aspects of fish biology form and function

295a
the actual locomotor organs they function to stabilise and manoeuveur the
fish. The median fins (dorsal and anal) prevent rolling and yawing in
the vertical axis while the paired fins (pectoral and ventral) prevent
the fish pitching horizontally. Turning is achieved chiefly by the
pectoral and ventral fins with body movements also playing some part. The
pectoral fins are nearly always used for braking; however, some fish
simply reverse their primary locomotory apparatus, e.g. the leather-
jacket (Parika scaber) reverses the direction of the undulations of the
dorsal and anal fins.
For most fish the shape and size of the pectoral fins, especially
the caudal fin, is a good index of speed, agility and mode of life
(figure 5). Fish with large square-cut or rounded tail fins as seen in
most reef fish are usually comparatively slow swimmers, but are capable
of sudden bursts of speed. A deeply forked and lunate tail, a narrow
caudal peduncle and small sickle-shaped pectoral fins are typical of the
fast-swimming pelagic fishes, e.g. the carangids, the tuna and. mackerel
and the snoek (Thrysites atun) . The midwater planktivorous fish (e.g.
two-spot demoiselles, Chromis dispilus and butterfly perch, Caesioperca
lepidoptera) have deeply forked tails and long oval pectoral fins,
allowing great manoeuverability. Hole dwelling and weed dwelling fish
such as the eels and syngnathids (seahorses and pipefish) have the
caudal and pectoral fins reduced in size and efficiency, and consequently
are poor swimmers. These fish also usually lack pelvic fins.
Fins also serve functions other than locomotion and may be
modified accordingly (figure 5). The dorsal and anal fins are capable
of being erected or depressed and are frequently used during aggressive
or courting displays. The spines and rays arfe supplied with muscles for
this purpose. The spines also provide the fish with some protection
against predators. These fins often also complement a fish' s camouflage.
For example, the long trailing fins of the butterfish (Odax pullus) and
the crested weedfish (Cristiceps aurantiacus) resemble"the weed in which
the fish live.
Many modifications are associated with seeking and obtaining food.
The lower rays of the pectoral fins can be drawn out and may form long
finger-like projections which act as tactile or sensory organs for
detecting food, e.g. red gurnard [Chelodonichthys kumu) and porae
[Chelodactylus douglasi). The first spine of the dorsal fin is greatly
extended in the anglerfish (Lophiformes) to form a 'line and bait 1