosed by calcareous plates called coccoliths.[419] Coccoliths are important microfossils. They are the largest global source of biogenic calcium carbonate, and make significant contributions to the global carbon cycle.[420] They are the main constituent of chalk deposits such as the white cliffs of Dover.

• 
  An elaborate mineral skeleton of a radiolarian made of silica.
• 
  Diatoms, major components of marine plankton, also have silica skeletons called frustules.
• 
  Coccolithophores have plates or scales made with calcium carbonate called coccoliths
• 
  Calcified test of a planktic foraminiferan

• 
  A diatom microfossil from 40 million years ago
• 
  Diatomaceous earth is a soft, siliceous, sedimentary rock made up of microfossils in the form of the frustules (shells) of single cell diatoms (click to magnify).
• 
  Illustration of a Globigerina ooze
• 
  Shells (tests), usually made of calcium carbonate, from a foraminiferal ooze on the deep ocean floor

The drainage basins of the principal oceans and seas of the world are marked by continental divides. The grey areas are endorheic basins that do not drain to the ocean.

Land interactions impact marine life in many ways. Coastlines typically have continental shelves extending some way from the shore. These provide extensive shallows sunlit down to the seafloor, allowing for photosynthesis and enabling habitats for seagrass meadows, coral reefs, kelp forests and other benthic life. Further from shore the continental shelf slopes towards deep water. Wind blowing at the ocean surface or deep ocean currents can result in cold and nutrient rich waters from abyssal depths moving up the continental slopes. This can result in upwellings along the outer edges of continental shelves, providing conditions for phytoplankton blooms.

Water evaporated by the sun from the surface of the ocean can precipitate on land and eventually return to the ocean as runoff or discharge from rivers, enriched with nutrients as well as pollutants. As rivers discharge into estuaries, freshwater mixes with saltwater and becomes brackish. This provides another shallow water habitat where mangrove forests and estuarine fish thrive. Overall, life in inland lakes can evolve with greater diversity than happens in the sea, because freshwater habitats are themselves diverse and compartmentalised in a way marine habitats are not. Some aquatic life, such as salmon and eels, migrate back and forth between freshwater and marine habitats. These migrations can result in exchanges of pathogens and have impacts on the way life evolves in the ocean.