aspects of fish biology form and function
aspects of fish biology form and function
aspects of fish biology form and function
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288a<br />
ASPECTS OF FISH BIOLOGY<br />
FORM AND FUNCTION<br />
Body shape, colouring <strong>and</strong> the degree <strong>of</strong> development <strong>of</strong> various<br />
body <strong>and</strong> sensory structures reveal much about a <strong>fish</strong> 1 s way <strong>of</strong> life.<br />
Fish may be streamlined for swiftness in open water, flat for hugging<br />
the bottom, have large eyes to see in the dark, or have a hard covering,<br />
spines or be well camouflaged for protection.<br />
Body shape<br />
Most <strong>fish</strong> belong to one <strong>of</strong> four basic categories (figure 3):<br />
(1) Streamlined <strong>and</strong> spindle-shaped <strong>fish</strong> such as the mackerels (Scomber<br />
australasicus) <strong>and</strong> tuna (Scombridae), king<strong>fish</strong> (Seriola gr<strong>and</strong>is) <strong>and</strong><br />
snoek (Thrysites atun). The bodies <strong>of</strong> these constantly moving, fast<br />
swimming pelagic <strong>fish</strong> are circular or elliptical in cross section <strong>and</strong><br />
thicker in front than behind (fusi<strong>form</strong>), a shape designed to cleave<br />
through the water. Contours are smooth <strong>and</strong> rounded with no projections<br />
which might <strong>of</strong>fer resistance to the water. The eyes are smooth, the gill<br />
covers close fitting <strong>and</strong> the body is covered with small scales. The<br />
dorsal <strong>and</strong> anal fins are able to be depressed into a groove for fast<br />
swimming.<br />
With water over 800 times denser than air a body shape such as this<br />
is important in order to swim with the greatest economy <strong>of</strong> energy. These<br />
<strong>fish</strong> rely on their speed to evade enemies <strong>and</strong> capture food. Departure<br />
from this body shape represents a loss in swimming efficiency <strong>and</strong> other<br />
<strong>form</strong>s display an array <strong>of</strong> devices to obtain food <strong>and</strong> being eaten.<br />
(2) A laterally compressed body (flattened from side to side) is typical<br />
<strong>of</strong> the reef associated <strong>fish</strong>, e.g red moki (Cheilodactylus spectabilis) ,<br />
parore (Girella tricuspidata) <strong>and</strong> paketi (Pseudoloabrus celidotus) . These<br />
are, usually relatively slow moving <strong>fish</strong> <strong>of</strong> small to moderate size, which<br />
remain close to the reef <strong>and</strong> depend on it for their food <strong>and</strong> shelter.
289a<br />
(3) Sedentary bottom-hugging <strong>fish</strong> are usually depressed from top to<br />
bottom- The best examples in this category are the stingray {Dasyatis<br />
brevicaudatus) ,<strong>and</strong> the eagle ray {Myliobatus tervuicaudatus) . Other<br />
sedentary <strong>fish</strong> have a flattened abdomen <strong>and</strong> the body is almost triangular<br />
in cross section, e.-g. hiwihiwi {Chironemus marmoratus) , blue cod<br />
{Parapercis colias) <strong>and</strong> the tripterygiids.<br />
These <strong>form</strong>s'are usually well camouflaged to avoid predators. They<br />
forage for relatively immobile prey or rely on camouflage <strong>and</strong>/or lures<br />
to take faster moving prey by surprise, e.g scorpion<strong>fish</strong> {Scorpaena<br />
cardinalis) <strong>and</strong> the spotted stargazer {Genyagnus monopterygius).<br />
sun<strong>fish</strong> {Mola mola)<br />
•*<br />
parore {Girella tricuspidata)<br />
mackerel {Scomber australasicus)<br />
Figure 3: Differences in body <strong>form</strong>.<br />
eel {Conger<br />
wilsoni)<br />
hiwihiwi (Chironemus marmoratus)<br />
seahorse<br />
{Hippocampus<br />
abdominalis)
291a<br />
e.g. the tropical trigger<strong>fish</strong> (Balistidae). The bright colours <strong>of</strong> most<br />
cleaner<strong>fish</strong> (e.g. the crimson cleaner, Suezichthys sp.) are thought to<br />
advertise the presence <strong>of</strong> the cleaner to other <strong>fish</strong>.<br />
' I Colour patterning may also enable individuals to recognise other<br />
members <strong>of</strong> their own species, which is particularly important for mating<br />
<strong>and</strong> territorial defence. Many species display bright colour patches<br />
during courtship displays or aggressive encounters. These may be found<br />
on the dorsal, anal or caudal fins <strong>and</strong> are only revealed when the fins<br />
are extended during a display, e„g. the bright blue spot on the first<br />
dorsal fin <strong>of</strong> the tripterygiid Gilloblennius decemdigitatus, <strong>and</strong> the<br />
yellow <strong>and</strong> black markings on the caudal fin <strong>of</strong> the male leatherjacket<br />
{Parika scaber). The black angei<strong>fish</strong> (Parma alboscapularis) ? turns on 1<br />
a white shoulder spot while courting, spawning <strong>and</strong> defending its<br />
territory.<br />
Colouration <strong>of</strong>ten differs between the sexes, particularly in those<br />
species that pair spawn <strong>and</strong> indulge in courtship displays. The male is<br />
usually more brightly coloured than the female. These differences may<br />
be apparent throughout the entire year, as seen in the labrid group, or<br />
may only occur during the breeding season. Male tripterygiids, for<br />
example, change from their normal drab colouring to contrasting or bright<br />
colours during the breeding season. In most pair spawning <strong>fish</strong> male<br />
colouration intensifies during the spawning season.<br />
Some species exhibit differences in colouring between adults <strong>and</strong><br />
juveniles. The juvenile black angei<strong>fish</strong> is entirely different in<br />
appearance from the adult, with its bright yellow body which is marked<br />
with bright blue dots <strong>and</strong> streaks. Other examples may simply reflect<br />
a difference in habitat between adults <strong>and</strong> juveniles. For example,<br />
