Benthos

Macrobenthos, prefix from Ancient Greek makrós 'long', comprises the larger, visible to the naked eye, benthic organisms greater than about 1 mm in size. Some examples are polychaete worms, bivalves, echinoderms, sea anemones, corals, sponges, sea squirts, turbellarians and larger crustaceans such as crabs, lobsters and cumaceans.[8]

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  Seagrass
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  Echinoderms
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  Sea squirts
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  Aquatic worm, snail, midge larvae and amphipods

Meiobenthos, prefix from Ancient Greek omeíkhō 'halfway, middle', comprises tiny benthic organisms that are less than about 1 mm but greater than about 0.1 mm in size. Some examples are nematodes, foraminiferans, tardigrades, gastrotriches and smaller crustaceans such as copepods and ostracodes.

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  Foraminiferan
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  Gastrotrich
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  Copepod
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  Tardigrade (water bear)

Microbenthos, prefix from the Greek mikrós 'small', comprises microscopic benthic organisms that are less than about 0.1 mm in size. Some examples are bacteria, diatoms, ciliates, amoeba, flagellates.

Benthic diatom

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  Diatoms
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  Ciliate
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  Dinoflagellates

Marine microbenthos are microorganisms that live in the benthic zone of the ocean – that live near or on the seafloor, or within or on surface seafloor sediments. The word benthos comes from Greek, meaning "depth of the sea". Microbenthos are found everywhere on or about the seafloor of continental shelves, as well as in deeper waters, with greater diversity in or on seafloor sediments. In shallow waters, seagrass meadows, coral reefs and kelp forests provide particularly rich habitats. In photic zones benthic diatoms dominate as photosynthetic organisms. In intertidal zones changing tides strongly control opportunities for microbenthos.

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  Elphidium a widespread abundant genus of benthic forams
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  Heterohelix, an extinct genus of benthic forams

• Marine microanimals
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  Darkfield photo of a gastrotrich, 0.06-3.0 mm long, a worm-like animal living between sediment particles
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  Armoured Pliciloricus enigmaticus, about 0.2 mm long, live in spaces between marine gravel

Both foraminifera and diatoms have planktonic and benthic forms, that is, they can drift in the water column or live on sediment at the bottom of the ocean. Either way, their shells end up on the seafloor after they die. These shells are widely used as climate proxies. The chemical composition of the shells are a consequence of the chemical composition of the ocean at the time the shells were formed. Past water temperatures can be also be inferred from the ratios of stable oxygen isotopes in the shells, since lighter isotopes evaporate more readily in warmer water leaving the heavier isotopes in the shells. Information about past climates can be inferred further from the abundance of forams and diatoms, since they tend to be more abundant in warm water.[9]

The sudden extinction event which killed the dinosaurs 66 million years ago also rendered extinct three-quarters of all other animal and plant species. However, deep-sea benthic forams flourished in the aftermath. In 2020 it was reported that researchers have examined the chemical composition of thousands of samples of these benthic forams and used their findings to build the most detailed climate record of Earth ever.[10][11]

Some endoliths have extremely long lives. In 2013 researchers reported evidence of endoliths in the ocean floor, perhaps millions of years old, with a generation time of 10,000 years.[12] These are slowly metabolizing and not in a dormant state. Some Actinomycetota found in Siberia are estimated to be half a million years old.[13][14][15]

Zoobenthos, prefix from Ancient Greek zôion 'animal', animals belonging to the benthos.

Phytobenthos, prefix from Ancient Greek phutón 'plant', plants belonging to the benthos, mainly benthic diatoms and macroalgae (seaweed).

Endobenthos (or endobenthic), prefix from Ancient Greek éndon 'inner, internal', lives buried, or burrowing in the sediment, often in the oxygenated top layer, e.g., a sea pen or a sand dollar.

Epibenthos (or epibenthic), prefix from Ancient Greek epí 'on top of', lives on top of the sediments, e.g., like a sea cucumber or a sea snail crawling about. Unlike other epiphytes.

Hyperbenthos (or hyperbenthic), prefix from Ancient Greek hupér 'over', lives just above the sediment, e.g., a rock cod.

Benthic macro-invertebrates play a critical role in aquatic ecosystems. These organisms can be used to indicate the presence, concentration, and effect of water pollutants in the aquatic environment. Some water contaminants—such as nutrients, chemicals from surface runoff, and metals[16]—settle in the sediment of river beds, where many benthos reside. Benthos are highly sensitive to contamination, so their close proximity to high pollutant concentrations make these organisms ideal for studying water contamination.[17]

Benthos can be used as bioindicators of water pollution through ecological population assessments or through analyzing biomarkers. In ecological population assessments, a relative value of water pollution can be detected. Observing the number and diversity of macro-invertebrates in a waterbody can indicate the pollution level. In highly contaminated waters, a reduced number of organisms and only pollution-tolerant species will be found.[18] In biomarker assessments, quantitative data can be collected on the amount of and direct effect of specific pollutants in a waterbody. The biochemical response of macro-invertebrates' internal tissues can be studied extensively in the laboratory. The concentration of a chemical can cause many changes, including changing feeding behaviors,[19] inflammation, and genetic damage,[20] effects that can be detected outside of the stream environment. Biomarker analysis is important for mitigating the negative impacts of water pollution because it can detect water pollution before it has a noticeable ecological affect on benthos populations.[21]