aspects of fish biology form and function

288a
ASPECTS OF FISH BIOLOGY
FORM AND FUNCTION
Body shape, colouring and the degree of development of various
body and sensory structures reveal much about a fish 1 s way of life.
Fish may be streamlined for swiftness in open water, flat for hugging
the bottom, have large eyes to see in the dark, or have a hard covering,
spines or be well camouflaged for protection.
Body shape
Most fish belong to one of four basic categories (figure 3):
(1) Streamlined and spindle-shaped fish such as the mackerels (Scomber
australasicus) and tuna (Scombridae), kingfish (Seriola grandis) and
snoek (Thrysites atun). The bodies of these constantly moving, fast
swimming pelagic fish are circular or elliptical in cross section and
thicker in front than behind (fusiform), a shape designed to cleave
through the water. Contours are smooth and rounded with no projections
which might offer resistance to the water. The eyes are smooth, the gill
covers close fitting and the body is covered with small scales. The
dorsal and anal fins are able to be depressed into a groove for fast
swimming.
With water over 800 times denser than air a body shape such as this
is important in order to swim with the greatest economy of energy. These
fish rely on their speed to evade enemies and capture food. Departure
from this body shape represents a loss in swimming efficiency and other
forms display an array of devices to obtain food and being eaten.
(2) A laterally compressed body (flattened from side to side) is typical
of the reef associated fish, e.g red moki (Cheilodactylus spectabilis) ,
parore (Girella tricuspidata) and paketi (Pseudoloabrus celidotus) . These
are, usually relatively slow moving fish of small to moderate size, which
remain close to the reef and depend on it for their food and shelter.

289a
(3) Sedentary bottom-hugging fish are usually depressed from top to
bottom- The best examples in this category are the stingray {Dasyatis
brevicaudatus) ,and the eagle ray {Myliobatus tervuicaudatus) . Other
sedentary fish have a flattened abdomen and the body is almost triangular
in cross section, e.-g. hiwihiwi {Chironemus marmoratus) , blue cod
{Parapercis colias) and the tripterygiids.
These forms'are usually well camouflaged to avoid predators. They
forage for relatively immobile prey or rely on camouflage and/or lures
to take faster moving prey by surprise, e.g scorpionfish {Scorpaena
cardinalis) and the spotted stargazer {Genyagnus monopterygius).
sunfish {Mola mola)
•*
parore {Girella tricuspidata)
mackerel {Scomber australasicus)
Figure 3: Differences in body form.
eel {Conger
wilsoni)
hiwihiwi (Chironemus marmoratus)
seahorse
{Hippocampus
abdominalis)

291a
e.g. the tropical triggerfish (Balistidae). The bright colours of most
cleanerfish (e.g. the crimson cleaner, Suezichthys sp.) are thought to
advertise the presence of the cleaner to other fish.
' I Colour patterning may also enable individuals to recognise other
members of their own species, which is particularly important for mating
and territorial defence. Many species display bright colour patches
during courtship displays or aggressive encounters. These may be found
on the dorsal, anal or caudal fins and are only revealed when the fins
are extended during a display, e„g. the bright blue spot on the first
dorsal fin of the tripterygiid Gilloblennius decemdigitatus, and the
yellow and black markings on the caudal fin of the male leatherjacket
{Parika scaber). The black angeifish (Parma alboscapularis) ? turns on 1
a white shoulder spot while courting, spawning and defending its
territory.
Colouration often differs between the sexes, particularly in those
species that pair spawn and indulge in courtship displays. The male is
usually more brightly coloured than the female. These differences may
be apparent throughout the entire year, as seen in the labrid group, or
may only occur during the breeding season. Male tripterygiids, for
example, change from their normal drab colouring to contrasting or bright
colours during the breeding season. In most pair spawning fish male
colouration intensifies during the spawning season.
Some species exhibit differences in colouring between adults and
juveniles. The juvenile black angeifish is entirely different in
appearance from the adult, with its bright yellow body which is marked
with bright blue dots and streaks. Other examples may simply reflect
a difference in habitat between adults and juveniles. For example,
juvenile paketi {Pseudolabrus celidotus) and leatherjackets [Parika
scaber) are usually the colour of the seaweed in which they shelter and
the adult colouration is assumed when they leave this habitat.
Colour patterns are generally consistent within a species or a
group such as males, females and juveniles. However, there is enough
variation to allow recognition of individual fish for behavioural studies.
The pattern and shape of different lines and spots, and even body scars
are usually used.
Teleosts are able to change their colours rapidly, and in some\
cases completely. Colour change is achieved by expansion
or contraction
of certain chromatophores, which effectively reduces or increases the