juvenile paketi {Pseudolabrus celidotus) <strong>and</strong> leatherjackets [Parika<br />
scaber) are usually the colour <strong>of</strong> the seaweed in which they shelter <strong>and</strong><br />
the adult colouration is assumed when they leave this habitat.<br />
Colour patterns are generally consistent within a species or a<br />
group such as males, females <strong>and</strong> juveniles. However, there is enough<br />
variation to allow recognition <strong>of</strong> individual <strong>fish</strong> for behavioural studies.<br />
The pattern <strong>and</strong> shape <strong>of</strong> different lines <strong>and</strong> spots, <strong>and</strong> even body scars<br />
are usually used.<br />
Teleosts are able to change their colours rapidly, <strong>and</strong> in some\<br />
cases completely. Colour change is achieved by expansion<br />
or contraction<br />
<strong>of</strong> certain chromatophores, which effectively reduces or increases the
292a<br />
concentration <strong>of</strong> that colour. Expansion "<strong>of</strong> the chromatophores usually<br />
darkens the <strong>fish</strong> whereas contraction makes them pale.<br />
A <strong>fish</strong> gains considerable protective advantage in being able to<br />
change colour intensity to match that <strong>of</strong> its surrounding environment.<br />
For example, the cobble blenny (Forsterygion capito) is almost white with<br />
a black lateral stripe when found in sediment or s<strong>and</strong>y areas, but on the<br />
the darker background <strong>of</strong> rocks <strong>and</strong> cobbles it changes to a mottled<br />
greenish-black on the back <strong>and</strong> sides with the dark stripe being almost<br />
obliterated. The parore {Givella tricuspidata) changes colour at night<br />
when it rests on the substratum. This can also be observed during the<br />
day with pink rnaomao (Caprodon longimanus) which changes from the almost<br />
uni<strong>form</strong> pink colouring it exhibits while swimming in midwater to a pale<br />
pink with large white blotches while resting on the bottom.<br />
Rapid colour changes are also observed during aggressive encounters<br />
or courtship displays. During aggressive interactions both <strong>fish</strong> may<br />
darken considerably in colour. Where there is a definite dominant/<br />
subordinant situation the dominant <strong>fish</strong> is usually dark whereas the<br />
other <strong>fish</strong> is considerably paler than normal. Courting males will <strong>of</strong>ten<br />
temporarily display very dark or intense colours.<br />
Fins <strong>and</strong> locomotion<br />
Fish are propelled through the water by their fins, body movements<br />
or a combination <strong>of</strong> the two. Four basic swimming methods can be<br />
observed (figure 4) .<br />
(1) Anguilli<strong>form</strong>: Segments <strong>of</strong> the body musculature alternatively<br />
contract <strong>and</strong> relax throwing the body into an S-shaped curve. A series<br />
<strong>of</strong> undulations pass the full length <strong>of</strong> the body, the main thrust coming<br />
from the action <strong>of</strong> the tail or tail fin against the surrounding water.<br />
Swimming efficiency is greatly increased if the tail is laterally<br />
compressed. This is the typical method <strong>of</strong> locomotion <strong>of</strong> the eels<br />
(Anguilli<strong>form</strong>es) <strong>and</strong> can also be seen in the ungainly movements <strong>of</strong> the<br />
cod, e.g. Lotella rhacinus.<br />
(2) Carangi<strong>form</strong>: The body undulations which produce movement are<br />
confined to the rear third <strong>of</strong> the <strong>fish</strong>'s body. The tail provides the<br />
main source <strong>of</strong> locomotive power. Most pelagic <strong>fish</strong> <strong>and</strong> reef <strong>fish</strong> swim<br />
in this manner. The carangids are the typical examples, e.g. king<strong>fish</strong><br />
(Seriola gr<strong>and</strong>is).
293a<br />
(3) Labri<strong>form</strong>: Some <strong>fish</strong>, particularly those <strong>of</strong> slow to moderate speeds<br />
use only their pectoral fins for swimming. The body muscles <strong>and</strong> tail<br />
fin are only used when short bursts <strong>of</strong> speed are required. This is the<br />
characteristic mode <strong>of</strong> locomotion <strong>of</strong> the labrids <strong>and</strong> is also used by<br />
butter<strong>fish</strong> (Odax pullus) <strong>and</strong> blue cod {Parapercis colias) . Movement<br />
is achieved for these species by simple synchronised flapping <strong>of</strong> the<br />
broad, rounded pectoral fins. Others use their pectoral fins with a<br />
wave-like motion, the fins beating synchronously as in the stingray<br />
{Dasyatis brevicaudatus), or asynchronously as seen in the eagle ray<br />
{Myliobatus tenuicaudatus) The pomacentrids use the pectoral fins with<br />
an oar-like motion, bringing the fin forward edgewise <strong>and</strong> pulling it<br />
back broadside on.<br />
(4) Balisti<strong>form</strong>: Undulations <strong>of</strong> the s<strong>of</strong>t-rayed dorsal <strong>and</strong> anal fins<br />
provide the locomotive power for many <strong>fish</strong>. This method is typified by<br />
the balistids (trigger<strong>fish</strong>) <strong>and</strong> monacanthids (leatherjackets) , e.g<br />
Parika scaber, <strong>and</strong> is also used by the john dory {Zeus faber) <strong>and</strong> the<br />
syngnathids (seahorses <strong>and</strong> pipe<strong>fish</strong>).<br />
Figure 4: Parts <strong>of</strong> the body <strong>and</strong> fins used for propulsion.<br />
Practically all <strong>fish</strong> adopt the horizontal position when swimming.<br />
A few do not. Seahorses (e.g. Hippocampus abdominalis) swim in an<br />
upright position, although the juveniles initially swim horizontally.<br />
Fish may even swim upside down. A species <strong>of</strong> freshwater cat<strong>fish</strong> from<br />
the Nile <strong>and</strong> other African rivers can be found swimming leisurely at the<br />
surface, belly upwards.
