Sharks The Animal Answer Guide

6 Sharks: The Animal Answer Guide

The chimaeras have beaklike tooth plates that are worn and regrown as

the animal grows. Their buck teeth and large eyes give many chimaeras a

rabbitlike appearance, which is why one alternative name for the group is

rabbitfish (not to be confused with the Siganidae, a family of tropical reef

fishes also called rabbitfishes). Holocephalans are also called ratfishes and

ghost sharks.

Elasmobranchs are more or less covered by scales called denticles that

look like elongate plates with microscopic ridges, curved spikes, or pedes-

tals protruding up from the surface. These denticles (technically, “placoid

scales”) may not overlap like the round, flat scales of bony fishes such as bass

or perch or goldfish but are instead scattered around the body and packed

together. Studies of developing embryonic sharks show that the scales are

derived from the same tissues as the teeth in the mouth of a shark, whereas

scales in bony fishes develop from bony layers in the skin. Holocephalans

and many batoids are almost scaleless. All the chondrichthyan fishes except

manta rays have their mouths and nasal (nostril) openings on the underside

of their head; in bony fishes, the mouth is typically at the front end of the

head and the nostrils are on the upper snout.

The major groups also differ in the way they swim. Sharks (but not

skates and rays) swim by passing waves of muscular contraction from their

head down to their tail, or by swinging their tail back and forth (see “How

do sharks swim?” in chapter 2). Skates and rays swim using their pectoral

(paired side) fins. Skates pass waves of contraction along the outer edges

of these fins; rays similarly pass waves along the edges but also flap their

large pectoral fins, much like birds flap their wings. Chimaeras use both

A close-up photo of shark skin,
showing the dermal denticles, or
placoid scales. The head of the
shark would be toward the top of
the photo. Streamlining is increased
by the ridges on the scales and their
flared nature, each scale consisting
of a flattened top sitting on a pedes-
tal. NOAA (National Oceanic and Atmospheric
Administration) Teacher at Sea Program, http://
teacheratsea.files.wordpress.com/ 2011 / 08 /
dermal-denticles-magnified.jpg

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