292a
concentration of that colour. Expansion "of the chromatophores usually
darkens the fish whereas contraction makes them pale.
A fish gains considerable protective advantage in being able to
change colour intensity to match that of its surrounding environment.
For example, the cobble blenny (Forsterygion capito) is almost white with
a black lateral stripe when found in sediment or sandy areas, but on the
the darker background of rocks and cobbles it changes to a mottled
greenish-black on the back and sides with the dark stripe being almost
obliterated. The parore {Givella tricuspidata) changes colour at night
when it rests on the substratum. This can also be observed during the
day with pink rnaomao (Caprodon longimanus) which changes from the almost
uniform pink colouring it exhibits while swimming in midwater to a pale
pink with large white blotches while resting on the bottom.
Rapid colour changes are also observed during aggressive encounters
or courtship displays. During aggressive interactions both fish may
darken considerably in colour. Where there is a definite dominant/
subordinant situation the dominant fish is usually dark whereas the
other fish is considerably paler than normal. Courting males will often
temporarily display very dark or intense colours.
Fins and locomotion
Fish are propelled through the water by their fins, body movements
or a combination of the two. Four basic swimming methods can be
observed (figure 4) .
(1) Anguilliform: Segments of the body musculature alternatively
contract and relax throwing the body into an S-shaped curve. A series
of undulations pass the full length of the body, the main thrust coming
from the action of the tail or tail fin against the surrounding water.
Swimming efficiency is greatly increased if the tail is laterally
compressed. This is the typical method of locomotion of the eels
(Anguilliformes) and can also be seen in the ungainly movements of the
cod, e.g. Lotella rhacinus.
(2) Carangiform: The body undulations which produce movement are
confined to the rear third of the fish's body. The tail provides the
main source of locomotive power. Most pelagic fish and reef fish swim
in this manner. The carangids are the typical examples, e.g. kingfish
(Seriola grandis).

293a
(3) Labriform: Some fish, particularly those of slow to moderate speeds
use only their pectoral fins for swimming. The body muscles and tail
fin are only used when short bursts of speed are required. This is the
characteristic mode of locomotion of the labrids and is also used by
butterfish (Odax pullus) and blue cod {Parapercis colias) . Movement
is achieved for these species by simple synchronised flapping of the
broad, rounded pectoral fins. Others use their pectoral fins with a
wave-like motion, the fins beating synchronously as in the stingray
{Dasyatis brevicaudatus), or asynchronously as seen in the eagle ray
{Myliobatus tenuicaudatus) The pomacentrids use the pectoral fins with
an oar-like motion, bringing the fin forward edgewise and pulling it
back broadside on.
(4) Balistiform: Undulations of the soft-rayed dorsal and anal fins
provide the locomotive power for many fish. This method is typified by
the balistids (triggerfish) and monacanthids (leatherjackets) , e.g
Parika scaber, and is also used by the john dory {Zeus faber) and the
syngnathids (seahorses and pipefish).
Figure 4: Parts of the body and fins used for propulsion.
Practically all fish adopt the horizontal position when swimming.
A few do not. Seahorses (e.g. Hippocampus abdominalis) swim in an
upright position, although the juveniles initially swim horizontally.
Fish may even swim upside down. A species of freshwater catfish from
the Nile and other African rivers can be found swimming leisurely at the
surface, belly upwards.

