Physical Characteristics
Size
Sharks show great diversity in size. Whale shark (Rhincodon typus)
may grow to 13.7-m (45-ft.) while Midwater shark (Squaliolus laticaudus)
and Pygmy ribbontail catshark (Eridacnis radcliffei) are only 22 to 25 cm
(8 to 10 in).
Body shape
Sharks typically have a fusiform body (rounded
and tapering at both ends). This body shape
reduces drag and requires a minimum of energy to swim.
Coloration
Sharks are generally drably countershaded.
Its a type of
camouflage in which the dorsal side is darker
than the ventral side. The dark top blends in with the dark
ocean depths when viewed from above. The light
ventral side blends in with the lighter surface
of the sea when viewed from below. The result is
that predators or prey do not see a contrast
between the animal and the environment.
Some coastal benthic sharks are
camouflaged to blend in with the ocean bottom.
Most stripes and other markings are juvenile
colors that fade or disappear with age, as in the
case of the Tiger shark (Galeocerdo cuvier).
Some, such as Wobbegongs (family
Orectolobidae) and the Whale shark, keep their
markings throughout their life.
The markings of some species change as the
shark ages. Young Zebra sharks (Stegostoma
fasciatum) have dark saddles and bands which
fade to uniformly distributed spots on
adult species. Thus adult Stegostoma fasciatum
are called Australian leopard sharks.
Fins
Sharks have five different types of fins, which are rigid, supported by cartilaginous rods:
- One or two dorsal fins stabilize the shark. In
some species, dorsal fins have spines.
- Paired pectoral fins lift the shark as it swims.
- Paired pelvic fins stabilize the shark.
- A single anal fin provides stability in species
- The caudal fin propels the shark.
Head
Eyes
Eyes are lateral on sharks, dorsal on batoids.Some species have an eyelidlike structure called a
nictitating membrane. The nictitating membrane protects
the eye from being injured by thrashing prey while the
shark is feeding. Eye size and position vary, depending on the
particular habitat or behavior of the species. In
general, deep-water sharks have bigger eyes than
shallow-water sharks.
Nostrils
Sharks and batoids have ventral external nostrils. Some species have nasal barbers, sensory projections
near the nostril
Mouth.
On both sharks and batoids, the mouth is usually
ventral. It is located at the tip of the snout in the
whale shark, megamouth shark (Megachasma pelagios),
frilled sharks (Chlamydoselachus spp.), and some
carpet sharks (family Parascylliidae). The mouth may have
labial folds or furrows.
Teeth are modified, enlarged placoid scales. Sharks
have numerous rows of teeth attached at their bases by
connective tissue. Several rows of replacement teeth
continually develop behind the outer row(s) of functional
teeth. As the functional teeth fall out, replacement
teeth take their place. Some species of sharks may shed as many as 30,000
teeth in a lifetime.
Gill slits
Sharks have five to seven pairs of lateral gill slits, while Batoids have five pairs of ventral gill slits.
Spiracles
Some species of elasmobranchs have small openings called
spiracies behind the eyes at the top of the head. These
openings bring oxygen-carrying water into the gill chamber.
Spiracles originate from rudimentary first gill slits and are
reduced or absent in active, fast-swimming sharks.
Scales
Sharks and batoids have placoid scales, also called
dermal denticles (dermal=skin, denticles=teeth). Placoid
scales have the same structure as a tooth, consisting of
three layers: an outer layer of vitro-dentine (an
enamel), dentine, and a pulp cavity. Placoid scales are
arranged in a regular pattern in sharks and an irregular
pattern in batoids.
Unlike other types of scales, placoid scales do not get
larger as the fish grows. Instead, the fish grows more
scales.
Like teeth, the shape of the scales is variable among
species and can be used to identify the species.
Placoid scales gave rise to teeth, stingrays'spines, and
the dorsal spines of horn sharks (Heterodontus spp.)
and dogfishes (family Squalidae).
As a shark or batoid swims, placoid scales may create a
series of vortices or whirlpools behind each scale. This
enables a shark to swim efficiently.
European cabinetmakers used the rough skin of a shark as
sandpaper, called shagreen. With the denticles removed,
shark skin is also used for leather.
Spines
Most rays (order Myliobatiformes) have one or more
venomous spines on the tail. Spines are an adaptation for
defending the animal against predators and are not used
aggressively. Shallow-water species do, however, pose a
risk to unwary bathers because if a ray is disturbed, its
natural reflex is to lift its spine.
Skates (family Rajidae) and guitarfish (family
Rhinobatidae) may have rows of short spines or prickles
on the back.
Some shark species, such as horn sharks (family
Heterodontidae) and dogfishes, have spines associated
with their dorsal fins. These spines are an adaptation
for defense against predators.
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