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Chondrichthyes (cartilaginous fish, including sharks, rays
& chimaeras)
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What is a cartilaginous fish?
Cartilaginous fishes (Class Chondrichthyes,
from Greek chondros, cartilage, and ichthos, fish) are a class of
aquatic, gill-breathing, finned vertebrates (animals with a
vertebral column or `backbone'), equivalent to the bony fishes
(Class Osteichthyes: osteos, bone, and ichthos, fish). Cartilaginous
fishes include the living sharks, rays, and chimaeras, and have true
upper and lower jaws, a sensory snout that overhangs the mouth and
nostrils on the underside of the head, teeth in conspicuous
transverse rows or in fused tooth plates that are replaced from
inside the mouth, no bony plates on the head, scales in the form of
small, toothlike dermal denticles or placoid scales, fins without
bony fin rays, and a simplified internal cartilaginous skeleton
without bone.
All cartilaginous fishes have a pair each of
symmetrical pectoral and pelvic fins on the sides of the body, which
correspond to the forelimbs and hands or wings, and hindlimbs and
feet of amphibians, reptiles, birds and mammals (tetrapods or
`four-footed' land vertebrates). All sharks and chimaeras have a
caudal fin on the end of the tail, which may be reduced or absent in
rays. An anal fin, on the underside of the tail between the paired
pelvic fins and the caudal fin, is present in many sharks and
chimaeras but absent in others, while all rays and skates lack this
fin. Most sharks and all chimaeras have two dorsal fins on the back
between the head and caudal fin while a few sharks have only one
dorsal fin and rays vary with two, one, or no dorsal fins. Some
sharks have fin spines on the front edges of their dorsal fins.
All cartilaginous fishes have 4 to 7 pairs of
gill slits on the sides or underside of the head, which
independently open to the exterior in sharks and rays but in
chimaeras are covered by a soft gill cover similar to the hard, bony
gill cover of bony fishes. All living cartilaginous fishes have
internal fertilization of eggs through paired claspers (copulatory
organs) on the pelvic fins of males, and are egg-layers or
live-bearers. Cartilaginous fishes collect urea and other metabolic
waste products in the blood and body fluids for osmotic balance, and
have a specialized rectal gland for removal of excess salt in their
body fluids.
There are over 900 named species of
cartilaginous fishes, and at least 1100 known species including
species new to science that are being described by researchers in
museums and other research organizations. The living cartilaginous
fishes are subdivided into two living groups, the large Subclass
Elasmobranchii (elasmos, plates, and branchos, gills), which
includes several groups of fossil sharks and the dominant Subcohort
Neoselachii (neos, new, and selachos, a shark) or `modern' SHARKS
and RAYS, and the small Subclass Holocephali (chimaeras). Modern
sharks and rays (neoselachians) have 5 to 7 pairs of external gill
slits, upper jaws not fused to the skull and usually capable of
protrusion from the mouth, rows of teeth not fused into tooth
plates, an erect 1st dorsal fin which cannot fold backwards, and no
accessory claspers on the head or pelvic fins.
What is a shark?
Sharks are cylindrical or flattened
neoselachians with 5 to 7 gill slits on the sides of their heads,
moderate-sized pectoral fins that are not attached to the head above
the gill slits, a large, stout tail with a large caudal fin, one or
two dorsal fins with or without spines, and an anal fin variably
present or absent. The term shark is also used loosely for members
of many fossil elasmobranch groups that are not members of the
Neoselachii but have a sharklike form.
There are over 370 species of living sharks
worldwide, almost entirely in marine waters. A few requiem sharks
(Carcharhinidae), including the Zambezi shark (Carcharhinus leucas)
regularly ascend tropical rivers and occur in lakes with access to
the sea.
What is a ray?
Rays are flattened neoselachians derived from
sharks, `winged sharks' that have their pectoral fins expanded
forwards and fused to the sides of their heads over the gill
openings, so that their gill openings are on the undersides of their
heads. They have short, flat bodies, 5 or 6 gill openings, tails
that vary from large, thick, and sharklike to slender and whiplike,
2, 1 or no dorsal fins which lack spines when present, no anal fin,
and the caudal fin varying from large and sharklike to absent. The
pectoral fins of rays supplement or replace the caudal fin as
propulsive organs, and are greatly enlarged in the more specialized
rays, particularly the stingrays and skates, which have reduced
slender tails and caudal fins.
The skeletons of rays are more complex than
those of sharks, with attachments of the pectoral girdle to the
vertebral column and the anterior basals of the pectoral fins to the
skull or neurocranium, and fusion of the vertebral column between
cranium and pectoral girdle into a tube. This arrangement serves to
support the broadened pectoral fins and to resist their propulsive
thrust.
At least 500 species of rays occur worldwide,
mostly in marine waters. The sawfishes (Family Pristidae) and
whiptailed stingrays (Family Dasyatidae) readily enter fresh water
and are found in tropical rivers and lakes, while some whiptailed
stingrays are confined to fresh water in Africa and Asia. The river
stingrays (Family Potamotrygonidae) are confined to freshwater river
systems in tropical South America.
Chimaeras are cartilaginous fishes belonging to
the Subclass Holocephali (holos, entire, and kephalos, head), in
reference to the fusion of the upper jaws to the skull in these
fishes. These compressed, often silvery cartilaginous fishes differ
from sharks and rays in having 4 pairs of gill slits, which are
covered by a soft gill cover and communicate to the exterior via a
single pair of external gill slits. Chimaeras have largely naked
skin, with denticles missing over most of the body, their teeth
fused into three pairs of ever-growing tooth plates like rodent
incisors (hence the names ratfish or rabbitfish for some of the
species), the first dorsal fin with a spine that can be erected or
depressed, and a simplified gut with the stomach merged with the
intestine. In addition to the pelvic claspers, male chimaeras have
an unpaired clasper (frontal tentaculum) on the forehead and paired
claspers (prepelvic tentaculum) in front of the pelvic fins, all of
which have hooklike dermal denticles to help the male hold the
female during copulation. Chimaeras propel themselves with their
large, fan-shaped pectoral fins.
Over 31 species of chimaeras exist, and most
live in deep water on the continental slopes. Some chimaeras,
including the local St. Joseph or elephant fish (Callorhinchus
capensis), occur on the continental shelves and may range close
inshore, but none are found in fresh water. All lay eggs in
distinctive spindle-shaped, finned egg-cases.
What to do if you find a rare or unusual species
If you obtain an unusual or rare cartilaginous
fish, particularly one that does not fit any of the species covered
in Biodiversity Explorer, please contact any of the organizations
below. If at
all possible save the specimen and bring it fresh or preferably
frozen to the nearest research organization. Small cartilaginous
fishes can be preserved in a solution of one part concentrated (40%)
formaldehyde to 9 parts water; if formaldehyde is not available use
50% isopropyl or n-propyl alcohol or 75% ethyl alcohol as a
preservative. If the animal is too big to keep intact, take
photographs of it, take its weight, total length, precaudal length
(for sharks and chimaeras), and disk width for rays, and cut off its
head intact or remove its jaws and save them. Photographs should be
taken of side views of the entire animal and the undersides of the
heads for normally shaped sharks and chimaeras; photograph the upper
and lower surfaces of angel sharks and rays. If you record your fish
on videotape or motion picture film take shots of it from several
angles.
- Societies concerned with cartilaginous fish
- Museums and other organisations
Text by Leonard J.V. Compagno, David A. Ebert
and Malcolm J. Smale
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