295a<br />
the actual locomotor organs they <strong>function</strong> to stabilise <strong>and</strong> manoeuveur the<br />
<strong>fish</strong>. The median fins (dorsal <strong>and</strong> anal) prevent rolling <strong>and</strong> yawing in<br />
the vertical axis while the paired fins (pectoral <strong>and</strong> ventral) prevent<br />
the <strong>fish</strong> pitching horizontally. Turning is achieved chiefly by the<br />
pectoral <strong>and</strong> ventral fins with body movements also playing some part. The<br />
pectoral fins are nearly always used for braking; however, some <strong>fish</strong><br />
simply reverse their primary locomotory apparatus, e.g. the leather-<br />
jacket (Parika scaber) reverses the direction <strong>of</strong> the undulations <strong>of</strong> the<br />
dorsal <strong>and</strong> anal fins.<br />
For most <strong>fish</strong> the shape <strong>and</strong> size <strong>of</strong> the pectoral fins, especially<br />
the caudal fin, is a good index <strong>of</strong> speed, agility <strong>and</strong> mode <strong>of</strong> life<br />
(figure 5). Fish with large square-cut or rounded tail fins as seen in<br />
most reef <strong>fish</strong> are usually comparatively slow swimmers, but are capable<br />
<strong>of</strong> sudden bursts <strong>of</strong> speed. A deeply forked <strong>and</strong> lunate tail, a narrow<br />
caudal peduncle <strong>and</strong> small sickle-shaped pectoral fins are typical <strong>of</strong> the<br />
fast-swimming pelagic <strong>fish</strong>es, e.g. the carangids, the tuna <strong>and</strong>. mackerel<br />
<strong>and</strong> the snoek (Thrysites atun) . The midwater planktivorous <strong>fish</strong> (e.g.<br />
two-spot demoiselles, Chromis dispilus <strong>and</strong> butterfly perch, Caesioperca<br />
lepidoptera) have deeply forked tails <strong>and</strong> long oval pectoral fins,<br />
allowing great manoeuverability. Hole dwelling <strong>and</strong> weed dwelling <strong>fish</strong><br />
such as the eels <strong>and</strong> syngnathids (seahorses <strong>and</strong> pipe<strong>fish</strong>) have the<br />
caudal <strong>and</strong> pectoral fins reduced in size <strong>and</strong> efficiency, <strong>and</strong> consequently<br />
are poor swimmers. These <strong>fish</strong> also usually lack pelvic fins.<br />
Fins also serve <strong>function</strong>s other than locomotion <strong>and</strong> may be<br />
modified accordingly (figure 5). The dorsal <strong>and</strong> anal fins are capable<br />
<strong>of</strong> being erected or depressed <strong>and</strong> are frequently used during aggressive<br />
or courting displays. The spines <strong>and</strong> rays arfe supplied with muscles for<br />
this purpose. The spines also provide the <strong>fish</strong> with some protection<br />
against predators. These fins <strong>of</strong>ten also complement a <strong>fish</strong>' s camouflage.<br />
For example, the long trailing fins <strong>of</strong> the butter<strong>fish</strong> (Odax pullus) <strong>and</strong><br />
the crested weed<strong>fish</strong> (Cristiceps aurantiacus) resemble"the weed in which<br />
the <strong>fish</strong> live.<br />
Many modifications are associated with seeking <strong>and</strong> obtaining food.<br />
The lower rays <strong>of</strong> the pectoral fins can be drawn out <strong>and</strong> may <strong>form</strong> long<br />
finger-like projections which act as tactile or sensory organs for<br />
detecting food, e.g. red gurnard [Chelodonichthys kumu) <strong>and</strong> porae<br />
[Chelodactylus douglasi). The first spine <strong>of</strong> the dorsal fin is greatly<br />
extended in the angler<strong>fish</strong> (Lophi<strong>form</strong>es) to <strong>form</strong> a 'line <strong>and</strong> bait 1
structure used attract prey.<br />
king<strong>fish</strong> (Seriola gr<strong>and</strong>is)<br />
gurnard<br />
(Chelodonichthys kumu)<br />
long-snouted pipe<strong>fish</strong><br />
(Stigmatopora macropterygia)<br />
butter<strong>fish</strong> {Odax pullus)<br />
296a<br />
two-spot demoiselle (Chromis<br />
eagle ray<br />
dispilus)<br />
(My liobatus tenuicaudatus)<br />
porae<br />
(Cheilodactylus douglasi)<br />
mottled blenny<br />
(Forsterygion<br />
Q^'Sr^a®^ varium)<br />
Figure 5: Various conditions <strong>of</strong> the dorsal, pectoral <strong>and</strong> ventral fins.<br />
The pelvic fins <strong>of</strong> the bottom dwelling <strong>fish</strong> such as the<br />
tripterygiids <strong>and</strong> blue cod (Parapercis colias) are reduced <strong>and</strong> thickened<br />
to act as props for the <strong>fish</strong> resting on the substratum. The lower rays <strong>of</strong><br />
the pectoral fins are usually unbranched <strong>and</strong> thickened. Some bottom<br />
living <strong>fish</strong> <strong>of</strong> shallow turbulent waters are able to cling, grasp or<br />
anchor themselves to the substratum to prevent being buffeted against<br />
the rocks. The hiwihiwi (Chironemus marmoratus) has the lower pectoral
297a<br />
rays detached <strong>and</strong> thickened to allow the <strong>fish</strong> to grasp the rock surface.<br />
The sucking disc <strong>of</strong> the cling<strong>fish</strong> is <strong>form</strong>ed by the pelvic fins. The<br />
seahorses <strong>and</strong> other members <strong>of</strong> the family Syngnathidae are unique in<br />
that their caudal peduncle is prehensile <strong>and</strong> is able to be curled around<br />
objects to anchor the <strong>fish</strong>.<br />
The claspers <strong>of</strong> the male sharks <strong>and</strong> rays are specialised pelvic<br />
fins. Copulatory structures may also be <strong>form</strong>ed from the anal fin in<br />
other species, e.g. the South American cypridonts.<br />
In addition to the size, shape <strong>and</strong> special modifications,<br />
variations in number, positioning <strong>and</strong> composition (the number <strong>of</strong> spines<br />
<strong>and</strong> rays) <strong>of</strong> the fins are particularly useful for <strong>fish</strong> identification.<br />
The fins are membranous structures with supporting spines <strong>and</strong> rays. In<br />
some <strong>fish</strong> the fins are covered with skin (e.g. moray eels) or scales<br />
(e.g. kyphosids such as silver drummer, typhosus Sydney anus) . Fish may<br />
possess one two or three dorsal fins which are supported with varying<br />
combinations <strong>of</strong> spiny <strong>and</strong> s<strong>of</strong>t rays. There is usually only a single<br />
anal fin which is usually composed mainly <strong>of</strong> s<strong>of</strong>t rays with a few<br />
anterior spines. Some <strong>fish</strong> have distinct s<strong>of</strong>t <strong>and</strong> spiny rayed portions<br />
<strong>of</strong> their anal fin, e.g. john dory {Zeus faber) <strong>and</strong> horse mackerel<br />
(Traahurus novae-zel<strong>and</strong>iae). Two separate anal fins are unusual, but<br />
are found in the northern hemisphere cods (Gadus) . The caudal fin is<br />
always situated terminally <strong>and</strong> is rarely spined. The pectoral fins are<br />
usually situated just behind the gill opening <strong>and</strong> show very little<br />