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

structure used attract prey.
kingfish (Seriola grandis)
gurnard
(Chelodonichthys kumu)
long-snouted pipefish
(Stigmatopora macropterygia)
butterfish {Odax pullus)
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two-spot demoiselle (Chromis
eagle ray
dispilus)
(My liobatus tenuicaudatus)
porae
(Cheilodactylus douglasi)
mottled blenny
(Forsterygion
Q^'Sr^a®^ varium)
Figure 5: Various conditions of the dorsal, pectoral and ventral fins.
The pelvic fins of the bottom dwelling fish such as the
tripterygiids and blue cod (Parapercis colias) are reduced and thickened
to act as props for the fish resting on the substratum. The lower rays of
the pectoral fins are usually unbranched and thickened. Some bottom
living fish of shallow turbulent waters are able to cling, grasp or
anchor themselves to the substratum to prevent being buffeted against
the rocks. The hiwihiwi (Chironemus marmoratus) has the lower pectoral

297a
rays detached and thickened to allow the fish to grasp the rock surface.
The sucking disc of the clingfish is formed by the pelvic fins. The
seahorses and other members of the family Syngnathidae are unique in
that their caudal peduncle is prehensile and is able to be curled around
objects to anchor the fish.
The claspers of the male sharks and rays are specialised pelvic
fins. Copulatory structures may also be formed from the anal fin in
other species, e.g. the South American cypridonts.
In addition to the size, shape and special modifications,
variations in number, positioning and composition (the number of spines
and rays) of the fins are particularly useful for fish identification.
The fins are membranous structures with supporting spines and rays. In
some fish the fins are covered with skin (e.g. moray eels) or scales
(e.g. kyphosids such as silver drummer, typhosus Sydney anus) . Fish may
possess one two or three dorsal fins which are supported with varying
combinations of spiny and soft rays. There is usually only a single
anal fin which is usually composed mainly of soft rays with a few
anterior spines. Some fish have distinct soft and spiny rayed portions
of their anal fin, e.g. john dory {Zeus faber) and horse mackerel
(Traahurus novae-zelandiae). Two separate anal fins are unusual, but
are found in the northern hemisphere cods (Gadus) . The caudal fin is
always situated terminally and is rarely spined. The pectoral fins are
usually situated just behind the gill opening and show very little
variation in this positioning. They usually consist of simple or
complex (branched) soft rays and seldom possess spines. The ventral fins
usually consist of soft rays and a few anterior spines. The positioning
of these fins on the body varies considerably and three broad categories
can be distinguished. The pelvic fins of the sharks and more primitive
teleosts (e.g. piper, Eeporhampus ihi) are situated in the middle of the
abdomen (abdominal). Bottom dwelling fish such as the blennioids and
blue cod (Parapercis colias) characteristically have their ventral fins
set in the region of the throat (jugular), and often forward of the level
of the pectoral fins. In the majority of teleosts in the Reserve the
ventral fins are situated in the chest region (thoracic).
Some species of fish possess extra fins and structures that
assist in locomotion. The fast swimming pelagic fishes may gain extra
stability from lateral scutes (e.g. carangids such as trevaiiy, Caranx

