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An Introduction to the Classification of Elasmobranchs

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An Introduction to the Classification of Elasmobranchs An introduction to the classification of elasmobranchs 17 Rekha J. Nair and P.U Zacharia Central Marine Fisheries Research Institute, Kochi-682 018 Introduction eyed, stomachless, deep-sea creatures that possess an upper jaw which is fused to its cranium (unlike in sharks). The term Elasmobranchs or chondrichthyans refers to the The great majority of the commercially important species of group of marine organisms with a skeleton made of cartilage. chondrichthyans are elasmobranchs. The latter are named They include sharks, skates, rays and chimaeras. These for their plated gills which communicate to the exterior by organisms are characterised by and differ from their sister 5–7 openings. In total, there are about 869+ extant species group of bony fishes in the characteristics like cartilaginous of elasmobranchs, with about 400+ of those being sharks skeleton, absence of swim bladders and presence of five and the rest skates and rays. Taxonomy is also perhaps to seven pairs of naked gill slits that are not covered by an infamously known for its constant, yet essential, revisions operculum. The chondrichthyans which are placed in Class of the relationships and identity of different organisms. Elasmobranchii are grouped into two main subdivisions Classification of elasmobranchs certainly does not evade this Holocephalii (Chimaeras or ratfishes and elephant fishes) process, and species are sometimes lumped in with other with three families and approximately 37 species inhabiting species, or renamed, or assigned to different families and deep cool waters; and the Elasmobranchii, which is a large, other taxonomic groupings. It is certain, however, that such diverse group (sharks, skates and rays) with representatives revisions will clarify our view of the taxonomy and phylogeny in all types of environments, from fresh waters to the bottom (evolutionary relationships) of elasmobranchs, leading to a of marine trenches and from polar regions to warm tropical better understanding of how these creatures evolved. waters with over 950 different species. Chimaeras are big- Fig. 1. Main parts of an Elasmobranch fish Summer school on recent advances in marine biodiversiy conservation and management 118 Rekha J. Nair and P.U Zacharia Fig. 2. Shark tail Fig. 3. Eye of sharkw (Source: FAO Species Catalogue No.8, vol. 1) Class Chondrichthyes (cartilaginous • Labial furrows when present reduced or absent on lower fishes) jaw Subclass Holocephali (chimaeras and fossil relatives) • Teeth without enlarged anterior or posterior teeth without Order Chimaeriformes (chimaeras or silver sharks) a gap or small intermediate teeth between anterior and Subclass Neoselachii (modern sharks and batoids) lateral teeth in the upper jaw Cohort Selachii (modern sharks) • 2 families • Family Chlamydoselachidae (Genus Chlamydoselachus) Superorder Squalomorphii (Squalomorph • Family Hexanchidae (Genera Hexanchus, Heptranchias) sharks) Order Hexanchiformes (cow and frilled sharks) Order Squaliformes - dogfish sharks Order Squaliformes (dogfish sharks) • Two dorsal fins, with or without spines Order Squatiniformes (angel sharks) • Anal fin absent Order Pristiophoriformes (sawsharks) • Five gill slits • Spiracles present Superorder Galeomorphii (Galeomorph • Nictiating lower eyelid absent sharks) • Lateral-line canal closed Order Heterodontiformes (bullhead sharks) • 7 families - Echinorhinidae*, Oxynotidae, Squalidae, Order Lamniformes (mackerel sharks) Etmopteridae, Centrophoridae, Somniosidae, Dalatiidae Order Orectolobiformes (carpet sharks) • As per Nelson (2006), Family Echinorhinidae is placed in Order Carcharhiniformes (ground sharks) a separate order Echinirhiniformes Cohort Batoidea (batoids) Order Torpediniformes (electric rays) Order Squatiniformes – angel sharks. Order Pristiformes (sawfishes) • Marine, temperate to tropical, found along continental Order Rajiformes (skates and guitarfishes) shelves and upper slopes Order Myliobatiformes (stingrays) • Atlantic, and Pacific • Body ray like Order Hexanchiformes (cow and frilled • Eyes dorsal sharks) • Two spineless dorsal fins • One dorsal fin, without spine,trunk cylindrical, head • No anal fin slightly depressed • Five gill openings • Anal fin present • Spiracle large • Six or seven gill slits present on sides of head, • Mouth almost terminal posteriormost in front of pectoral fin origins • Nostrils terminal with barbels on anterior margin. • Eyes without nictitating fold Maximum length up to 2 m. • Spiracle present but small, well behind eye • Family Squatinidae– angel sharks. • Nostrils without barbels • Nasoral grooves separate from mouth Order Pristiophoriformes– saw sharks. • Mouth large, arched and