Introducing Sharks, Skates, Rays, and Chimaeras 7
their bodies and their fins to swim, passing a wave of muscular contractions
down the body but also flapping their pectoral fins. When not moving,
fishes need a way of keeping from sinking in the water. Bony fishes have a
gas-filled bladder for maintaining buoyancy; chondrichthyans use a large,
oil-rich liver to achieve the same end.
Chimaeras are chondrichthyan fishes but not elasmobranchs. They are
in a separate subclass, the Holocephali, with one modern order, the Chi-
maeriformes (and six extinct orders known from fossils that are very dis-
similar to modern forms). Aside from their cartilage skeletons, chimaeras
share few similarities with sharks and rays, and they almost seem like a
hodgepodge assembled from spare parts. The name, in fact, comes from
Greek mythology, in which a chimaera was a monster constructed of un-
likely parts.
Sharks can be told apart from skates and rays fairly easily. A major dif-
ference is that in selachian sharks, the gill slits are on the sides above and
ahead of the pectoral fins, whereas in batoid skates and rays, the gills are on
the belly (ventral) side below the pectoral fins and behind where the front
end of the pectorals attach to the body. Also, most sharks have a relatively
narrow attachment point for their relatively narrow pectoral fins, and the
front edge of the pectoral fin is attached behind the head. In batoids, the
pectorals are broadly attached, sometimes as wide as the body is long; and
the front edge of the pectoral fin may be attached to the head, giving the
fins a winglike appearance and producing an overall disklike shape to the
body. Chimaeras’ pectoral fins are not as broad as those of rays, and the at-
tachment to the body is just behind the head and quite narrow, more like
that of a bony fish such as a seabass. Teeth also differ among groups. Ba-
toids have flat, pavement-like teeth that carpet the mouth, which means
multiple rows function to crush prey. Selachians usually have more pointed
teeth around their jaws, with the outer row the most functional. Chimae-
ras’ teeth are fused into a beak. Although body shape would seem an obvi-
ous identifier—selachians are torpedo-shaped and round in cross-section,
A juvenile Pacific Spookfish, Rhi-
nochimaera pacifica. This relatively
large chimaera can grow to 1. 6 m
( 5. 3 ft) long. The fleshy snout, whose
function remains a mystery, be-
comes more narrow and pointed in
adults. Photographer unknown