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

299a
swim bladder reduces the problems of lift and has freed the paired fins
for manoeuvering and braking functions.
Mouth, jaws and teeth
The size and structure of a fish 1 s mouth, jaws and teeth are
convenient features for classification, species identification and can
also be used as clues to feeding habits and food consumed.
The usual situation in bony fishes is that the mouth is terminal
(i.e. situated at the end of the snout), the jaws are equal or near equal
in length and the snout is short. Departures from this form are usually
designed to aid in food capture. The mouth may be situated on the under-
side of the head as in the sharks and rays and some teleosts (e.g. the
mimic blenny, Rlagiotremus tapeinosoma) . In some fish the mouth is large
and set at an oblique angle (e.g. the spotted stargazer, Genyagnus
monopterygius) and the jaws may be extremely protractile to enable rapid
and surprise capture of small mobile prey (e.g. john dory, Zeus faber
(figure 6)). A small mouth situated at the end of an elongated snout is
ideal for picking small crustacea and other animal from cracks and crevices
and from amongst encrusting invertebrate growth and seaweeds (e.g. the
seahorses and pipefish (syngnathids) and boarfish (pentacerotids)). The
elongated lower jaw of the piper (Reporhampus ihi) is thought to act as an
extension of the lateral line system and to help these nocturnal feeders
to detect their minute planktonic prey in the dark.
Figure 6i Head of the john dory {Zeus faber) with the mouth retracted (A)
and protracted (B).

300a
The lips of fishes' mouths are often fleshy and thickened into
several folds. As they are usually well supplied with sense cells, this
increases the sensory surface area. This is typical of the labrids
and is the character from which their german common name 1 Lippenfische 1
(or lipfish) was derived. The cheilodactylids, such as the red moki
(Cheilodactylus spectabilis) use their large thick lips to suck
prey from the rocks.
Fish teeth are primarily outgrowths of the skin. The sharks and
rays possess teeth in their jaws only. These are similar in structure to
the scales of their bodies. The teeth are not directly attached to the
jaws and are constantly being replaced. Sharks 1 teeth are profuse and
well structured for grasping, tearing and cutting. The teeth of the
voracious, predatory species like the mako shark (Isuvus oxyvhinchus) vary
in size and shape from large and triangular to slender and awl-like. The
sluggish bottom feeding sharks and the rays generally have small blunt
teeth, which are arranged in pavement fashion i,n several rows, for
crushing hard-shelled prey.
Bony fish possess teeth in their jaws and often also on the tongue,
the bones on the roof of the mouth (vomerine and palatine teeth), the throat
(pharyngeal teeth) and even the outside of the head (figure 7). The teeth
are usually firmly attached, although in some they are moveable (e.g.
parore, Give 1 la tviauspidata). They are rarely planted in sockets in
the jaw bones as in the balistids and monacanthids (e.g. leatherj ackets
Pavika scabev) .
A. lower jaw and floor of mouth; B. upper jaw and roof of
mouth.

301a
The fish eaters (piscivores) usually possess strong flat, closely
set teeth, which may be acutely sharp and pointed and ideally suited for
capturing and holding live fish (e.g. the snoek, Thrysites atun).
However, some piscivores such as the kingfish (Seriola grandis) have
relatively fine brush-like teeth. These still meet the requirements of
grasping and holding struggling prey. Others like the blue cod
(Parapercis colias) have surprisingly small teeth, or even toothless
mouths; however, thev have sharp well developed pharyngeal teeth.
Invertebrate feeders and herbivores exhibit a vast array of teeth, the
type dependinq on the food they eat. Small pointed cone-shaped teeth
at the front of the jaw are used to pick invertebrates from the substratum.
Some species also possess blunt molar-like teeth further back on the jaw
(the sparids e.g. Chrysophrys auratus) , or in the throat (the labrids,
e.g. banded wrasse, Pseudolabrus fucicola) to crush hard shells. The
herbivores usually have bands of small notched teeth in the jaws and some
have a series of chisel-like incisors for cutting or scraping algae from
the rocks. The teeth of the plankton eaters are small and feeble, or may
be absent altogether. These fish typically possess elaborate structures
known as gill filaments, which strain the microscopic organisms
out of the water they take into their mouths before it passes over the
gills. These double rows of stiff, interlocking appendages are situated
on the inner marqins of the gill arch (see figure 9). In most fish they
exist as bony knobs but in the planktivorous fish they are long, numerous
and closely set with many secondary and tertiary branches.
Generally the teeth are single structures. However in several
groups, the teeth in the jaws are fused together. The teeth of the
parrotfish (scarids) are fused to form a beak for cutting off pieces of
coral and seaweed. In other groups the fused teeth form a single or
double plate in each jaw as in the families Diodontidae and Tetradontidae.
These plates are sharp at the edge and also provide a broad crushing
surface within.
Skin9 scales and spines .
Fish have a skin composed of two layers. The outer layer, or
epidermis, is composed of cells which are constantly being warn away and
replaced by new ones developing at the base. Underneath is the dermis,
a thick laver of connective tissue, muscle fibres and mucous glands.
Fish also have an outer covering of scales. When these are absent the

