Batoidea

Batoids are flat-bodied, and, like sharks, are cartilaginous fish, meaning they have a boneless skeleton made of a tough, elastic cartilage. Most batoids have five ventral slot-like body openings called gill slits that lead from the gills, but the Hexatrygonidae have six.[2] Batoid gill slits lie under the pectoral fins on the underside, whereas a shark's are on the sides of the head. Most batoids have a flat, disk-like body, with the exception of the guitarfishes and sawfishes, while most sharks have a spindle-shaped body. Many species of batoid have developed their pectoral fins into broad flat wing-like appendages. The anal fin is absent. The eyes and spiracles are located on top of the head. Batoids have a ventrally located mouth and can considerably protrude their upper jaw (palatoquadrate cartilage) away from the cranium to capture prey.[3] The jaws have euhyostylic type suspension, which relies completely on the hyomandibular cartilages for support.[4] Bottom-dwelling batoids breathe by taking water in through the spiracles, rather than through the mouth as most fish do, and passing it outward through the gills.

Batoids reproduce in a number of ways. As is characteristic of elasmobranchs, batoids undergo internal fertilization. Internal fertilization is advantageous to batoids as it conserves sperm, does not expose eggs to consumption by predators, and ensures that all the energy involved in reproduction is retained and not lost to the environment.[5] All skates and some rays are oviparous (egg laying) while other rays are ovoviviparous, meaning that they give birth to young which develop in a womb but without involvement of a placenta.[6]

The eggs of oviparous skates are laid in leathery egg cases that are commonly known as mermaid's purses and which often wash up empty on beaches in areas where skates are common.

Capture-induced premature birth and abortion (collectively called capture-induced parturition) occurs frequently in sharks and rays when fished.[6] Capture-induced parturition is rarely considered in fisheries management despite being shown to occur in at least 12% of live bearing sharks and rays (88 species to date).[6]

Most species live on the sea floor, in a variety of geographical regions – mainly in coastal waters, although some live in deep waters to at least 3,000 metres (9,800 ft). Most batoids have a cosmopolitan distribution, preferring tropical and subtropical marine environments, although there are temperate and cold-water species. Only a few species, like manta rays, live in the open sea, and only a few live in freshwater, while some batoids can live in brackish bays and estuaries.

Most batoids have developed heavy, rounded teeth for crushing the shells of bottom-dwelling species such as snails, clams, oysters, crustaceans, and some fish, depending on the species. Manta rays feed on plankton.

Radiation of cartilaginous fishes, based on Michael Benton, 2005.[7]

Batoids belong to the ancient lineage of cartilaginous fishes. Fossil denticles (tooth-like scales in the skin) resembling those of today's chondrichthyans date at least as far back as the Ordovician, with the oldest unambiguous fossils of cartilaginous fish dating from the middle Devonian. A clade within this diverse family, the Neoselachii, emerged by the Triassic, with the best-understood neoselachian fossils dating from the Jurassic. The oldest confirmed ray is Antiquaobatis, from the Pliensbachian of Germany.[8] The clade is represented today by sharks, sawfish, rays and skates.[9]

The classification of batoids is currently undergoing revision; however, molecular evidence refutes the hypothesis that skates and rays are derived sharks.[10] Nelson's 2006 Fishes of the World recognizes four orders. The Mesozoic Sclerorhynchoidea are basal or incertae sedis; they show features of the Rajiformes but have snouts resembling those of sawfishes. However, evidence indicates they are probably the sister group to sawfishes.[11] Phylogenetic tree of Batoidea:[12]

Order	Image	Common name	Family	Genera	Species				Comment
					Total
Myliobatiformes		Stingrays and relatives	10	29	223	1	16	33	[lower-alpha 1][13]
Rajiformes		Skates and relatives	5	36	270	4	12	26	[lower-alpha 2]
Torpediniformes		Electric rays	4	12	69	2		9	[lower-alpha 3][14]
Rhinopristiformes		Shovelnose rays and relatives	1	2	5-7	3-5	2		[lower-alpha 4][15]

