What makes a fish a fish?

• Although fish are vertebrates - like humans, birds, cats, dogs and elephants - they are also almost like an alien in a lot of ways.
  • Fish are designed to live in water.
  • They move through the water and balance not with hands and feet but with fins.
  • For most fish, they breathe not through lungs but by using gills to extract oxygen from the water.
  • They are cold-blooded, with their body temperature usually the same as the surrounding water.
• And while we can survive in their world with the help of wetsuits and breathing equipment or submarines, they can't live in our world - they are literally like a fish out of water.

Body Shape

• There is enormous diversity in fish.
• The traditional image of a fish shape isn't reflected in the many variations that exist.
• Each species is adapted to a particular habitat and role in the wild.
• The body shape of the fish gives us a picture of the habitat and lifestyle of that fish.
• Fish with tall bodies that are laterally compressed (from side to side), such as Angelfish and Discus, are generally adapted to life in slow-moving waters or to life among rocks and dense vegetation.
  • A slender, torpedo-shaped body is designed for quick movement through the water, such as on the Lepidiolamprologus kendalii right.
    • This body shape reduces drag and requires a minimum amount of energy to swim. This body shape is recognised as the 'typical fish shape.'
    • Many predatory fish are torpedo-shaped, to allow for quick action to chase and catch prey.
    • Most 'prey' fish are also torpedo-shaped to allow them to make a quick escape.
  • Elongated body shapes are associated with eels and eel-like fish. The elongated body is horizontally 'stretched'.
    • The Spiny Eel is an example of a fish with an elongated body shape. The Khuli Loach is another with an elongated body.
    • The elongated body is not common among freshwater fish, but is found more in marine fish - with the Moray Eel probably being the best known of these.
  • A depressed (flattened) body shape (from top to bottom), is also found in bottom-dwelling fish.
    • Borneo Suckers (Family Cobiditae) Stingrays (right) and Freshwater Sole are good examples of fish with a depressed body shape.
    • As with the elongated shape, the depressed shape is not that common in freshwater fish.
  • Some bottom-dwelling fish, such as Humpheads (Steatocranus casuarius) have modified a swim baldder that give them less bouyancy.
    • These fish typically live in rheophilic (rapids) areas, and less bouyancy means that they can stay on the bottom away from the turbulence of the rapids and in the relatively calm waters.
    • This also gives them a comical hopping motion as they move along the bottom.

Mouth

• The position and shape of the mouth tell us a lot about how the fish feed:
  • Fish that live and feed near the surface have a superior (upturned) mouth.
  • They have a straight dorsal surface and an upturned mouth that they use for gathering food from the surface.
  • Examples of fish with a superior mouth are:
    • African Butterflyfish (Pantodon buchholzi)
    • Killifish
    • Hatchetfish
    • Archerfish (Toxotes jaculatrix)
    • Saratoga (Scleropages sp)
    • Arowana (Osteoglossum bicirrhosum - right)
    • Gourami
    • Gobies
• Bottom-dwelling fish have and inferior mouth (downwards facing) and flattened bellies (right) as opposed to the flattened backs of surface-dwelling fish.
  • The flattened ventral surface helps bring them into close contact with the substrate where their food is.
  • These are used to help them locate food in the often dark and muddy waters of their habitat.
  • Many bottom-dwelling fish also have whiskers or barbels around their mouth.
  • Examples of fish with an inferior mouth are:
    • Loaches
    • Catfish
    • Flying Foxes
    • Algae-eaters
    • "Sharks"
    • Distochodus
• Fish that live and feed in mid-water have a terminal mouth, which means it doesn't point up or down but is located at the end of the head.
  • These fish generally snatch their food as it falls through the water.
  • This is what is thought of as a "normal" fish mouth. The majority of fish have a terminal mouth (right).
  • Examples of fish with a terminal mouth are:
    • Cichlids
    • Goldfish
    • Bitterlings
    • Perch
    • Barbs

Gills and other Breathing Organs

• Fish, like people, need oxygen to survive. Because they live in water, they need to extract the oxygen they need from the water.
• Fish take water in through their mouth.
  
• The water is passed over gill membranes before it is expelled past the gill covers at each side of the head.
• As the water comes in contact with the gill membranes, the dissolved oxygen is absorbed by the tiny blood vessels in the gills and transported around the body by the heart.
  