302a
skin may be thick and leathery (e.g. sunfish, Mold mold) or thickly coated
with mucous (e.q. clinqfish) for protection.
The form of the scales, spines and other related structures varies
considerably and provides an important character for classification.
The sharks have a primitive type of scale. Their placoid scales are
tooth-like structures, each consistinq of a central spine coated with
enamel and with an intermediate layer of dentine. These scales do not
increase in size as the fish qrows; instead, new scales are continually
beinq added.
The teleosts are covered with thin bony scales that overlap each
other like the tiles on the roof of a house. The scales increase in size
as the fish grows. These may have a comb-like serrated rear margin
(ctenoid scales) or a smooth rear margin (cycloid scales) (figure 8) A
few fishes, the. sturgeons (Acipenseridae) and some garfishes (Exocoetidae),
possess ganoid scales. These are hard thick bony scales which fit against
each other like the bricks of a wall, and often form ridges of armour
along the back and sides of the fish.
Figure 8: The different types of scales
In general, teleosts with softrayed fins have cycloid scales, for
example members of the orders Salmoniformes and Antheriniformes and the

303a
cod family, Gadidae. The spiny rayed fishes usually have ctenoid scales *
Occasionally both ctenoid and cycloid scales are found on the same fish.
Scales vary widely in size, from the minute scales of the mackerels
(e.g. Scomber australasicus) to the large scales of the labrids (e.g. the
pigfish, Bodianus oxycephalies) and the pomacentrids (e.g. the black
angelfish, Parma alboscapularis). The eels (e.g. the yellow moray,
Gyrrmothorax prasinus) have tiny well separated scales which are deeply
embedded in the skin. ,In other species the scales lie very close to the
surface of the skin and are easily rubbed off (e.g. goatfish Upeneichthys
porosus).
Scales may be modified in various ways. The scales of the porcupine
fish (Allomy cterus whitleyi) are strong spines, the roots of which are in
contact with one another thus giving the fish extra protection. Those of
the leatherjacket (Parikā scaber) have become reduced and coalesced to
form a tough sandpapery skin. The scutes, or keeled scales, of many fast-
swimming fish such as the carangids, give extra strength to the narrow
caudal peduncle and also act as stabalizers. In the syngnathids
(seahorses and pipefish) the scales have been replaced by a series of
jointed bone-like rings.
A series of pored scales, or in some cases notched scales (e.g.
many tripterygiids), the lateral line scales, form an external line which
extends from behind the fish f s head along each side of the body down to
the tail. The number and type of the scales and the shape of this line
are often used in fish classification. The lateral line may run straight
along the midline of the sides, or may be curved to follow either the
contour of the back or belly. Usually the lateral line runs continuously
to the tail but it also may be interrupted or incomplete. Fish usually
possess only one lateral line; however some have several lines of
pored scales (e.g. rockfish, Acanthoclinus quadridactylus) or a branched
lateral line (e.g. the gemfish, Rexea solandri) . Others such as the
clupeioids, clingfish and yellow-eyed mullet {Aldrichetta forsteri) have
no external lateral line at all.
In most fish the mucous glands in the skin secrete a protective
coating of slime over the scales. This acts as a barrier to the entry
of parasites, bacteria, fungi and other disease organisms, and it also
reduces friction as the fish moves through the water and amongst seaweeds.
Weed dwelling fish such as the marblefish (Aplodactylus meandratus) are

