Sharks The Animal Answer Guide

Introducing Sharks, Skates, Rays, and Chimaeras 5

even species is the shape and nature of the cartilaginous skull (chondro-

cranium). Anyone interested in this important subject should consult the

general references listed in the bibliography.

But several differences and characteristics are worth exploring. For ex-

ample, all elasmobranchs have multiple external gill slits for expelling the

water they breathe, whereas the holocephalan chimaeras have a single ex-

ternal gill slit. The gill arches are the internal cartilage structures on which

the gills are supported. Water flows in the mouth, through the gill arches,

and out the gill slits. The vast majority of sharks have five gill arches and

five gill slits. But variety is the spice of life, and a few sharks and rays de-

part from the norm. Fortunately, this departure is usually reflected in their

names, both common and scientific. Hence, sixgill sharks are Hexanchus

(Greek for six openings), sevengill sharks are Heptranchias (seven open-

ings), and the sixgill stingrays are in the family Hexatrygonidae (“six” plus

a root word for stingrays that refers to their harplike shape; the adjective

and noun form would be “hexatrygonid”). The Sixgill Sawshark, Pliotrema

warreni, is not as conveniently named.

All chondrichthyan sharklike fishes have internal fertilization, the males

possessing paired penis-like structures called claspers made from modified

pelvic fins. In chimaeras the claspers have four lobes. Rays and most sharks

are live-bearers, whereas some sharks and all skates and chimaeras are egg-

layers. All sharklike fishes produce young that are miniature versions of the

adult, which is to say that the young don’t have a larval life stage. All holo-

cephalans are marine, but many sharks, skates, and rays spend part or all of

their lives in brackish or fresh water.

Another difference between the two main groups—elasmobranchs and

holocephalans—involves their teeth, or dentition. Elasmobranchs have

many teeth and replace old teeth with new teeth throughout their lives.

Body parts of a generalized shark. Many sharks lack spines in front of the dorsal fins; some lack the second, usually
smaller dorsal fin; and a few have six or seven gill slits. Wikimedia Commons, http://en.wikipedia.org/wiki/File:Parts_of_a_shark.svg