• Where the waters of a natural habitat are deficient in oxygen, the fish native to that area have evolved with an auxillary breathing apparatus.
  • This is known as the Labyrinth organ and is found in Anabantid fish (eg. Gouramies and Fighting Fish).
  • The Labyrinth organ consists of rosette-shaped plates that contain hundreds of blood vessels to absorb oxygen inhaled from atmospheric air.
  • In Anabantids,the Labyrinth organ is located in the head behind the gills.
  • Corydoras and other Callichthid catfish (eg. Hoplos) are also able to breathe atmospheric air and they process it in the back part of the gut.
  • The Australian Lungfish has a lung as its supplemantary breathing organ that its uses during intense activity or when the gills become clogged with mud.
  • They are the only Lungfish that is not restricted to only breathing air.
• The operculum protects the delicate gill structures from external damage and opens and closes to allow more or less water to pass over the gills.

Osmotic regulation

• Whenever two fluids that have different strengths are separated by a membrane such as skin, the water from the weaker fluid passes through the membrane to dilute the stronger in a process known as osmosis.
• In fresh water fish, the internal body fluids are stronger in concentration than the fresh water they swim in, which means that the water passes through the skin into the body.
• If the fish did nothing to counteract this, it would swell up like a balloon.
• Freshwater fish have to excrete large amounts of water to remain in balance. This process is known as osmotic regulation.

Fins and muscles

• Fish swim in water, which is 800 times denser than air. Typically, fish depend on rapid movements of their tail to propel them through the water.
• All this swimming demands large muscles and the swimming muscles in fish make up 40 - 65 percent of the body weight of a fish.
• Swimming is a coordinated switching of contraction and relaxation of muscles from one side of the body to the other.
• Most fish have the following fins, dorsal, caudal, anal, pectoral and pelvic fins. Some fish have an extra fin - an adipose fin.
• Fins are multi-purpose limbs. Fish use their fins for moving through the water, stability, spawning, nest-building and as tactile organs.
• Fins are rays that are webbed with tissue. The fin rays may be hard or soft.
  • Hard rays are rigid and inflexible.
  • Soft rays are articulated and branched, giving them flexibility.
  • The fins are folded and extended by using small muscles.
• Fish have paired and unpaired fins.
  • The unpaired fins are the dorsal, anal, caudal and, in some fish, the adipose fins.
  • The paired fins are the pelvic and pectoral fins.
• Each fin type has a special purpose:
• The Caudal Fin
  • The caudal fin, or tail, is used to propel the fish through the water.
  • The power in the caudal fin is generated by muscles in a series of strong, wave-like motions along the length of the body.
    • Fish with forked and crescent-shaped tails are fast swimmers.
    • Fish with rounded, truncated and fan-shaped tails may be slow-moving but capable of quick dashes.
    • Some fish have extensions on the caudal fin.
      • These extensions, while serving no apparent purpose, are usually a good indication that the fish is a male.
• The Anal Fin
  
  • The anal fin is located on the underside of the body just forward of the tail.
  • The anal fin stabilises the fish while it is swimming, like the keel on a boat .
  • In some fish, most notably the Livebearers, the anal fin is used during spawning.
  • In male fish, the usual fan-shaped anal fin is folded into a rod-like structure called a gonopodium. The male injects sperm through the gonopodium into the female's vent when breeding.
• The Pelvic Fins
  • The paired pelvic fins are located forward of the anal fin.
  • These are used for further stability when swimming.
  • The pelvic fins are also referred to as the ventral fins as they are located on the underside of the fish.
  • Some fish have modifications to their pelvic fins:
    • In Gouramies , the pelvic fins have modified into thread-like fins that are used as tactile organs. You will often see a Gourami touching other fish or objects in the tank with their pelvic fins.
    • The pelvic fins of Corydoras are used to hold eggs during spawning (called the Pelvic Basket).
    • The pelvic fins of Gobies are fused to create a suction cup that helps them cling to rocks in fast flowing current.
• The Pectoral Fins
  • The paired pectoral fins are located just behind the gill cover and are used for maneouvering the fish.
  • The pectoral fins are used for abrupt changes in direction and speed. They can act as brakes or hydroplanes to assist with gliding through the water.
    • Many catfish have the ability to lock their pectoral fins (by using a complex lock and release network of bones) at right angles to their body as a defence against predation.
    • Some catfish also have spines on their pectoral fins as an added defence, which can inflict a painful sting.
• The Dorsal Fin
  • The unpaired dorsal fin is located on the back of fish. The dorsal fin helps to balance fish when they are swimming.
  • The dorsal fin is made up of hard rays (at the front of the fin) and soft rays.
  • Some fish also have spines on their dorsal fin.
  • Some fish have no dorsal fin or a small dorsal fin (eg. Knifefish and Ranchu Goldfish), while others have two dorsal fins (eg. Rainbowfish and Gobies).
• The Adipose Fin
  • The adipose fin is a small fin found between the dorsal and caudal fins of some fish - predominantly Characins and some Catfish.
  • The adipose fin has no rays and is just fleshy tissue.
  • The purpose of the adipose fin is unknown.
• Many fish are also selectively bred to give us longer finnage and other features not seen in the wild form.
  • Veiltail Angelfish, Siamese Fighting Fish, Fantail Goldfish, Delta-tail Guppies and Lyretail Mollies are some examples of selective breeding for beautiful fins that aren't seen in the wild.

