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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 fish1 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 (Che i l odac ty lus 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.

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(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).

mackerel {Scomber australasicus)

sunfish {Mola mola) eel {Conger

wilsoni)

hiwihiwi (Chironemus marmoratus)

• *

parore {Girella tricuspidata)

seahorse {Hippocampus

abdominalis)

Figure 3: Differences in body form.

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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 on1

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

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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).

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(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.

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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 bait1

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structure used attract prey.

kingfish (Seriola grandis)

gurnard

(Chelodonichthys kumu)

long-snouted pipefish (Stigmatopora macropterygia)

butterfish {Odax pullus)

two-spot demoiselle (Chromis

dispilus)

eagle ray (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

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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

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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 fish1s 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

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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 fish1s 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).

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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 1Lippenfische1

(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. Sharks1 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.

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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

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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

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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 (Par ikā 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 fs 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

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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 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: 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

hearing organs of life

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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 (Cera t i as )

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

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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

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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.

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