The Basic Fish    The Outer Covering    The Fins    Bodily functions

The Basic Fish

The traditional torpedo-shaped image of a fish doesn't reflect the many variations of fish shape that exist. Fishes come from a variety of locations, and their bodies have become adapted to suit these different environments. A fish's shape will tell you a good deal about its living style: what type of system it inhabits, how it feeds, and what kind of swimmer it is.

The mouth

The structure of a fish's mouth can reveal its feeding habits. Fishes can be divided into three feeding groups: top-, midwater- and bottom-feeders.

Top-swimmers' mouths:

This type of fish has a straight dorsal surface, and an upturned, scooplike mouth for gathering floating insects.

Midwater-swimmers' mouths:

Species that swim in midwater have mouths at the very tip of their snouts, and generally snatch their food as it falls through the water. A few have underslung mouths fringed with rasplike folds, enabling them to graze on algae.

Bottom-dwellers' mouths:

These fishes have underslung mouths with flattened ventral surfaces which can be brought into close contact with the riverbed where much of their food lies.

The gills

The fish's equivalent of lungs, these delicate membrane layers diffuse oxygen into the fish's bloodstream. They are protected from damaging particles in the water by an arch of gill-rakers.

THE ANATOMY OF A FISH

This cutaway diagram shows the principle organs and structures found in the fish:

Mouth position Superior, terminal and inferior mouths indicate surface-, midwater- and bottom-feeders respectively.

BODY SHAPE AND PURPOSE

The basic "fusiform" fish shape has evolved to suit different living conditions such as the rate of waterflow in the habitat and the location of food.

Disc-shaped body The narrow, laterally, compressed shape of this Symphysodon discus is suited to slowmoving or stationa waters. This type of fish often lives in reeds.

Thin, deep-sectioned body The body of this Carnegiella strigata contains muscles which enable it to skim over the water. Flat-bottomed body This shape helps riverbed dwellers like Corydoras aeneus to hug the bottom.

Cylindrical body The shape of this Danio malabaricus suits fast-flowing waters.

Straight topped body A fish which has a straight dorsal profile, like this Aphyosemion gardneri, swims just below the surface.

The Outer Covering

Fishes have two layers of skin: a thin outer layer known as the epidermis, and a thicker inner layer known as the dermis. In most cases, the fish's body is encased in a scaly exterior. These overlapping plates, which grow out of the skin, provide streamlining and protection against injury. The scales are covered in a thin mucus layer that protects against parasites and gives "slip-ability".

Colour as camouflage

Fishes are colour-shaded from top to bottom, with a dark top and a light underside, camouflaging them against the riverbed from predators above. Other colour patterns serve as species and sex recognition, offer camouflage within the fish's natural habitat, and/or give visual warnings to other species that the fish may be poisonous. Some marine fishes have colour patterns that mimic those of species that they prey on. In some species the eye is hidden in a black area, and a false "eye" is featured elsewhere in the colour scheme to give diversionary protection if the fish is attacked.

SCALE TYPES:

Fishes' scales are either placoid or elasmoid. Placoid scales are found in Sharks and Rays, and resemble small teeth-like projections from the skin. Aquarium fishes have elasmoid scales, which rise directly from the dermis, and these may be of two types: ctenoid or cycloid. Some fishes have one sort only, others have both types. Ctenoid scales These scales have comb or teeth-like extensions to the rear edge. Cycloid scales This type of scale is round and smooth. Scutes Armoured Catfishes don't have scales; instead, their bodies are covered with two (or three) rows of overlapping bony plates known as scutes. "Naked" fishes African Catfishes have neither scales nor scutes, merely skin, and for this reason are often known as "Naked Catfishes". 

How is the fish's colour formed?

Colour is produced in two ways - by light reflection and by pigmentation. Iridescent fishes owe their "sparkle" to light reflecting back from a layer of guanin just beneath the skin. Colour changes are made in the pigmentation cells, and can be brought about by excitement, fear or hormonal activity. Pencilfishes have a nocturnal colour pattern, and change back to their normal coloration at daylight.

Juvenile coloration

The majority of juvenile fishes look like smaller versions of their parents. However, the colouring and patterning of juvenile marine Angelfishes change radically as the fishes mature.