Scales (or not)

• Most fish have a protective covering on their body in the form of scales.
• Scales are thin, bone-like plates that cover the skin of most fish and allow them to glide through the water.
• They also protect fish from parasites and scrapes.
• There are five main types of scales:
  • CYCLOID - which have smooth edges (right). The word cycloid comes from the Greek word cyclo meaning circle.
  • CTENOID - have tiny teeth along the rear edge of each scale and give the body a roughish feel. The word ctenoid comes from the Greek word cteno meaning comb, and refers to the comb-like ctenii or teeth on the rear margin of the scale.
  • PLACOID - are found in sharks and rays, and can vary greatly in external appearance. They do not increase in size as the fish grows, instead new scales are added. Placoid scales are often referred to as denticles. Placoid scales consist of a flattened rectangular base plate which is embedded in the fish, and variously developed structures, such as spines, which project posteriorly on the surface. The spines give many species a rough texture. .
  • GANOID - are hard diamond-shaped scales common on sturgeon, bichirs, paddlefish, gars and bowfins. Cycloid and ctenoid scales are derived from ganoid scales that have lost the ganoine and thinned the bony embedded plate. Ganoid scales have articulating peg and socket joints between them. They are modified cosmoid scales which consist of a bony basal layer, a layer of dentine, and an outer layer of ganoine (an inorganic bone salt).
  • COSMOID - are found in the Lungfish (family Ceratodidae) and some fossil fishes. Cosmoid scales are similar to placoid scales and probably evolved from the fusion of placoid scales. They consist of two basal layers of bone, a layer of dentine-like cosmine, and an outer layer of vitrodentine. As the fish grows each scale becomes larger as new bone is added to the basal layers.
• The scales are waterproof and overlap, forming a protective flexible armour.
• Glands embedded between the scales secrete mucus (slime) that covers the entire body to protect fish from infection.
• Scales lay in the direction of head to tail in all bony fish. This helps reduce drag through the water, as the edges of the scales are lying in the opposite direction to the water movement.
• Some catfish, such as Corydoras sp and Brochis sp (right), have bony plates or armour in place of regular scales.
• Fish such as Loaches, some catfish (Pimelodus sp and Synodontis sp), Knifefish (below) and Elephantnoses are scaleless or have very small scales embedded in the skin.

Colour

• Colour plays an important role for fish.
• Some species rely on colour for camouflage while others use colour to attract a mate or as a warning mechanism.
• Juvenile colours are often vastly different to adult colours, often being spotted, dappled or dull brown or silver to blend in with the surroundings (right - some juvenile and adult differences).
• The metallic sheen of some fish is caused by deposits of guanin, a waste product, immediately beneath the scales.
• Depending on the angle that the scales are laid down and the way the light hits them different colours will be reflected.
• Many wild fish have been selectively bred for aquariums to create brighter colours than those that occur naturally.
• Discus (Symphysodon sp), Goldfish and Siamese Fighting Fish (Betta splendens) are the best known examples of using selective breeding in this way.

Sensory organs....

