Kingdom:
Animalia
Phylum:
Chordata
Class:
Chondrichthyes
INTERESTING FACTS
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GENERAL Sharks belong to the class Chondrichthyes (kon-DRIK-thees), the cartilaginous fishes. Derived from greek words chondros, whick means cartilage and ichthyus, which means fish.
They are distinguished from the bony fishes by having a backbone, but the rest of the skeletal structure is only cartilage.
Sharks have five to seven open gill slits on the side of the head.
Also, unlike most bony fishes, sharks do not have well developed ribs or a swim bladder. Thus, sharks are slightly negatively buoyant. And despite the lift from a large oil-rich liver, they sink slowly unless they continue to swim.
165 species of sharks are found in Australian waters, slighty less than half the world total.
Sharks are elite prediators in the oceans with the oldest known fossil reported to be over 400 million years old, older than dinosaurs. For more that 100 million years, they have been nearly unchanged in form.
REPRODUCTION Most sharks bare live young with the exception of the Port Jackson shark and the Cat shark. These two sharks lay eggs with horny shells.
In most sharks the developing embryo is nourished by the yolk sac, although development is completed in the uterus. This is termed ovoviviparity. Some sharks, such as the gummy shark and whalers, have true or placental viviparity, where the yolk sac of the developing young is much reduced and nourishment of the embryo comes from the uterus via a placenta. The number of young varies with the species and the size of the female.
Most sharks are estimated to live to 20-30 years and become sexually mature at 6-7 years, while a few such as gummy and whaler sharks are mature at 2-3 years and live only 10-15 years. The White-spotted spurdog lives to 70 years with sexual maturity at 20.
STRUCTURE SKELETON SHAPE COLORATION FINS SCALES TEETH NOSTRILS EYES GILLS DISTRIBUTION Shallow-water sharks spend most of their lives on the continental shelves, in water less than 650 feet (200m) deep. Examples of shallow-water sharks are the grey reef, hound
sharks, nurse, wobbegong, reef whitetip, blacktip, hammerhead, zebra, angel, sandbar,
sand tiger, horn, Port Jackson, lemon, copper, bull, bonnethead, dusky, sharpnose, and
blacknose sharks.
Pelagic or open-sea sharks stay in the upper few hundred meters of the deep oceans,
occasionally coming nearshore. These include the oceanic whitetip, crocodile, shortfin
mako, longfin mako, whale, basking, silky, blue, porbeagle, and thresher sharks.
Deep-dwelling sharks, such as the megamouth, goblin, frilled, cookiecutter, and pygmy
sharks inhabit the deeper parts of the open oceans, usually not swimming close to the
surface.
Many people mistakenly believe that sharks are a significant threat
to human life. There are around 370 species of sharks with the majority being harmless. Most are less than 2m long, and many species are less than 1m long. The largest species, the whale and basking sharks, are plankton feeders, and no threat to humans. However shark numbers have been steadily declining over the years by extensive over fishing.
Sharks are found in most parts of the ocean, from the surface to depths of more than 2000m and from mid-ocean waters to the shallows off almost every coast in the world. In some areas, notably Central America and Africa and even tropical Australia, sharks swim many kilometres into fresh water.
Scalloped Hammerhead Shark
As well as sight, smell, hearing, touch and taste, sharks have a number of sophisticated prey locating sensory systems that include vibration detection and bio-electric reception. This latter sense, only discovered in the last 30 years, enables sharks to detect small electric fields given off by animals buried in the sand.
Sharks reproduce by internal fertilization with the males possessing a pair of claspers used to tranfer sperm during copulation, although only one maybe used at a time.
The female is also doubly equipped in that she has two uterine horns. When mating takes place, the male may clamp his jaws onto one of the female's pectoral fins as a way of immobilizing her.
Bizarre egg-eating and intra-uterine cannibalism occurs with the Australian Grey Nurse shark. The first hatched eats the other eggs and embryos in the uterus, results in only two well-developed pups being born. More than 80 embryos have found in a large Tiger sharks, while the Whale shark recently has been found to have as many as 300 young.
Sharks have certain anatomical features in common. Sharks belong to the group Chondrichthyes, which are fish with the following characteristics: jaws, paired fins, paired nostrils and a skeleton made of cartilage. Sharks along with most other chondrichthyan fishes, such as stingrays, electric rays, skates, sawfish and guitarfish, are collectively known as elasmobranchs. Elasmobranch means "strap gills," which refers to the five to seven gill slits an elasmobranch has on each side of its head.