variation in this positioning. They usually consist <strong>of</strong> simple or<br />
complex (branched) s<strong>of</strong>t rays <strong>and</strong> seldom possess spines. The ventral fins<br />
usually consist <strong>of</strong> s<strong>of</strong>t rays <strong>and</strong> a few anterior spines. The positioning<br />
<strong>of</strong> these fins on the body varies considerably <strong>and</strong> three broad categories<br />
can be distinguished. The pelvic fins <strong>of</strong> the sharks <strong>and</strong> more primitive<br />
teleosts (e.g. piper, Eeporhampus ihi) are situated in the middle <strong>of</strong> the<br />
abdomen (abdominal). Bottom dwelling <strong>fish</strong> such as the blennioids <strong>and</strong><br />
blue cod (Parapercis colias) characteristically have their ventral fins<br />
set in the region <strong>of</strong> the throat (jugular), <strong>and</strong> <strong>of</strong>ten forward <strong>of</strong> the level<br />
<strong>of</strong> the pectoral fins. In the majority <strong>of</strong> teleosts in the Reserve the<br />
ventral fins are situated in the chest region (thoracic).<br />
Some species <strong>of</strong> <strong>fish</strong> possess extra fins <strong>and</strong> structures that<br />
assist in locomotion. The fast swimming pelagic <strong>fish</strong>es may gain extra<br />
stability from lateral scutes (e.g. carangids such as trevaiiy, Caranx
298a<br />
georgianus), or a series <strong>of</strong> finlets on the dorsal <strong>and</strong> ventral surfaces<br />
<strong>of</strong> the caudal peduncle (e.g. the mackerel, Scomber australasicus, <strong>and</strong> the<br />
snoek, Thrysites atun) . Another type <strong>of</strong> fin, the adipose fin, is a<br />
characteristic <strong>of</strong> several groups <strong>of</strong> <strong>fish</strong>es such as the salmon, trout,<br />
graylings <strong>and</strong> lizard<strong>fish</strong> (e.g. Synodus sp.), all members <strong>of</strong> the order<br />
Salmoni<strong>form</strong>es, <strong>and</strong> the cat<strong>fish</strong> (Siluri<strong>form</strong>es). This fin is a small flap<br />
<strong>of</strong> fatty tissue covered with skin <strong>and</strong> without any supporting structures.<br />
The <strong>function</strong> <strong>of</strong> this fin is unknown.<br />
Swinribladder<br />
As pressure increases with depth a <strong>fish</strong> will sink unless it<br />
expends considerable energy swimming to maintain its position in the water<br />
column. Bony <strong>fish</strong> possess a swim bladder which acts as a hydrostatic<br />
organ <strong>and</strong> allows the <strong>fish</strong> to regulate its buoyancy. This is a long<br />
silvery bag found within the body cavity, just below the backbone.<br />
Bouyancy is controlled by the secretion <strong>of</strong> gases, via the bloodstream,<br />
to <strong>and</strong> from the swim bladder <strong>and</strong> this enables the <strong>fish</strong> to remain virtually<br />
weightless at any depth its selects. This ability enables the <strong>fish</strong> to<br />
utilise all its swimming energy in a forward driving force.<br />
The degree <strong>of</strong> development <strong>of</strong> this organ is related to the <strong>fish</strong> 1 s<br />
way <strong>of</strong> life. In bottom dwelling <strong>fish</strong> the swim bladder is absent or<br />
greatly reduced (e.g. the tripterygiids <strong>and</strong> the scorpion<strong>fish</strong>, Scorpaena<br />
cardinalis).. The midwater living oblique-swimming blenny (Forsterygion<br />
sp.C) has to swim continously to maintain its postion or it sinks to the<br />
bottom. Pressure changes with depth are most important for <strong>fish</strong> which<br />
make large vertical migrations in their search for food. These <strong>fish</strong><br />
usually have well developed swim bladders. For example, the king<strong>fish</strong><br />
(Seriola gr<strong>and</strong>is) is able to ascend <strong>and</strong> dive rapidly through 75m <strong>of</strong> water,<br />
the swim bladder is large <strong>and</strong> well developed <strong>and</strong> the skull <strong>and</strong> tissues<br />
are full <strong>of</strong> oil which provides a further aid to buoyancy for deep water<br />
swimming.<br />
The cartilaginous <strong>fish</strong>es, the sharks <strong>and</strong> rays, do not possess a<br />
swim bladder <strong>and</strong> therefore sink rapidly to the bottom as soon as they<br />
stop swimming. The body design <strong>of</strong> the sharks compensates to a certain<br />
extent. The large heterccercal tail <strong>and</strong> horizontally placed pectoral fins<br />
give the body some lift. However the relatively inflexible pectoral fins<br />
are capable <strong>of</strong> movement in the vertical plane only. This means the <strong>fish</strong><br />
must swerve to avoid collisions. In the teleosts the possession <strong>of</strong> a
299a<br />
swim bladder reduces the problems <strong>of</strong> lift <strong>and</strong> has freed the paired fins<br />
for manoeuvering <strong>and</strong> braking <strong>function</strong>s.<br />
Mouth, jaws <strong>and</strong> teeth<br />
The size <strong>and</strong> structure <strong>of</strong> a <strong>fish</strong> 1 s mouth, jaws <strong>and</strong> teeth are<br />
convenient features for classification, species identification <strong>and</strong> can<br />
also be used as clues to feeding habits <strong>and</strong> food consumed.<br />
The usual situation in bony <strong>fish</strong>es is that the mouth is terminal<br />
(i.e. situated at the end <strong>of</strong> the snout), the jaws are equal or near equal<br />
in length <strong>and</strong> the snout is short. Departures from this <strong>form</strong> are usually<br />
designed to aid in food capture. The mouth may be situated on the under-<br />
side <strong>of</strong> the head as in the sharks <strong>and</strong> rays <strong>and</strong> some teleosts (e.g. the<br />
mimic blenny, Rlagiotremus tapeinosoma) . In some <strong>fish</strong> the mouth is large<br />
<strong>and</strong> set at an oblique angle (e.g. the spotted stargazer, Genyagnus<br />
monopterygius) <strong>and</strong> the jaws may be extremely protractile to enable rapid<br />
<strong>and</strong> surprise capture <strong>of</strong> small mobile prey (e.g. john dory, Zeus faber<br />
(figure 6)). A small mouth situated at the end <strong>of</strong> an elongated snout is<br />
ideal for picking small crustacea <strong>and</strong> other animal from cracks <strong>and</strong> crevices<br />
<strong>and</strong> from amongst encrusting invertebrate growth <strong>and</strong> seaweeds (e.g. the<br />
seahorses <strong>and</strong> pipe<strong>fish</strong> (syngnathids) <strong>and</strong> boar<strong>fish</strong> (pentacerotids)). The<br />
elongated lower jaw <strong>of</strong> the piper (Reporhampus ihi) is thought to act as an<br />
extension <strong>of</strong> the lateral line system <strong>and</strong> to help these nocturnal feeders<br />
to detect their minute planktonic prey in the dark.<br />
Figure 6i Head <strong>of</strong> the john dory {Zeus faber) with the mouth retracted (A)<br />
<strong>and</strong> protracted (B).