A juvenile Angelfish In colour and patterning this young Pomacanthus imperator differs from the adult:

The Fins

The fins consist of rays which are webbed with tissue; these rays may be "hard" (non -articulated and quite rigid) or "soft" (having many articulations or branches). With the help of small muscles, fins can be folded or extended.

Fin types:

(Fins in diagram: Anal fin Adipos, fin Dorsal fin Caudal fin.)  A fish usually has seven fins: three singles - dorsal, caudal and anal - and two sets of paired - pelvic and pectoral:

The dorsal and anal fins

The dorsal and anal fins are used to keep the fish upright, acting like keels. In some species, the anal fin has become modified as a spawning aid. In male livebearers (see p. 245) the anal fin has developed into a rod-like tube that enables the sperm to be directed at the female's vent during spawning.  The anal fins of some Characins have tiny hooks on them, and it is believed that these help keep the male and female close together when spawning.

The caudal fin

Better known as the tail, this fin provides the "final drive" to push the fish through the water. Power is generated by muscles in a series of strong, wave-like motions along the length of the body.

The adipose fin

This small, extra single fin is carried by some fishes on their dorsal surface, THE OUTER COVERING/FINS between the proper dorsal fin and the caudal fin. It has no rays within its fatty tissue structure, and seems to serve no apparent purpose.

The paired fins

Fishes manoeuvre by means of the paired fins. These are the pectoral fins which are situated just behind the gill cover, and the pelvic or ventral fins which emerge just in front of the anal fin. These fins correspond roughly to the limbs of mammals, and can be used in a wide variety of ways.

CAUDAL FIN SHAPE

(The shape of the tail often indicates the swimming habits of the fish.)

Crescentic -  Found in some continuous, high-speed swimmers:

Emarginate - Found in slow-movers capable of fast dashes:

Forked - Usual in continuous, high-speed swimmers:

Rounded - Common in very slow-moving and cultivated varieties:

Truncate - Usual in slow-movers capable of few fast dashes:

Pointed - Found in some slow-moving and cultivated varieties.

Uses of pectoral fins:

Uses of pelvic fins:

"FANCY" CAUDAL FINS

Many Fancy Goldfishes and aquarium-developed strains of tropical fishes have exaggerated caudal fins known as Lace- or Veil-tails. Fishes with such fins are generally slow see p. 86
swimmers and aren't found in nature:

CAUDAL FIN EXTENSIONS

The presence of extended rays on the caudal fin of a fish such as a Swordtail or an Emperor Tetra is often a reliable indication that the fish is male. These extensions, which are found in wild fishes, serve no apparent purpose.

DEFENSIVE FINS

The long, hollow spines of the Lionfish are poisonous, and are used in selfdefence. Triggerfishes have a fin which can be locked in an erect position to prevent capture. This fin normally lies just in front of the dorsal fin.

Bodily functions

A basic knowledge of how your fishes function will help you to understand their needs and habits.

Respiration

Fishes breathe oxygen which is dissolved in the surrounding water. They do so by taking in water through the mouth and expelling it through the gills. As it passes across the delicate gill membranes, oxygen is absorbed into the blood and carbon dioxide expelled. A certain amount of ammonia may also be released via the gills, and in freshwater fishes some water is released too. Some fishes, notably Anabantoids (see p. 60), can breathe atmospheric air via a special labyrinthine chamber in the head behind the gills; others such as Corydoras Catfishes (see p. 74) can process it in the hind part of the gut. Fishes with suckermouths - such as 11 he Suckermouth Catfish (see p. 76) - breathe through extra slits behind the head, thus releasing their special mouth to carry out the more important duties it evolved for. These include feeding and maintaining position in fast-moving water by clinging onto rock surfaces.

THE LABYRINTH ORGAN

Found in Anabantoid fishes (see p.60), this organ consists of rosette-shaped plates which carry hundreds of blood vessels that absorb oxygen from inhaled atmospheric air:

Caudodorsal compartment
Gill arch  Pharyngeal opening

Do fishes sleep?

