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The
Diversity of The World of Life
Vertebrates
(Vertebrata)
Cartilaginous Fishes
(Chondrichthyes)
Representatives
Sharks,
Sawfish,
Skates,
& Rays (Elasmobranchii)
Ratfish & Chimaeras (Bradyodonti)
Biology
ENVIRONMENTS
Almost all cartilaginous fishes are marine species.
DESCRIPTION
See also vertebrates in general.
The streamlined, sometimes immense body of a cartilaginous
fish is covered by a sandpaper-like skin, with or without small, conical
scales. There are five to seven pairs of gill slits behind the
mouth, with jaws and bony teeth (presumably evolved from scales).
There is no tongue nor eyelids or tear ducts, as in terrestrial vertebrates.
There are fins: The vertical fins are typically rudder-like; the
side fins are typically "hydrofoil", providing lift, as the fish
is propelled forwards through the water (there is no swim bladder, as
in higher fish).
The body of a cartilaginous fish is
supported by the buoyancy of the water in which it lives and also by its "cartilaginous endoskeleton" (a rubbery,
not bony, skeleton inside the body).
Usually sharp-toothed, sharks have a streamlined body with a
primitive, "hetercercal" tail (the upper lobe is long; the lower lobe,
short).
Usually with grinding teeth, sawfish, skates, and rays have
a "dorsoventrally" flattened body (flattened from top to bottom)
with large "pectoral" fins (up front, on each side), often
wing-like. Rays typically have a very long, thin tail, sometimes
with a stinging barb at its base (not tip).
The ratfish has a scale-less body, tapering towards the
rear; its gills are covered by an "operculum" flap.
FEEDING HABITS
Various
cartilaginous fishes are scavengers or carnivores; some large species (whale sharks and
manta rays) are filter-feeders, gleaning microscopic plankton from the
water through which they swim.
MOTION
The head-to-tail waves of muscular contractions on opposite
sides of the backbone -- which is flexible because it is segmented into
"vertebrae" -- produce the efficient squirming, swimming motion of
fish, further aided by the typically paddle-like tail.
The fins of fish typically help provide stability and
guidance, propulsion and lift during swimming.
Many primitive fish keep from sinking by the overall
"hydrofoil" design of their body -- the upper surface of the
body is more curved
than the lower -- like the "airfoil" wing of a bird.
Cartilaginous fishes have no swim bladder, as inside higher
fishes.
Skates
and rays swim my the waving motions of their wing-like pectoral fins.
DIGESTION
Bones that supported the front-most gills in primitive fish
(which, like modern lampreys,
typically fed by sucking and rasping) evolved
into the jaws of more modern fish. Jaws with teeth (evolved from bony scales)
are more efficient in feeding, as by biting and chewing.
The teeth of sharks are continuously worn down and
replaced.
The saw-like snout of a sawfish tears up schools of fish for
easy eating.
Skates
and rays typically feed with their mouth in the sand or mud of the bottom.
Inside
a cartilaginous fish, there is typically a pharynx, esophagus, stomach,
and intestine, emptying out through a "cloaca" (a chamber for
the exit of digestive and excretory wastes as well as for the exit of
sperms and eggs). A pancreas and a liver secrete digestive juices
and other materials into the intestine, which has a "spiral
valve", to slow the passage of food and, thus, allow it extra time to
be digested.
RESPIRATION
Gases
dissolved in water are exchanged across the blood-filled gills, which open
on either side from the pharynx.
Skates
and rays can take-in oxygen-rich water while they feed, with their mouth
in the sand or mud of the bottom, by means of a pair of large
"spiracles", atop the head.
CIRCULATION
The
heart of a cartilaginous fish is typically below the gills. There is
a "spleen", typically towards the middle or rear of the body
cavity, filtering
the blood.
EXCRETION
There
are a pair of kidneys, each emptying urine by means of a "Wolffian
duct" into the cloaca. A rectal gland helps excrete excess
salts.
COORDINATION
Cartilaginous
fishes typically have fairly well developed senses, with a small, yet
functional brain.
REPRODUCTION
Male cartilaginous fishes have a "clasper" (a
modified portion of the rear, pelvic fin) with
which to "inseminate" (put sperms into) the female.
Unlike
the thin-skinned eggs of higher fishes, the eggs of cartilaginous fishes
are typically covered by a horny capsule. There is typically much
yolk, to nourish the developing embryo.
Some
cartilaginous fishes (such as skates) are "oviparous" -- the
eggs are laid in the environment and, thus, hatch outside the body of the
mother.
Some
cartilaginous fishes (such as most sharks) are "ovoviviparous" --
the eggs develop within a "uterus" (a modified portion of
the oviduct) of the mother, the embryo is nourished by the yolk sac
(within the egg), and the young are born (produced from the body of the
mother) looking like small adults.
And
some cartilaginous fishes (such as some other sharks) are
"viviparous" -- the eggs develop within a uterus of the mother,
the embryo is nourished by a "placenta" (an intermingling of the
yolk sac and the lining of the uterus), and the young are born looking
like small adults.
Vertebrates
(Vertebrata)
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