300a<br />
The lips <strong>of</strong> <strong>fish</strong>es' mouths are <strong>of</strong>ten fleshy <strong>and</strong> thickened into<br />
several folds. As they are usually well supplied with sense cells, this<br />
increases the sensory surface area. This is typical <strong>of</strong> the labrids<br />
<strong>and</strong> is the character from which their german common name 1 Lippenfische 1<br />
(or lip<strong>fish</strong>) was derived. The cheilodactylids, such as the red moki<br />
(Cheilodactylus spectabilis) use their large thick lips to suck<br />
prey from the rocks.<br />
Fish teeth are primarily outgrowths <strong>of</strong> the skin. The sharks <strong>and</strong><br />
rays possess teeth in their jaws only. These are similar in structure to<br />
the scales <strong>of</strong> their bodies. The teeth are not directly attached to the<br />
jaws <strong>and</strong> are constantly being replaced. Sharks 1 teeth are pr<strong>of</strong>use <strong>and</strong><br />
well structured for grasping, tearing <strong>and</strong> cutting. The teeth <strong>of</strong> the<br />
voracious, predatory species like the mako shark (Isuvus oxyvhinchus) vary<br />
in size <strong>and</strong> shape from large <strong>and</strong> triangular to slender <strong>and</strong> awl-like. The<br />
sluggish bottom feeding sharks <strong>and</strong> the rays generally have small blunt<br />
teeth, which are arranged in pavement fashion i,n several rows, for<br />
crushing hard-shelled prey.<br />
Bony <strong>fish</strong> possess teeth in their jaws <strong>and</strong> <strong>of</strong>ten also on the tongue,<br />
the bones on the ro<strong>of</strong> <strong>of</strong> the mouth (vomerine <strong>and</strong> palatine teeth), the throat<br />
(pharyngeal teeth) <strong>and</strong> even the outside <strong>of</strong> the head (figure 7). The teeth<br />
are usually firmly attached, although in some they are moveable (e.g.<br />
parore, Give 1 la tviauspidata). They are rarely planted in sockets in<br />
the jaw bones as in the balistids <strong>and</strong> monacanthids (e.g. leatherj ackets<br />
Pavika scabev) .<br />
A. lower jaw <strong>and</strong> floor <strong>of</strong> mouth; B. upper jaw <strong>and</strong> ro<strong>of</strong> <strong>of</strong><br />
mouth.
301a<br />
The <strong>fish</strong> eaters (piscivores) usually possess strong flat, closely<br />
set teeth, which may be acutely sharp <strong>and</strong> pointed <strong>and</strong> ideally suited for<br />
capturing <strong>and</strong> holding live <strong>fish</strong> (e.g. the snoek, Thrysites atun).<br />
However, some piscivores such as the king<strong>fish</strong> (Seriola gr<strong>and</strong>is) have<br />
relatively fine brush-like teeth. These still meet the requirements <strong>of</strong><br />
grasping <strong>and</strong> holding struggling prey. Others like the blue cod<br />
(Parapercis colias) have surprisingly small teeth, or even toothless<br />
mouths; however, thev have sharp well developed pharyngeal teeth.<br />
Invertebrate feeders <strong>and</strong> herbivores exhibit a vast array <strong>of</strong> teeth, the<br />
type dependinq on the food they eat. Small pointed cone-shaped teeth<br />
at the front <strong>of</strong> the jaw are used to pick invertebrates from the substratum.<br />
Some species also possess blunt molar-like teeth further back on the jaw<br />
(the sparids e.g. Chrysophrys auratus) , or in the throat (the labrids,<br />
e.g. b<strong>and</strong>ed wrasse, Pseudolabrus fucicola) to crush hard shells. The<br />
herbivores usually have b<strong>and</strong>s <strong>of</strong> small notched teeth in the jaws <strong>and</strong> some<br />
have a series <strong>of</strong> chisel-like incisors for cutting or scraping algae from<br />
the rocks. The teeth <strong>of</strong> the plankton eaters are small <strong>and</strong> feeble, or may<br />
be absent altogether. These <strong>fish</strong> typically possess elaborate structures<br />
known as gill filaments, which strain the microscopic organisms<br />
out <strong>of</strong> the water they take into their mouths before it passes over the<br />
gills. These double rows <strong>of</strong> stiff, interlocking appendages are situated<br />
on the inner marqins <strong>of</strong> the gill arch (see figure 9). In most <strong>fish</strong> they<br />
exist as bony knobs but in the planktivorous <strong>fish</strong> they are long, numerous<br />
<strong>and</strong> closely set with many secondary <strong>and</strong> tertiary branches.<br />
Generally the teeth are single structures. However in several<br />
groups, the teeth in the jaws are fused together. The teeth <strong>of</strong> the<br />
parrot<strong>fish</strong> (scarids) are fused to <strong>form</strong> a beak for cutting <strong>of</strong>f pieces <strong>of</strong><br />
coral <strong>and</strong> seaweed. In other groups the fused teeth <strong>form</strong> a single or<br />
double plate in each jaw as in the families Diodontidae <strong>and</strong> Tetradontidae.<br />
These plates are sharp at the edge <strong>and</strong> also provide a broad crushing<br />
surface within.<br />
Skin9 scales <strong>and</strong> spines .<br />
Fish have a skin composed <strong>of</strong> two layers. The outer layer, or<br />
epidermis, is composed <strong>of</strong> cells which are constantly being warn away <strong>and</strong><br />
replaced by new ones developing at the base. Underneath is the dermis,<br />
a thick laver <strong>of</strong> connective tissue, muscle fibres <strong>and</strong> mucous gl<strong>and</strong>s.<br />
Fish also have an outer covering <strong>of</strong> scales. When these are absent the
302a<br />
skin may be thick <strong>and</strong> leathery (e.g. sun<strong>fish</strong>, Mold mold) or thickly coated<br />
with mucous (e.q. clinq<strong>fish</strong>) for protection.<br />
The <strong>form</strong> <strong>of</strong> the scales, spines <strong>and</strong> other related structures varies<br />
considerably <strong>and</strong> provides an important character for classification.<br />
The sharks have a primitive type <strong>of</strong> scale. Their placoid scales are<br />
tooth-like structures, each consistinq <strong>of</strong> a central spine coated with<br />
enamel <strong>and</strong> with an intermediate layer <strong>of</strong> dentine. These scales do not<br />
increase in size as the <strong>fish</strong> qrows; instead, new scales are continually<br />
beinq added.<br />
The teleosts are covered with thin bony scales that overlap each<br />
other like the tiles on the ro<strong>of</strong> <strong>of</strong> a house. The scales increase in size<br />
as the <strong>fish</strong> grows. These may have a comb-like serrated rear margin<br />
(ctenoid scales) or a smooth rear margin (cycloid scales) (figure 8) A<br />
few <strong>fish</strong>es, the. sturgeons (Acipenseridae) <strong>and</strong> some gar<strong>fish</strong>es (Exocoetidae),<br />
possess ganoid scales. These are hard thick bony scales which fit against<br />
each other like the bricks <strong>of</strong> a wall, <strong>and</strong> <strong>of</strong>ten <strong>form</strong> ridges <strong>of</strong> armour<br />
along the back <strong>and</strong> sides <strong>of</strong> the <strong>fish</strong>.<br />
Figure 8: The different types <strong>of</strong> scales<br />
In general, teleosts with s<strong>of</strong>trayed fins have cycloid scales, for<br />
example members <strong>of</strong> the orders Salmoni<strong>form</strong>es <strong>and</strong> Antherini<strong>form</strong>es <strong>and</strong> the
303a<br />
cod family, Gadidae. The spiny rayed <strong>fish</strong>es usually have ctenoid scales *<br />
Occasionally both ctenoid <strong>and</strong> cycloid scales are found on the same <strong>fish</strong>.<br />
Scales vary widely in size, from the minute scales <strong>of</strong> the mackerels<br />
(e.g. Scomber australasicus) to the large scales <strong>of</strong> the labrids (e.g. the<br />
pig<strong>fish</strong>, Bodianus oxycephalies) <strong>and</strong> the pomacentrids (e.g. the black<br />
angel<strong>fish</strong>, Parma alboscapularis). The eels (e.g. the yellow moray,<br />
Gyrrmothorax prasinus) have tiny well separated scales which are deeply<br />
embedded in the skin. ,In other species the scales lie very close to the<br />
surface <strong>of</strong> the skin <strong>and</strong> are easily rubbed <strong>of</strong>f (e.g. goat<strong>fish</strong> Upeneichthys<br />
porosus).<br />
Scales may be modified in various ways. The scales <strong>of</strong> the porcupine<br />
<strong>fish</strong> (Allomy cterus whitleyi) are strong spines, the roots <strong>of</strong> which are in<br />
contact with one another thus giving the <strong>fish</strong> extra protection. Those <strong>of</strong><br />
the leatherjacket (Parikā scaber) have become reduced <strong>and</strong> coalesced to<br />
<strong>form</strong> a tough s<strong>and</strong>papery skin. The scutes, or keeled scales, <strong>of</strong> many fast-<br />
swimming <strong>fish</strong> such as the carangids, give extra strength to the narrow<br />
caudal peduncle <strong>and</strong> also act as stabalizers. In the syngnathids<br />
(seahorses <strong>and</strong> pipe<strong>fish</strong>) the scales have been replaced by a series <strong>of</strong><br />
jointed bone-like rings.<br />
A series <strong>of</strong> pored scales, or in some cases notched scales (e.g.<br />
many tripterygiids), the lateral line scales, <strong>form</strong> an external line which<br />
extends from behind the <strong>fish</strong> f s head along each side <strong>of</strong> the body down to<br />
the tail. The number <strong>and</strong> type <strong>of</strong> the scales <strong>and</strong> the shape <strong>of</strong> this line<br />
are <strong>of</strong>ten used in <strong>fish</strong> classification. The lateral line may run straight<br />
along the midline <strong>of</strong> the sides, or may be curved to follow either the<br />
contour <strong>of</strong> the back or belly. Usually the lateral line runs continuously<br />
to the tail but it also may be interrupted or incomplete. Fish usually<br />
possess only one lateral line; however some have several lines <strong>of</strong><br />
pored scales (e.g. rock<strong>fish</strong>, Acanthoclinus quadridactylus) or a branched<br />
lateral line (e.g. the gem<strong>fish</strong>, Rexea sol<strong>and</strong>ri) . Others such as the<br />
clupeioids, cling<strong>fish</strong> <strong>and</strong> yellow-eyed mullet {Aldrichetta forsteri) have<br />
no external lateral line at all.<br />
In most <strong>fish</strong> the mucous gl<strong>and</strong>s in the skin secrete a protective<br />
coating <strong>of</strong> slime over the scales. This acts as a barrier to the entry<br />
<strong>of</strong> parasites, bacteria, fungi <strong>and</strong> other disease organisms, <strong>and</strong> it also<br />
reduces friction as the <strong>fish</strong> moves through the water <strong>and</strong> amongst seaweeds.<br />
Weed dwelling <strong>fish</strong> such as the marble<strong>fish</strong> (Aplodactylus me<strong>and</strong>ratus) are
304a<br />
noticeably slippery, whereas several midwater swimming <strong>fish</strong> do not produce<br />
mucous <strong>and</strong> are rough <strong>and</strong> s<strong>and</strong>papery to feel (e.g. slender roughy,<br />
Hoplostethus elongatus, <strong>and</strong> pink rnaomao, Caprodon longimanus) .<br />