elongated, extending well • Marine (rarely in estuaries), temperate to tropical, behind eyes continental and insular shelves and slopes 16th February – 8th March 2015 Central Marine Fisheries Research Institute 119 An introduction to the classification of elasmobranchs Fig. 4 Measurements used for sharks (Source: FAO Species Catalogue No.8, vol. 1) Summer school on recent advances in marine biodiversiy conservation and management 120 Rekha J. Nair and P.U Zacharia • Western Atlantic in region of Bahamas, Florida, and ∗ Head nearly as long as trunk Cuba, Southwestern Indian off South Africa, and Western ∗ Snout with a greatly elongated with flattened blade like Pacific from Southern Australia to Japan. projections • Body shark like ∗ Jaws very protusible • Snout produced in along flat blade with teeth on each ∗ Gill openings short side ∗ Peduncle pit absent • One pair of long barbels ∗ Eye small • No dorsal fin spines ∗ Caudal fin long but ventral lobe not developed • Anal fin absent ∗ One genus, one species- Mitsukurina owstoni, Goblin • Spiracles large shark ∗Family Pristiophoridae - saw sharks. Family Pseudocarchariidae - crocodile sharks Superorder Galeomorphii (Galeomorph sharks) ∗ Eyes exceptionally large ∗ Mouth large, parabolic, ventral on head, jaws protrusible Order Heterodontiformes (bullhead ∗ Teeth large, anterior ones narrow, laterals compressed sharks) blade like • Two dorsal fins, each with a spine ∗ Gill openings extending onto dorsal surface of head • Anal fin present ∗ Pectoral fins small, pelvic large • Five gill slits ∗ Caudal peduncle with upper and lower precaudal pits • Spiracle present but small and with low lateral keel • Snout pig like ∗ One genus with one species- Pseudocarcharias • Nictitating fold absent kamoharai, Crocodile shark • Oviparous • One family - Heterodontidae Family Megachasmidae – megamouth • 1genus Heterodontus, with eight species sharks ∗ Head elongated, about length of trunk Order Lamniformes – mackerel sharks ∗ Mouth very large, terminal • Head conical, not expanded laterally ∗ Snout short and broadly rounded • Eyes usually on sides of head ∗ Gill opening moderately long but not extending onto • Nictitating eyelids, barbels absent dorsal surface of head last two gill slits over pectoral fin • Snout very short base • Spiracles usually present ∗ Teeth small, in numerous rows • Five gill slits, last two above pectoral fin origin ∗ One genus with one species Megachasma pelagios- • Two dorsal fins, without spines Megamouth shark • Anal fin present • 7 families with 10 genera and 15 species Family Alopiidae – thresher sharks * Family Odontaspididae - Sand Tiger Sharks ∗ Upper lobe of caudal fin long and curving, about as long * Family Pseudocarchariidae - Crocodile Sharks as rest of shark * Family Alopiidae – Thresher Sharks ∗ Last two gill openings above pectoral fin base * Family Cetorhinidae– Basking Sharks ∗ Gill openings short * Family Lamnidae - Mackerel Sharks ∗ Mouth small * Family Mitsukurinidae - Goblin Sharks ∗ Pectoral fins long and narrow * Family Megachasmidae – Megamouth Sharks ∗ One genus, Alopias, with three species Family Odontaspididae - sand tiger sharks Alopias superciliosus ∗ Eyes relatively small ∗ Head nearly flat between eyes, with a deep horizontal ∗ Gill openings all in front of pectoral fin, relatively large groove on nape on each side above gills but not extending onto dorsal surface of head ∗ Eyes very large, with orbits expanded onto dorsal surface ∗ Caudal peduncle with an upper precaudal pit, without a of head lateral keel ∗ 2 genera- Carcharias and Odontaspis, with three species Alopias pelagicus ∗ Carcharias taurus-, Odontaspis ferox, Odontaspis ∗ Head narrow, forehead nearly straight noronhai ∗ Eyes smaller, with orbits not expanded onto dorsal surface of head Family Mitsukurinidae - goblin sharks ∗ Pectoral fins nearly straight and broad-tipped 16th February – 8th March 2015 Central Marine Fisheries Research Institute 121 An introduction to the classification of elasmobranchs ∗ Sides above pectoral bases dark Alopias vulpinus ∗ Head broad, snout shorter ∗ Eyes smaller, with orbits not expanded onto dorsal surface of head ∗ Forehead strongly arched ∗ Sides above pectoral-fin bases marked with a white patch Family Cetorhinidae– basking sharks ∗ Gill openings exceptionally large, extending almost to the top of the head; ∗ Teeth small and numerous ∗ Mouth large • Spiracles present ∗ Eye small • Teeth very small, numerous, with a single medial cusp ∗ Gill rakers elongate and usually one or more cusplets on each side near the ∗ One genus with one species Cetorhinus maximus, basking center of mouth shark • Intestine with spiral valve • Lateral keels or precaudal pits absent on caudal peduncle Family Lamnidae - mackerel sharks • 16 genera- of which 7 genera are deep sea forms found ∗ Large sharks with pointed snouts and spindle-shaped
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