304a
noticeably slippery, whereas several midwater swimming fish do not produce
mucous and are rough and sandpapery to feel (e.g. slender roughy,
Hoplostethus elongatus, and pink rnaomao, Caprodon longimanus) .
Spines are usually associated with protection. They are found in
the fins, especially the dorsal fin, on the opercular and preopercular
bones (e.g. the redbanded perch, Ellerkeldia huntii, and the two-spot
demoiselle, Chromis dipilus) , on the tail (e.g. the stingrays and eagle
rays) or all over the body (e.g. the porcupine fish, Allomyctevus
whitleyi) The effectiveness of their protection is greatly increased
by the association of poison glands with the spines , as in the rays and
red scorpionfish (Scorpaena cardinalis) .
The senses
Fish possess the senses of smell, touch, taste, sight,
and electroreception. The.degree of development of the sense
and associated- structures is often related to the fish ! s mode of life
or habitat.
SMELL:
The olfactory organs in fishes, the nostrils, are essentially a
deep pit lined with sensory tissue. Most fish have two pair, one
situated on each side of the snout, excluding the pomacentrids which
usually have only one set of nostrils. The nostrils are never used for
breathing in fishes as they are in the terrestrial vertebrates. The
sense of smell plays and important part in finding food in some species,
especially the sharks which have poor sight.
TOUCH:
Cells sensitive to touch are found all over the body. Some fish
may also have special feelers to aid in the search for food, for
example the elongated lower pectoral fin rays of the porae (Cheilodaotylus
douglasi) and the red gurnard (Chelodonichthys kurrru) .
SIGHT:
hearing
organs
Fish eyes are very like our own in that there is a sensitive screen
(the retina) at the back of the eye and a lens which projects an image
onto that screen. However, unlike the human eye the iris does not expand

305a
and contract as the light conditions change. Most teleosts are thought
to have some sense of colour vision.
Fish have no true eyelids. The skin of the head becomes transparent
where it passes over the eye. Sharks possess a nictating membrane, which
is a freely moving membrane situated in the corner of the eye and can
be passed over the entire surface of the eye when required. This membrane
is often browninsh in colour and is used to protect the eye from intense
sunlight. Bottom dwelling fish such as the rays and flatfish, frequently
possess a thick dark lobe above the eyes which effectively shades them
from strong light.
There is some relationship between a fish's way of life and the
degree of development of the eyes. Fish living in murky waters usually
have small eyes, whereas nocturnal fishes usually have relatively large
eyes (e.g. slender roughy, Boplostethus elongatus, and bigeyes,
Pempheris adspersa). Many deep sea fish and cave dwellers are blind.
However, others have large well developed eyes and may also have the
ability to produce light themselves.
The majority of fish have their eyes situated on either side of
the head, giving them monocular vision. Some fish are capable of
focusing both eyes on the same object at the same time (binocular vision),
which is important for judging distances, especially when capturing food.
This may be achieved in various ways. The planktivorous fish, which pick
individual organisms out of the water, have their eyes set well forward
on the head (e.g. sweep, Scorpis aequipinnis, and blue rnaomao, Scorpis
violaceus) . Hunters such as the joh-n dory (Zeus faber) , have protrusible
eyes which can be rotated forwards in their sockets. The eyes of many
bottom dwelling fish are situated on top of the head (e.g. the spotted
stargazer, Genyagnus monopterygius) . The flatfish are unique among
teleost fishes in that both eyes are on the same side of the head.
Fish" with poor vision usually have a well developed sense of
smell, touch or taste.
HEARING:
Although fish do not have an outer ear, as is usual in terrestrial
animals, they can hear. Fish possess an inner ear enclosed in a chamber
in the hind part of the skull, on either side of the head. As water is
a better conductor of sound than air the outer and middle ears are not
needed to direct and magnify the sound waves. The ear is concerned with
both hearing and the maintenance of equilibrium.