Spines are usually associated with protection. They are found in<br />
the fins, especially the dorsal fin, on the opercular <strong>and</strong> preopercular<br />
bones (e.g. the redb<strong>and</strong>ed perch, Ellerkeldia huntii, <strong>and</strong> the two-spot<br />
demoiselle, Chromis dipilus) , on the tail (e.g. the stingrays <strong>and</strong> eagle<br />
rays) or all over the body (e.g. the porcupine <strong>fish</strong>, Allomyctevus<br />
whitleyi) The effectiveness <strong>of</strong> their protection is greatly increased<br />
by the association <strong>of</strong> poison gl<strong>and</strong>s with the spines , as in the rays <strong>and</strong><br />
red scorpion<strong>fish</strong> (Scorpaena cardinalis) .<br />
The senses<br />
Fish possess the senses <strong>of</strong> smell, touch, taste, sight,<br />
<strong>and</strong> electroreception. The.degree <strong>of</strong> development <strong>of</strong> the sense<br />
<strong>and</strong> associated- structures is <strong>of</strong>ten related to the <strong>fish</strong> ! s mode <strong>of</strong> life<br />
or habitat.<br />
SMELL:<br />
The olfactory organs in <strong>fish</strong>es, the nostrils, are essentially a<br />
deep pit lined with sensory tissue. Most <strong>fish</strong> have two pair, one<br />
situated on each side <strong>of</strong> the snout, excluding the pomacentrids which<br />
usually have only one set <strong>of</strong> nostrils. The nostrils are never used for<br />
breathing in <strong>fish</strong>es as they are in the terrestrial vertebrates. The<br />
sense <strong>of</strong> smell plays <strong>and</strong> important part in finding food in some species,<br />
especially the sharks which have poor sight.<br />
TOUCH:<br />
Cells sensitive to touch are found all over the body. Some <strong>fish</strong><br />
may also have special feelers to aid in the search for food, for<br />
example the elongated lower pectoral fin rays <strong>of</strong> the porae (Cheilodaotylus<br />
douglasi) <strong>and</strong> the red gurnard (Chelodonichthys kurrru) .<br />
SIGHT:<br />
hearing<br />
organs<br />
Fish eyes are very like our own in that there is a sensitive screen<br />
(the retina) at the back <strong>of</strong> the eye <strong>and</strong> a lens which projects an image<br />
onto that screen. However, unlike the human eye the iris does not exp<strong>and</strong>
305a<br />
<strong>and</strong> contract as the light conditions change. Most teleosts are thought<br />
to have some sense <strong>of</strong> colour vision.<br />
Fish have no true eyelids. The skin <strong>of</strong> the head becomes transparent<br />
where it passes over the eye. Sharks possess a nictating membrane, which<br />
is a freely moving membrane situated in the corner <strong>of</strong> the eye <strong>and</strong> can<br />
be passed over the entire surface <strong>of</strong> the eye when required. This membrane<br />
is <strong>of</strong>ten browninsh in colour <strong>and</strong> is used to protect the eye from intense<br />
sunlight. Bottom dwelling <strong>fish</strong> such as the rays <strong>and</strong> flat<strong>fish</strong>, frequently<br />
possess a thick dark lobe above the eyes which effectively shades them<br />
from strong light.<br />
There is some relationship between a <strong>fish</strong>'s way <strong>of</strong> life <strong>and</strong> the<br />
degree <strong>of</strong> development <strong>of</strong> the eyes. Fish living in murky waters usually<br />
have small eyes, whereas nocturnal <strong>fish</strong>es usually have relatively large<br />
eyes (e.g. slender roughy, Boplostethus elongatus, <strong>and</strong> bigeyes,<br />
Pempheris adspersa). Many deep sea <strong>fish</strong> <strong>and</strong> cave dwellers are blind.<br />
However, others have large well developed eyes <strong>and</strong> may also have the<br />
ability to produce light themselves.<br />
The majority <strong>of</strong> <strong>fish</strong> have their eyes situated on either side <strong>of</strong><br />
the head, giving them monocular vision. Some <strong>fish</strong> are capable <strong>of</strong><br />
focusing both eyes on the same object at the same time (binocular vision),<br />
which is important for judging distances, especially when capturing food.<br />
This may be achieved in various ways. The planktivorous <strong>fish</strong>, which pick<br />
individual organisms out <strong>of</strong> the water, have their eyes set well forward<br />
on the head (e.g. sweep, Scorpis aequipinnis, <strong>and</strong> blue rnaomao, Scorpis<br />
violaceus) . Hunters such as the joh-n dory (Zeus faber) , have protrusible<br />
eyes which can be rotated forwards in their sockets. The eyes <strong>of</strong> many<br />
bottom dwelling <strong>fish</strong> are situated on top <strong>of</strong> the head (e.g. the spotted<br />
stargazer, Genyagnus monopterygius) . The flat<strong>fish</strong> are unique among<br />
teleost <strong>fish</strong>es in that both eyes are on the same side <strong>of</strong> the head.<br />
Fish" with poor vision usually have a well developed sense <strong>of</strong><br />
smell, touch or taste.<br />
HEARING:<br />
Although <strong>fish</strong> do not have an outer ear, as is usual in terrestrial<br />
animals, they can hear. Fish possess an inner ear enclosed in a chamber<br />
in the hind part <strong>of</strong> the skull, on either side <strong>of</strong> the head. As water is<br />
a better conductor <strong>of</strong> sound than air the outer <strong>and</strong> middle ears are not<br />
needed to direct <strong>and</strong> magnify the sound waves. The ear is concerned with<br />
both hearing <strong>and</strong> the maintenance <strong>of</strong> equilibrium.