Chondrichthyans differ from osteichthyans, or bony fishes, whose skeletons are heavily calcified. A shark's skeleton is made mostly of cartilage than can be strengthened by deposits of minerals in areas subjected to special stress such as the jaws and vertebrae. Cartilage is an ideal tissue for sharks for several reasons. First, cartilage is lighter than bone, important for sharks which have no swim bladder. Most bony fish utilize a gas-filled swim bladder for buoyancy. Second, cartilage is a relatively flexible material, giving tensile force to swimming and turning movements. Many sharks can virtually turn on a dime, but their skeletal structure and fins are much less elaborately articulated than in bony fish. This combined with the lack of a swim bladder means that the bodies of sharks are on the whole less maneuverable than those of many bony fish. Lastly, cartilage can grow throughout the life of a shark.
Most sharks are characterized by a fusiform (rounded and tapering at both ends) body.
This streamlined, cylindrical body shape reduces drag and requires a minimum of
energy to swim.
Sharks are generally drably countershaded. Countershading is a type of cryptic
coloration in which the dorsal side (top) is darker than the ventral side (bottom). The
dark top of the countershaded shark blends in with the dark ocean depths when viewed
from above. The light bottom blends in with the lighter surface of the sea when viewed
from below. As a result, predators or prey do not see a contrast between the
countershaded shark and the environment.
Shark fins are rigid, supported by cartilagenous rods and proteinaceous fibers. Sharks
have five different kinds of fins. Paired pectoral fins lift the shark as it swims. Paired pelvic fins as well as one or two dorsal fins stabilize the shark. In some species, dorsal fins have spines. A single anal fin provides stability in some species where it is present.
The caudal or tail fin propels the shark. Sharks, as do all fish, use their body and tail in a side-to-side motion.
Sharks, like all elasmobranchs, have placoid scales, also called dermal denticles. The
word "denticles" indicates a relation to teeth, and sharks' teeth indeed originate from
this same layer of tissue. Placoid scales have the same structure as a tooth, consisting
of three layers: an outer enamel layer, dentine and a central pulp cavity. Placoid scales
are arranged in a regular pattern in sharks. Unlike other types of scales such as ctenoid
scales of bony fish, placoid scales do not get larger as the fish grows. Instead, the shark grows more scales. Unfortunately, this means that a shark's scales cannot be used to tell the age of the shark, like they can in bony fish. Ctenoid scales of bony fish overlap to provide both protection and suppleness. The placement of a shark's placoid scales reduces the friction of the water for fast-moving sharks by channeling the water flow over the body. A shark can wound its prey by breaking the prey's skin with its placoid scales. Like teeth, the shape of the scales is variable among shark species.
Shark teeth are in parallel rows. Several rows of replacement teeth develop continuously
throughout life behind the outer row of functional teeth. Studies by Mote scientists have
shown that, on an average, a nurse shark will replace each front-row tooth every ten
days to two weeks in the summer when actively feeding, and every one to two months in
the winter when they are less active. A degree of feeding specialization is evidenced by
the shape and size of the teeth. Thin, pointed teeth, found in the mako, are for grabbing
and holding prey; serrated, wedge-shaped teeth for cutting are found in the great white
shark; and small conical teeth found in the nurse shark are for crushing the shells of
crabs and mollusks on which these bottom dwellers feed. A rough formula for calculating
the size of a shark, using its teeth, is to measure the length of one side of the tooth in inches, then multiply by ten to calculate the total length of the shark in feet. However, this estimate only applies to large triangular-shaped teeth.
Sharks have paired external nostrils on the underside of the snout. These nasal
chambers are used only for smell, not for obtaining oxygen. Some species like the nurse
shark have sensory projections near the nostrils and mouth called nasal barbels.
Shark eyes are similar to those of humans in that the eyeball is a rigid structure,
containing a cornea, iris, pupil, lens and retina. Eyes are lateral (located on the side of the head) on sharks. Some species have an eyelid-like structure called a nictitating
membrane, which protects the eye from being injured by thrashing prey while the shark
is feeding. Eye size and position vary with species. Deep-water sharks generally have
larger eyes than shallow-water sharks. The eyes of deep-water sharks are often
emerald green and adapted to perceive the luminescence of other animals that also
dwell where little sunlight reaches.
Bony fish have one lateral gill opening on each side of the head while sharks have five
to seven gill slits on each side. As water passes over the gills, oxygen is absorbed by
blood in the gills. Some sharks have small openings called spiracles behind the eyes at
the top of the head. Spiracles are rudimentary first gill slits and are reduced or absent in most active, fast-swimming sharks.
Sharks inhabit tropical and temperate seas as well as some cold and polar seas and
even some freshwater lakes. Many juvenile sharks spend early portions of their lives in
bays or estuaries which serve as nursery grounds. Shark migration may be short or
long. Food availability, environmental cycles or reproductive cycles probably determine
most migrations. Females of many species migrate to specific locations to lay eggs or
give live birth to pups.