Modern neoselachian diversity
Nearly 950 species of neoselachians exist today, including 403 described sharklike species and 534 skates and rays (Nelson 2006) (Fig. 12.2).
Sharks (subdivision Selachii) can generally be distinguished from rays (subdivision Batoidea) by the following features. Sharks have: (i) gill openings on the sides of the body; (ii) the anterior edge of the pectoral fin not attached to the side of the head; (iii) the anal fin present in galeomorphs but absent in squalomorphs (except for the five species of hexanchiforms); and (iv) small lateral spiracles compared with large dorsal spiracles in rays. Rays in contrast have: (i) ventral gill openings; (ii) the anterior edge of the enlarged pectoral fin attached to the side of the head; (iii) the anal fin absent; and (iv) the intake of water for breathing chiefly through an enlarged dorsal spiracle (except in water column species).
Among the sharks, the requiem or ground sharks (Carcharhiniformes) make up more than half the species and are particularly diverse in tropical and subtropical, nearshore habitats. Offshore, pelagic sharks include lamniform species such as mako, White, thresher, and Basking sharks, whereas the squaliform dogfishes are particularly successful in the North Atlantic, North Pacific, and deepsea regions. The batoids are concentrated in four orders, the torpediniform torpedo rays, the pristiform sawfishes, the rajiform skates, and the myliobatiform stingrays. Skates are most diverse and abundant in deep water and at high latitudes, whereas stingrays are most diverse in tropical, inshore waters. Most skates have one or two dorsal fins and long, slender claspers that are depressed at their distal end, whereas stingrays have a serrated tail spine (the “sting”), lack dorsal fins, and have short, stout claspers that are cylindrical or only moderately depressed.
Taxonomic distribution and representative orders of the c. 950 species of modern sharks, skates, and rays. (A) Sharklike fishes in nine orders constitute 40% of modern euselachian species, with the carcharhiniform (ground or requiem) sharks outnumbering all other orders combined. The echinorhiniform bramble sharks, with two species, are not shown. (B) Raylike batoids make up 60% of the Euselachii, dominated by skates and stingrays; the four recognized orders are shown. Guitarfishes (Rhinobatidae, Rhinidae) are diverse members of the Rajiformes. Adapted from Compagno (1990b), used with permission.
Amidst this diversity, certain general patterns emerge that emphasize the unique traits and fascinating adaptations of elasmobranchs. These trends include: (i) large size; (ii) a marine habitat; (iii) mobility; (iv) slow metabolism and slow growth; (v) predatory feeding habits; (vi) reliance on non-visual senses; (vii) low fecundity and precocial (independent) young; and (viii) vulnerability to exploitation (see Compagno 1990b; Gruber 1991).