307a<br />
Production <strong>of</strong> sound, light <strong>and</strong> electricity<br />
As well as perceiving, <strong>fish</strong> are also able to produce light, sound<br />
<strong>and</strong> electricity. Light production appears mainly in deep sea <strong>fish</strong>.<br />
However, some relatively shallow dwelling <strong>fish</strong> also have this ability.<br />
Light producing cells may be found all over the body or they may be<br />
concentrated into large patches, particularly in the head region. In<br />
many species the light is due to organs containing luminous bacteria.<br />
The appearance <strong>of</strong> this light may be controlled by the movement <strong>of</strong> a special<br />
fold <strong>of</strong> skin or chromatophores over whole or part <strong>of</strong> the organ. Some<br />
sharks <strong>and</strong> teleosts have self-luminous photophores which are <strong>form</strong>ed from<br />
modified mucous. In these <strong>fish</strong> the light is able to be flashed on <strong>and</strong><br />
<strong>of</strong>f.<br />
The <strong>function</strong> <strong>of</strong> light production is uncertain, but the lights<br />
<strong>of</strong>ten show distinctive patterns <strong>and</strong> they may serve in the recognition<br />
<strong>of</strong> species <strong>and</strong>/or sex. They may also be used to startle attackers, <strong>and</strong><br />
in some cases to illuminate prey. In the deep sea angler<strong>fish</strong> (Ceratias)<br />
the luminous tip <strong>of</strong> the fin is used as a lure.<br />
Several <strong>fish</strong> are able to make drumming, grunting, growling or<br />
hissing sounds. Horse mackerel (Trachurus novae-zel<strong>and</strong>iae) , sun<strong>fish</strong><br />
(Mola mola) <strong>and</strong> leatherjackets (Parika soaher) stridulate, grinding the<br />
bones <strong>of</strong> the upper <strong>and</strong> lower pharyngeal teeth together. Others use the<br />
spines <strong>of</strong> their dorsal, anal or ventral fins, the gill covers or the<br />
jaw bones. Expulsion <strong>of</strong> air from the swim bladder produces a grunt.<br />
The red gurnard (Chelodonichthys kvmu) have special muscles lying in the<br />
wall <strong>of</strong> the swim bladder <strong>and</strong> are able to produce a drumming sound.<br />
Nearly all animals emit a weak electric field when in seawater.<br />
This originates from diverse sources such as swimming movements, muscle<br />
<strong>and</strong> heart activity <strong>and</strong> the voltage potential between body fluids <strong>and</strong><br />
seawater <strong>and</strong> between different parts <strong>of</strong> the body. Several species can<br />
actively produce a much greater electric field» These include the star-<br />
gazers (e.g. Genyagnus monopterygius), the torpedo rays (e.g. Torpedo<br />
faivchildi) <strong>and</strong> some eels <strong>and</strong> cat<strong>fish</strong>. These <strong>fish</strong> possess electric<br />
organs which may differ in <strong>form</strong> <strong>and</strong> position in different species, but<br />
all have similar microscopic structure. They consist <strong>of</strong> jelly filled<br />
hexagonal-shaped cells. The current output varies depending on the
308a<br />
species <strong>and</strong> the size <strong>of</strong> the <strong>fish</strong>, but has been recorded as large as<br />
200 volts.<br />
Electrical discharge is usually associated with the capture <strong>of</strong><br />
food. However, it is obviously also a good method <strong>of</strong> defence. Many <strong>fish</strong><br />
with electrical properties live in murky waters or have poor eyesight,<br />
so the ability to produce an electric current may also be important in<br />
electrolocation <strong>and</strong> communication.<br />
Gills <strong>and</strong> respiration<br />
Bony <strong>fish</strong> typically have four gills situated on each side <strong>of</strong> the<br />
head. These consist <strong>of</strong> four bony rods, or gill bars, placed one behind<br />
the other to <strong>form</strong> a series <strong>of</strong> arches. Attached to the hind edge <strong>of</strong> each<br />
gill bar is a double row <strong>of</strong> gill filaments (figure 9). Each filament is<br />
is thrown into a large number <strong>of</strong> smaller folds, which greatly increases<br />
the surface area <strong>of</strong> the gill exposed to the water* The gills <strong>of</strong> the bony<br />
<strong>fish</strong> open into a general chamber which is protected by a moveable flap,<br />
the gill cover or operculum.<br />
Figure 9: The head <strong>of</strong> a <strong>fish</strong> showing the first gill arch in its position behind<br />
the operculum (A) <strong>and</strong> a section <strong>of</strong> the gill arch (B) supporting the<br />
gill filaments on its hind edge <strong>and</strong> gill rakers on its front edge.<br />
The gill structure <strong>of</strong> the sharks <strong>and</strong> rays is similar but each arch<br />
is separated from its neighbour by a partition, the septum, <strong>and</strong> each gill
309a<br />
opens to the exterior through its own gill slit (figure 10). Normally<br />
there are five gill slits on each side <strong>of</strong> the head, but in one genus<br />
(Heptranchias) there are seven gill slits <strong>and</strong> in three genera<br />
(Chlamydoselaches, Hexaohus <strong>and</strong> Pliotrema) there are six= The skeletal<br />
structure supporting the gills is cartilaginous . The spiracles <strong>of</strong> the<br />
sharks <strong>and</strong> rays are vestigial gill clefts.<br />
The majority <strong>of</strong> <strong>fish</strong> draw water in through the mouth- The cavity<br />
<strong>of</strong> the mouth <strong>and</strong> the cavity in which the gills lie act as a double-<br />
chambered pumpo . Water is sucked in through the mouth, the mouth is<br />
closed <strong>and</strong> compressed to force water into the gill chamber <strong>and</strong> the out<br />
to the exterior. A flap <strong>of</strong> skin on the jaw acts as a valve to prevent<br />
the escape <strong>of</strong> water when the mouth cavity is compressed, while the closed<br />
gill cover prevents an inflow <strong>of</strong> water from the rear <strong>of</strong> the system. The<br />
two chambers work slightly out <strong>of</strong> phase to produce a continuous flow <strong>of</strong><br />
water over the gills.<br />
In certain fast swimming <strong>fish</strong> such as the tuna <strong>and</strong> mackerel<br />
(e.g. Scomber australasicus) , the pumping system is dispensed with except<br />
when resting. These <strong>fish</strong> swim with their mouths open allowing water to<br />
flow freely into the mouth <strong>and</strong> over the gills.
310a