Early Eocene fossil stingray Heliobatis radians

Order Torpediniformes

• Family Hypnidae (coffin rays)
• Family Narcinidae (numbfishes)
• Family Narkidae (sleeper rays)
• Family Torpedinidae (torpedo rays)

Order Rhinopristiformes

• Family Glaucostegidae (giant guitarfishes)
• Family Platyrhinidae* (fanrays)
• Family Pristidae (sawfishes)
• Family Rhinidae (wedgefishes)
• Family Rhinobatidae (guitarfishes)
• Family Trygonorrhinidae (banjo rays)
• Family Zanobatidae* (panrays)

* the placement of these families is uncertain

Order Rajiformes

• Family Anacanthobatidae (legskates)
• Family Arhynchobatidae (softnose skates)
• Family Gurgesiellidae (pygmy skates)
• Family Rajidae (skates)

Order Myliobatiformes

• Family Aetobatidae (pelagic eagle rays)
• Family Dasyatidae (whiptail stingrays)
• Family Gymnuridae (butterfly rays)
• Family Hexatrygonidae (sixgill stingrays)
• Family Myliobatidae (devilrays)
• Family Plesiobatidae (giant stingarees)
• Family Potamotrygonidae (Neotropical stingrays)
• Family Rhinopteridae (cownose rays)
• Family Urolophidae (stingarees)
• Family Urotrygonidae (round stingrays)

According to a 2021 study in Nature, the number of oceanic sharks and rays has declined globally by 71% over the preceding 50 years, jeopardising "the health of entire ocean ecosystems as well as food security for some of the world's poorest countries". Overfishing has increased the global extinction risk of these species to the point where three-quarters are now threatened with extinction.[16][17][18] This is notably the case in the Mediterranean Sea - most impacted by unregulated fishing - where a recent international survey of the Mediterranean Science Commission concluded that only 38 species of rays and skates still subsisted.[19]

All sharks and rays are cartilaginous fish, contrasting with bony fishes. Many rays are adapted for feeding on the bottom. Guitarfishes are somewhat between sharks and rays, displaying characteristics of both (though they are classified as rays).

Comparison of Elasmobranchid fish

Characteristic	Sharks	Guitar fish	Rays
Shape	laterally compressed spindle	dorsoventrally compressed (flattened) disc
Spiracles	not always present		always present
Habitat	usually pelagic surface feeders, though carpet sharks are demersal bottom feeders	demersal / pelagic mix	usually demersal bottom feeders
Eyes	usually at the side of the head	usually on top of the head
Gill openings	on the sides	ventral (underneath)
Pectoral fins	distinct	not distinct
Tail	large caudal fin whose primary function is to provide main forward propulsion		varies from thick tail as extension of body to a whip that can sting to almost no tail.
Locomotion	swim by moving their tail (caudal fin) from side to side	Guitar fish and sawfish have a caudal fin like sharks	swim by flapping their pectoral fins like wings

Wikimedia Commons has media related to Batoidea.

• List of cartilaginous fish#clade Batoidea
• Rhenanida

• McEachran, J.D.; Dunn, K.A.; Miyake, T. (1996). "Interrelationships of the batoid fishes (Chondrichthyes: Batoidea)". Interrelationships of Fishes. Academic Press.
• Nelson, Joseph S. (2006). Fishes of the World (4th, illustrated ed.). John Wiley & Sons. ISBN 9780471756446.

• "Shark references". – database of bibliography of living/fossil sharks and rays (see Chondrichtyes: Selachii) with more than 15 000 listed papers and many download links.
• "Rays Fact Sheet". Rays fact sheet (PDF). Fisheries (Report). Recreational fishing. Perth, Australia: Government of Western Australia. Archived from the original (PDF) on 13 May 2013.