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Production of sound, light and electricity
As well as perceiving, fish are also able to produce light, sound
and electricity. Light production appears mainly in deep sea fish.
However, some relatively shallow dwelling fish also have this ability.
Light producing cells may be found all over the body or they may be
concentrated into large patches, particularly in the head region. In
many species the light is due to organs containing luminous bacteria.
The appearance of this light may be controlled by the movement of a special
fold of skin or chromatophores over whole or part of the organ. Some
sharks and teleosts have self-luminous photophores which are formed from
modified mucous. In these fish the light is able to be flashed on and
off.
The function of light production is uncertain, but the lights
often show distinctive patterns and they may serve in the recognition
of species and/or sex. They may also be used to startle attackers, and
in some cases to illuminate prey. In the deep sea anglerfish (Ceratias)
the luminous tip of the fin is used as a lure.
Several fish are able to make drumming, grunting, growling or
hissing sounds. Horse mackerel (Trachurus novae-zelandiae) , sunfish
(Mola mola) and leatherjackets (Parika soaher) stridulate, grinding the
bones of the upper and lower pharyngeal teeth together. Others use the
spines of their dorsal, anal or ventral fins, the gill covers or the
jaw bones. Expulsion of air from the swim bladder produces a grunt.
The red gurnard (Chelodonichthys kvmu) have special muscles lying in the
wall of the swim bladder and are able to produce a drumming sound.
Nearly all animals emit a weak electric field when in seawater.
This originates from diverse sources such as swimming movements, muscle
and heart activity and the voltage potential between body fluids and
seawater and between different parts of the body. Several species can
actively produce a much greater electric field» These include the star-
gazers (e.g. Genyagnus monopterygius), the torpedo rays (e.g. Torpedo
faivchildi) and some eels and catfish. These fish possess electric
organs which may differ in form and position in different species, but
all have similar microscopic structure. They consist of jelly filled
hexagonal-shaped cells. The current output varies depending on the

308a
species and the size of the fish, but has been recorded as large as
200 volts.
Electrical discharge is usually associated with the capture of
food. However, it is obviously also a good method of defence. Many fish
with electrical properties live in murky waters or have poor eyesight,
so the ability to produce an electric current may also be important in
electrolocation and communication.
Gills and respiration
Bony fish typically have four gills situated on each side of the
head. These consist of four bony rods, or gill bars, placed one behind
the other to form a series of arches. Attached to the hind edge of each
gill bar is a double row of gill filaments (figure 9). Each filament is
is thrown into a large number of smaller folds, which greatly increases
the surface area of the gill exposed to the water* The gills of the bony
fish open into a general chamber which is protected by a moveable flap,
the gill cover or operculum.
Figure 9: The head of a fish showing the first gill arch in its position behind
the operculum (A) and a section of the gill arch (B) supporting the
gill filaments on its hind edge and gill rakers on its front edge.
The gill structure of the sharks and rays is similar but each arch
is separated from its neighbour by a partition, the septum, and each gill

309a
opens to the exterior through its own gill slit (figure 10). Normally
there are five gill slits on each side of the head, but in one genus
(Heptranchias) there are seven gill slits and in three genera
(Chlamydoselaches, Hexaohus and Pliotrema) there are six= The skeletal
structure supporting the gills is cartilaginous . The spiracles of the
sharks and rays are vestigial gill clefts.
The majority of fish draw water in through the mouth- The cavity
of the mouth and the cavity in which the gills lie act as a double-
chambered pumpo . Water is sucked in through the mouth, the mouth is
closed and compressed to force water into the gill chamber and the out
to the exterior. A flap of skin on the jaw acts as a valve to prevent
the escape of water when the mouth cavity is compressed, while the closed
gill cover prevents an inflow of water from the rear of the system. The
two chambers work slightly out of phase to produce a continuous flow of
water over the gills.
In certain fast swimming fish such as the tuna and mackerel
(e.g. Scomber australasicus) , the pumping system is dispensed with except
when resting. These fish swim with their mouths open allowing water to
flow freely into the mouth and over the gills.

310a