Sharks
Sharks
PDF created with pdfFactory Pro trial version www.avalanche.com.au
1
Sharks
CLASSIFICATION
Sharks belong to the vertebrate class of animals known as Chondrichthyes which
include the sharks, rays and skates. Unlike other fishes they have a skeleton of cartilage
rather than bone which gives rise to their more common name of Cartilaginous fishes.
CARTILAGINOUS FISHES
(Chondrichthyes)
Sharks (Selachii)
Skates & Rays (Batoidea)
There are approximately 370 known species of shark. These can be divided into eight
major orders depending on certain characteristics. A representative of each order is
shown below. Notice the diverse characteristics of each.
Angel sharks (Squatiniformes)
Bullhead sharks
(Heterodontiformes)
Requiem sharks (Carcharhiniformes)
Carpet sharks (Orectolobiformes)
Mackerel sharks (Laminiformes)
Sawsharks (Pristiophoroforms)
Frilled or Cow sharks (Hexanchiformes)
Dogfish sharks (Squaliformes)
Figure 2: Examples of the eight orders of shark.
PDF created with pdfFactory Pro trial version www.avalanche.com.au
2
Sharks
SHARKS Vs BONY FISHES
Sharks, although similar to bony fishes, have several fundamental differences. The most
important of these include:
(i)
Skeleton: The shark's skeleton is constructed of cartilage. This provides the
shark with greater buoyancy and flexibility that would a skeleton constructed of
solid bone.
(ii)
Liver: The shark has an enlarged liver, containing an oil called squalene. This oil
is both a rich source of vitamin A and provides the shark with additional buoyancy.
(iii)
Gill slits: Sharks normally have 5 to 7 pairs of external gill slits. These replace
the protective gill cover found on bony fishes.
(iv)
Skin: Rather than flat protective scales and a slime coating, such as those found
on bony fishes, sharks have small, hard, teeth-like placoid scales or denticles.
(iv)
Tail structure: The tail of a shark is heterocercal which means that the upper
and lower lobe of the tail are different in shape and size.
(v)
Reproduction: Sharks have three methods of giving birth. Some sharks lay eggs
(oviparous), some give birth to live young (viviparous) and in others, the eggs
develop internally until they hatch and are released from the female's uterus
(ovoviviparous).
(vi)
Digestion: The shark's small intestine have been modified into a small, spiral
shaped organ called the spiral valve. This allows more room in the body cavity for
large organs such as the liver, without affecting the streamlined shape of the
shark's body.
PDF created with pdfFactory Pro trial version www.avalanche.com.au