• Like people, fish have the five senses of sight, touch, taste, smell and hearing.
• Their sense of taste and smell is far better than ours.
• Hearing
  • Fish have ears, although there is no external opening to the ear.
  • The fish ear is less complicated than our ear, as it has no middle or outer ear.
  • They only need the equivalent of the inner ear because water is denser than air and sounds and vibrations are more easily detected. Sound and vibration move five times faster through water than through air.
  • Fish are very sensitive to noise - and this is one of the reasons you should never tap the glass of the aquarium.
  • Bony fish have an inner ear for equilibrium, detecting acceleration, and hearing.
  • A fish's soft body tissue has about the same acoustic density as water; sound waves travel through soft tissue to the ear.
  • Some fish species have adaptations for enhanced sound reception at higher frequencies.
  • In some fish, the swim bladder lies against the ear and acts as an amplifier to enhance sound detection.
  • In some species, such as Goldfish (Carassius auratus) and catfish, a series of small bones, known as the Weberian Apparatus, connects the swim bladder to the inner ear.
• Lateral line
  • The lateral line is a sixth sense for fish.
  • Like the ear, the lateral line senses vibrations.
  • It functions mainly in distance perception and detecting low-frequency vibrations and directional water flow.
  • The lateral line system is made up of a series of fluid-filled canals just below the skin of the head and along the sides of the body of the bony fish.
  • The canal is open to the surrounding water through tiny pores.
  • The lateral line canals contain a number of sensory cells, which are tiny hairlike structures similar to the hairs in our ears.
  • Water movement created by turbulence, currents, or vibrations of other fish displaces these hairlike projections and stimulates the sensory cells.
  • This stimulation triggers a nerve impulse to the brain.
  • In some fish this line is incomplete.
• Eyesight
  • In most fish, vision is monocular, meaning they can see in two directions, but they can't focus both eyes on the same object at the same time.
  • Fish with eyes set high on their head have more binocular vision than those with lower eyes.
  • Fish can focus on objects up to 45 centimetres away, but they don't have to see something to know it's there. They can detect things that are much further away through the lateral line system.
  • The eyesight in some species of bony fish is well developed.
  • Fish have a basic vertebrate eye, with a large lens that helps retain a large portion of the light entering the eye.
  • Unlike our eyes, the lens of the fish eye isn't altered - instead the lens itself moves back and forward.
    • Goldfish (Carassius auratus) have excellent eyesight up to nearly 5 metres.
    • Nocturnal fish generally have larger eyes than fish that are active in the daytime. This is to gather the available light for efficiently.
    • Several species of fish have accurate eyesight both above and below the surface of the water. Archerfish are probably the best known example.
    • Some species of fish have no eyes. The blind cavefish uses other senses to help them find prey.
    • Suckermouth Catfish (Loricarids) have an unusual feature.They have a flap of skin across the eye that can be expanded or contracted to alter the amount of light entering the eye (right).
    • Fish, especially those that live in shallow-water habitats, probably have colour vision.
    • Because they live in water, fish don't need eyelids. The water cleans their eyes and keeps them from drying out.
    • Armoured catfish such as Corydoras have the ability to roll their eye, making it look like they winking.
• Taste
  • Fish have taste buds inside their mouths and some species have taste buds in pits along the head and ventral side of the body.
  • Catfish have taste buds in the barbels alongside their mouth (right).
  • Taste may be responsible for the final acceptance or rejection of food.
• Smell
  • Olfactory cells in the nasal sac detect tiny amounts of chemicals in solution. The sense of smell varies among species, although it is a generally well developed sense in all fish.
  • Fish have two nostrils, located on either side of the head. The nostrils are used solely for smelling not breathing.
• Electroreception
  • Some fish in the families Gymnotidae (Knifefish) and Mormyridae (Elephantnose) produce a low-voltage electric current that sets up an electric field around the fish.
  • Electric organs are made up of cells called electrocytes that have evolved from muscle cells.
  • Electroreception is an adaptation for detecting prey and for navigating in murky water.
  • Tiny skin organs on the fish detect disruptions in the electric field that are caused by prey or inanimate objects.
  • Knifefish (such as the Black Ghost Knifefish right) and Elephantnoses are weakly electric.
  • Other fish (Electric Catfishand Electric Eels ) produce stronger electric currents that they use to stun and immobilise their prey fish.
  • The maximum voltage recorded is 550 volts, from an Electric Eel measuring 2 metres in length. The discharge from a large fish is enough to knock a person down, but not enough to kill.
• Next we'll have a look at the various habitats and adaptations of fish that allow them to live in some of the extreme parts of the world that they do.

 

Habitat & Adaptation

 

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