aspects of fish biology form and function

298a
georgianus), or a series of finlets on the dorsal and ventral surfaces
of the caudal peduncle (e.g. the mackerel, Scomber australasicus, and the
snoek, Thrysites atun) . Another type of fin, the adipose fin, is a
characteristic of several groups of fishes such as the salmon, trout,
graylings and lizardfish (e.g. Synodus sp.), all members of the order
Salmoniformes, and the catfish (Siluriformes). This fin is a small flap
of fatty tissue covered with skin and without any supporting structures.
The function of this fin is unknown.
Swinribladder
As pressure increases with depth a fish will sink unless it
expends considerable energy swimming to maintain its position in the water
column. Bony fish possess a swim bladder which acts as a hydrostatic
organ and allows the fish to regulate its buoyancy. This is a long
silvery bag found within the body cavity, just below the backbone.
Bouyancy is controlled by the secretion of gases, via the bloodstream,
to and from the swim bladder and this enables the fish to remain virtually
weightless at any depth its selects. This ability enables the fish to
utilise all its swimming energy in a forward driving force.
The degree of development of this organ is related to the fish 1 s
way of life. In bottom dwelling fish the swim bladder is absent or
greatly reduced (e.g. the tripterygiids and the scorpionfish, Scorpaena
cardinalis).. The midwater living oblique-swimming blenny (Forsterygion
sp.C) has to swim continously to maintain its postion or it sinks to the
bottom. Pressure changes with depth are most important for fish which
make large vertical migrations in their search for food. These fish
usually have well developed swim bladders. For example, the kingfish
(Seriola grandis) is able to ascend and dive rapidly through 75m of water,
the swim bladder is large and well developed and the skull and tissues
are full of oil which provides a further aid to buoyancy for deep water
swimming.
The cartilaginous fishes, the sharks and rays, do not possess a
swim bladder and therefore sink rapidly to the bottom as soon as they
stop swimming. The body design of the sharks compensates to a certain
extent. The large heterccercal tail and horizontally placed pectoral fins
give the body some lift. However the relatively inflexible pectoral fins
are capable of movement in the vertical plane only. This means the fish
must swerve to avoid collisions. In the teleosts the possession of a