Because they don't have eyelids, and therefore can't close their eyes, it is sometimes assumed that fishes don't sleep. However, they need rest, and this takes the form of suspended animation, where the fish lies motionless for several hours. Some marine species such as the Wrasses (Labridae) may bury themselves in the coral sand or spin "sleeping bags" -cocoons of mucus each night to sleep in.

Excretion

In addition to the usual disposal of waste. products from digestive processes, fishes excrete ammonia from the gills. Freshwater species also excrete water from the gills. Moreover, fishes deposit waste products such as guanin within their own bodies (usually just under the skin). It is these guanin deposits which contribute iridescence to fishes' colourings.

Body fluid levels

Strange as it may seem, fishes have a drinking problem, despite being surrounded by water.

Saltwater fishes:
The concentration of blood salts in marine species is lower than the salt concentration of the surrounding water. Due to a phenomenon known as osmosis, water is continuously lost from fishes' bodies, and to make up for this loss they must drink. By drinking copiously, passing very small quantities of urine and excreting excess salt they maintain their body-fluid levels.

Freshwater fishes:
The situation is reversed in freshwater, where the blood concentration is higher than the surrounding water. In this instance, water is always being absorbed into the body. So, to avoid bursting, freshwater fishes have to excrete large amounts of water (up to 10 times their body weight daily!). They do this in two ways: as urine, and through the gills.

The sense of smell

Fishes smell through their nostrils, which, unlike ours, aren't used for breathing at all. They consist of two or four openings on the front of the snout, connected directly to the olfactory system. The piscine sense of smell assists in the detection of pheromones - for example, creating a "fear reaction" when one fish in a shoal releases the fear pheromone into the water -and helps to locate food or spawning areas.

The sense of taste

In fishes, the taste buds are primarily concentrated in the mouth, tongue and lips. However, they may also occur over other parts of the body, the pelvic fins and, of course, on the barbels of bottomdwelling species.

Sight

In most species, vision is monocular they can see in two directions, but can't focus both eyes on the same object at the same time. However, where the eyes are located high on the tip of the head some degree of binocular vision may occur they can focus both eyes on the same object at once, giving a stereoscopic effect. Fishes can only focus up to 45 ems, but they can detect things much further off via their "lateral line system". Fishes are able to respond to colours, but may be confused by varying brightnesses.

EYE STRUCTURE

Unlike our eyes, where the curvature of the lens is altered in order to focus the image on the retina, in a fish's eye the lens shape isn't altered; instead, the lens itself is moved backwards or forwards.

Eye flap Suckermouth catfishes (seep. 76) have an unusual feature: a flap of skin protruding across the eye that can be expanded or contracted, enabling them to alter the amount of ligh t entering the eye:

The eyelid

Unlike terrestrial animals, fishes have no eyelids because they have no need to keep their eyes moistened.

Hearing

Fishes' "ears" are much less complicated than ours, being made up of the equivalent of our inner ear only, since the connecting apparatus of the human middle and outer ear is unnecessary. The reason for this is that water is a very dense medium, and therefore sounds or vibrations - which travel five times faster than through air - are more easily detected.
Although fishes can hear sounds over much the same frequency range as we can, different species are sensitive to only parts of that range. Fishes' hearing is supported by their ability to sense vibrations. These may be detected by the swim-bladder and passed on to the ear by means of the Weberian Ossicles (a series of interconnecting bones), or by a tubular structure, resembling a doctor's stethoscope, that connects the swimbladder with the ear.

The lateral line system

This system is the fish's "sixth sense", enabling it to detect vibrations and currents. It gets its name because the inputs to the vibration-sensing nerves are
visible as a line Of tiny perforations or "portholes" through a row of scales along the lateral line of a fish. This line is often incomplete, or may be extended right over the top of the gills to the nostrils.

The swim-bladder

With the exception of a few bottomdwellers such as Gobies, most aquarium fishes have a swim-bladder; this organ enables them to maintain their position at any level in the water. In some fishes it serves other purposes: it may act as an amplifier for any sound that the fish is able to produce, or it may supplement the ear.

NAVIGATIONAL AIDS

Lateral line navigation
Fishes that live in dark, muddy waters or that lack eyes - for example, this Astyanax mexicanus, the Blind Cave Fish - navigate perfectly well using the lateral line system to inform them of obstacles, other fishes or moving food in their vicinity: