Fish coloration

various tropical fish colours

Coloration in fish

How do fish colours come about?

various tropical fish coloursFish coloration is formed by the reflection and absorption of various parts of the spectrum of light inside specialised cells. This is achieved by various layers of colour forming cells, such as crystals in the skin(iridophores), pigment cells(Chromatophores) and underlying flesh colours. Pigment cells are of various types.

Chromatophores can be classed according to colour under white light: xanthophores (yellow), erythrophores (red), iridophores (reflective), leucophores (white), melanophores (black/brown), and cyanophores (blue).

Combinations of these pigments and optical effects can produce many distinct colours.
Fish can also alter their brightness, colour and patterns. These colour changes are mostly autonomous and are effected by nerve impulses and hormonal releases during mating or fright. The nerve impulses and hormonal releases affect the chromatophores. Coloured chromatophores are branch liked structures that contract to remove the colour or expand fully to show full colour. Iridophores can reflect light thereby creating a metallic effect. Some iridophores refract light. This has the effect of creating a colour where that colour is not available as a pigment. Blue is often created in this way. Very few species of fish have blue pigmentation.

regal angelfish fish coloration
regal angelfish fish coloration

The pigments found in the chromatophores cannot be created by the fish but must be extracted from their diet. It has been demonstrated that when certain fish are denied a certain pigment from its diet that the fish will change colour.

Why do fish have colour?

Fish can change their colours to blend in with their environment called background adaptation, in terms of brightness and colour. They can also change their colours depending on mood and agression or submission.

Colouration in fish is vital for a fish’s survival. It plays a role in camouflage, fish recognition, mate selection ,mood display and warning of poisonous or danger. Most preyed upon fish have camouflage themselves by having a coloration, pattern or shade that closely matches it background environment. Fish can recognise each other as being from the same species and which gender and colour plays an important role in this. When in breeding mood usually the male will colour up with brilliant coloration. The brightest colours tend to attract the best females and is used as a warning against rivals. Fish can display dominance and submission by darkening or lightening their colours. In some species such as in many Malawi cichlids, submissive males will take on the colour of the females to avoid attack from dominant males. Some fish do the opposite of camouflage and in fact have developed very striking colours as a warning of being poisonous or dangerous.

yellow box fish
yellow box fish from Maldives

In the tropical fish keeping hobby colour is one of the most important factors that determine whether a particular species of fish makes its way into the hobby. Hobbyists favour fish with brilliant colours and patterns. Hobbyists by selective breeding have created new “sports” with enhanced or even new colours. These fish are more valuable and score higher in fish keeping shows.

Experienced tropical fish keepers recognise that fish do change coloration according to their environment, water conditions and diet. Having a darker gravel, feeding foods with high levels of pigmentation and providing the correct ph, hardness and salt levels all help in getting the best out of fish colours. Some vitamins and amino acids that produce pigmentation have a short shelf life and will not survive in sufficient quantities in dried fish food. Only live food and vegetable matter can provide sufficient pigmentation to aquarium kept fish.  Stressed fish will lose coloration as well as fish under bright lighting in bare aquariums or fish undergoing medical treatment.

Countershading is where there a fish’s body is dark when viewed from above and is light when viewed from below. This is for camouflage from predators above the water who will find it difficult to see a dark backed fish against the dark of the ocean, while the light bellied underneath of a fish makes it difficult for predators underneath to see the fish against the bright sky.

When in courtship mood fish usually enhance their colours to their maximum level of vividness. Usually male fish have better blue and red coloration and are generally more colourful than their female counterparts.

Young fish in most species are grey,green or black and have few distinctive markings. This is a form of camouflage because most young fish are preyed upon by adult fish. Young fish spend most of their youth near river or lake banks or near muddy bottoms between algae and plants and so have colours that resemble their bushy or earthy surroundings. Some sea fish have young that undergo a larval stage where they drift along with plankton. Most of these young are transparent to blend in with other plankton to avoid being eaten.

Fish anatomy

fish anatomy

River fish are usually streamlined

Anatomy of fish

The shape of a fish’s body tells you a lot about its behaviour in the wild. Long stream-lined fish are usually lake or river fish. Being streamlined helps a fish to swim faster in rapid moving waters or open bodies of water such as a large lake. Short or stocky fish are usually from ponds or live near the bed of a river. Side flattened fish usually reside in in slow moving waters with lots of vegatation or roots. The body shape helps these fish swim between plants and reeds. Top flattened fish usually swim on or near the river bed or floor of a pond. Fish with mouth barbels usually swim near the floor of the river or pond and use these barbels to seek food on the murky bottom.

Mouth Anatomy of fish

The shape of a fish’s mouth tells you which part of the water it feeds from. Fish with upturned mouths feed from the water’s surface. Fish with forward pointing mouths feed mostly from midwater. Fish with down pointing mouths feed near the pond floor or river bed. Fish with mouths underneath the head feed off the floor or off algae attached to encrusted rocks.

Fish scales

The scales on a fish’s body provides physical protection from injury. These scales are quite tough and overlap each other to form a tough but flexible armour. The whole of the fish’s scaly body and fins and all body parts are covered in a slimy mucous. This mucous is being continuously secreted by the fish and is used to wash away any bacteria, fungus or virus from invading the body of the fish.

Fins of fish

Fins are used by fish for propulsion, steering, stability and braking. Fins have a secondary function as flags and signals to other fish. Fins when held spread are usually a sign that the fish is healthy, while a fish that is unhealthy will tend to clamp its fins closed.
Some fins are single while other fins are paired. All fins have some purpose.

The single (unpaired) fins are:

  • Dorsal fin – This fin is used for stability
  • Tail fin – This together with the tail is the main means of forward propulsion
  • Anal fin – This is used for stability and in male livebearers is adapted into a reproductive organ.
  • Adipose fin – This is found in some species such as the characins or tetras. This fin appears on the top of the fish in between the dorsal fin and the tail fin.

The doubled fins

  • Pectoral fins – Pectoral fins lie on either side of the body behind the gills and are attached near the bottom of the fish’s body. They are used for braking, manoeuvring and reversing.
  • Pelvic fins – These lie forward of the fishes anus. On most fish these are attched mid body near the bottom of the fish’s body. The pelvic fins helps the fish to rise and descend through the water. They also help the fish turn sharply and assist braking.

Swim Bladder

The swim bladder in fish is an air filled sac that is within the fish’s body that aids in buoyancy and maintaining the fish’s level in the water. By relaxing or contracting the muscles of the swim bladder a fish can compress the air sac or expand the air sac. When the fish decides to compress the swim bladder it will sink. When the fish expands the swim bladder it will float.

Fish Senses

Fish have the same five senses we humans have, plus they also have a specialised sixth sense, the lateral line. The lateral line runs along the middle of the fish from begind the gills to the tail on both sides. The lateral line consists of small pits in the middle of each midline scale. The water filled pits contain small neuromasts which are minute finger shaped structures that contain hairs. When currents enter the pits the neuromasts move, sending a nerve signal to the brain of the fish. This detection of minute changes in flow and water pressure allows the fish to sense its surroundings in a sonar like way. This sense is used in shoaling and detecting minute water currents from prey, other fish or objects. Blind cave fish rely on the lateral line to ‘see’.

Sense of taste in fish

Of course fish don’t have tongues but they do have taste buds in and around their mouths. Many fish also have taste buds all over their body. Many bottom feeding fish have taste buds in their barbels.

Sense of smell in fish

Fish smell through their nostrils. Some fish have two sets of nostrils that allows water to flow through them. Water is pumped into the intake nostril and expelled out of the ottake nostril. The resulting flow of water that passes across receptor cells in the nostril cavity which detects chemical messages and sends signals to the fish’s brain. Fish use their sense of smell to swim away from some smells that may be harmful, such as that of a predator. Or they may swim towards a smell of food or towards potential mate.

Sense of hearing in fish

Although fish do not have ears they do have an internal ear mechanism. Sound actually travels faster through water than through air. Sound travels through water as a series of vibrations. These vibrations travel through the fish’s body mostly undisturbed. However in the fish’s inner ear there are tiny bones called otoliths. Because the otoliths are denser than the surrounding flesh, they vibrate. This vibration is detected by nerves attached to these bones. The nerves then send signals to the brain. In some species sound is amplified by the use of the fish’s swim bladder.
Some fish can make sounds by grinding their teeth or drumming their swim bladder.

Sense of sight in fish

Sight is an important sense for fish as light does penetrate to beneath the waters surface. The eyes of fish have much in common with amphibians, reptiles, birds and mammals. Fish’s eyes tend to be more spherical though. Most species have colour vision and some fish can even see ultraviolet light. Fish focus their vision by moving the lense closer or further from the retina. Most fish do not have binocular vision. However, the hammerhead shark does have a good binocular vision because it has its eyes set far apart but focussed forward.

Sense of touch in fish

Fish do have a sense of touch that spans through their who skin. Scaleless fish such as some catfish have an even better sense of touch.