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Fish physiology

  Sensory organs and communication - breathing and circulation - swimming and buoyancy

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anatomy = the doctrine of the structure of living beings, the location and structure of their organs and tissues.

physiology = researches the functions and life processes of the plant and animal body and its individual parts (cells, tissues, organs).

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Sensory organs and communication

  The eyes of crepuscular and nocturnal predatory fish (and especially deep-sea species) are very large. The shark also has small mirror plates (tapetum lucidum) behind the retina, which reflect weak light and thus increase the sensitivity to light. Sharks can therefore perceive moving shapes particularly well in twilight.

Fish are generally farsighted, with the greatest sharpness in the middle field. Fish that live in relatively shallow water can see colors.

The limit of the light perception of the fish is approx. 550m depth. Few specialists can still perceive weak residual light at a depth of 1100 m.


Clark's anemonefish - Amphiprion clarkii

Oriental gurnard - Dactyloptena orientalis


Noise, Perception and Generation: Water has a density about a thousand times higher than air. Sound can therefore travel five times faster under water than in the air: 1500m per second. One problem, however, is that the water is constantly in motion and this creates complex sound reflections - on the surface of the water, on thermoclines with different temperatures or salt concentrations, and on the sea floor. It is therefore particularly difficult to communicate over great distances. Dolphins and whales have solved this problem by constantly varying the pitch and frequency of their signals (more information).

Fish can perceive sounds with their lateral line organ and their inner ears. In many fish there is also a connection between the inner ear and swim bladder, so that vibrations of the bladder are transmitted to the ear.

Fish produce sounds in very different ways. They grind their teeth, they swallow air and expel it through the anal opening of the swim bladder, or they rub thorns and fin rays against each other. In some of these mechanisms, the swim bladder acts as a resonance chamber that amplifies the sounds. Such sounds can be so loud that you can hear them even above water. In some fish, special muscles have developed around the swim bladder that contract quickly and produce a drum sound. Since these muscles are only developed by the males in some species, these tones probably play an essential role in courtship and matching.

If the diver holds his breath in order to hear well at all, the following noises can be heard in the water: clicks and grunts (soldier fish and wrasse), growls (gurnards), knocks (angelfish), quick sounds (catfish), drum sounds (Drummers), toktok lutes (anemonefish), croaks (toad fish).

The anemonefish creates various clicking and chirping noises by rubbing its teeth together. Researchers have found that the sounds are made when the hyoid bone lowers and the jaw closes at the same time. The teeth clash together, transferring energy to the jaws, which radiate the sounds into the water.

Some sounds are very loud - the biggest noisemaker in the reef is the pistol shrimp (Alpheidae, actually shrimp), it is only 3 to 5 cm tall, but can generate a noise of 150 to 200 decibels. That is about the volume of a jet taking off!

  Many fish are remarkably sensitive to olfactory stimuli. Sharks, for example, can still perceive blood in a dilution of 1: 1 million. The olfactory organs are well developed in most fish. The olfactory organs of fish consist of a chamber through which water flows in one direction. The nostrils lie on the muzzle and have a close connection with the olfactory receptors under the skin. Some fish also have bizarre nostrils, such as horn sharks or moray eels. It is believed that the hammerhead shark, for example, with its nostrils that are far apart, can smell "two-dimensionally".

Defense through poisons and bad tasting substances

Red speckled red mullet (Parupeneus heptacanthus)

 Some fish and many molluscs (such as squid), flower animals and sponges can excrete poisons or foul-tasting substances, which deter predators. Other fish, such as the frogfish, are known to attract their prey with chemical compounds.

Recognizing chemical stimuli is also an important survival factor for fish. Chunks of food, even with low concentrations of toxic substances, are recognized and immediately spat out. When attacked by a fish, some soft corals emit warning substances to which they in turn react sensitively.

Conversely, some fish use chemical substances to protect themselves from predators, such as the parrotfish, which secretes a mucous cocoon at night, which obviously forms an odor barrier. Moray eels cannot smell the sleeping parrotfish now.


 Many fish have only a very rudimentary tongue. It looks more like a bulge and consists of connective tissue. Tongues are often toothed. A large part of the fish gulps the food intact up to the stomach. The taste test must therefore be carried out before it is taken into the mouth. In fish, taste buds are therefore not only in the mouth, but also on the lips, in the barbels, on exposed fin rays, in the fins and are distributed over the whole body and head. The catfish, for example, can perceive its food from a distance of 5 m with the taste buds on its barbels and on the body.

Brown = scales / blue = openings of the lateral line organ / pink = muscles / red = nerves

 The most important sense organ of the fish, which apart from them only a few amphibians have is that Lateral line organ (or Lateral line). It is a remote sense of touch and allows you to determine currents and to evaluate water vibrations, such as those caused by prey, swarm companions or sexual partners, to determine who is causing them and where it is. The fish can also feel pressure waves that are thrown back from an obstacle.

Halfway up the flanks of the fish sit a series of special, openwork scales that run from the head to the tail fin. The visible pores lead to a liquid-filled canal running longitudinally behind the scales, in which the actual sensory organs are located. These sensors are sunk in pits and consist of hair cells that generate a nerve signal when the fine hairy appendages are bent. In this way, water movements can be perceived and their prey and predators can be located.

In addition, some fish have converted the sideline system into electroreceptors and even use this modified system for geomagnetic navigation at times. This means that migratory fish can orientate themselves on the magnetic fields of the earth.



It is difficult to orientate yourself in murky water and at night. Some fish therefore produce electric fields in order to "see". With the help of electrical fields, they can not only recognize conspecifics, possible prey and predators, but are even able to differentiate between materials. The electric field deforms where it meets an object with a different electrical conductivity than water, such as a stone or an animal. There are fish (the Nile pike or elephantnose fish, a freshwater fish) that can perceive a distortion of less than 1 percent. The fish can not only capture the object in three dimensions, but also capture the distance and catalog it in terms of shape and electrical properties.

The sideline organ of these fish has been transformed to acquire the ability to electrolocate. The hair sensory cells (see above) were converted, the hairy appendages were lost and the sensors were stored deeper.

Sharks and rays have receptors that sense the low-frequency electric fields (below 50 Hertz) that surround all living things. The organs (Lorenzini ampoules) are located on the front of the head of sharks. They are fine channels filled with a jelly-like substance that end in ampoules. They are connected to the pores in the skin. With these very sensitive organs, sharks and rays can locate prey buried in the sand. The Nilhecht, on the other hand, perceives the high-frequency fields (100 to several 1000 Hertz) that it generates itself and thus orientates itself in its environment.

White border sky-gazers dug in the sand - Uranoscopus sulphureus


Some species of saltwater fish also generate considerable electrical currents: torpedo and electric rays produce pulses of up to 230 volts and over 30 amps. The kidney-shaped organs for generating the electric shocks are located on both sides of the electric ray's head and are usually quite easy to see. These electrical organs evolved from modified eye muscles.

You can use it to stun your prey or scare off enemies. For example, they lie flat on sole, electrify it or at least disorient it and then eat it. Another tactic is to bury yourself in the sand and then poke up when prey is within reach. The electrical charge is released shortly before the attack. Other species of rays have weakly electrical organs in their tail area that send out pulses of different shapes and durations from species to species. These pulses serve less for stunning prey fish and more for orientation.

The sky-gazer lurks, half buried in the ground, for small fish, which he sucks in by opening his mouth. During this process, the organ fires a burst of pulses (up to 50 volts), but this is not enough to stun the prey. The purpose of these discharges has therefore not yet been clarified, but it is assumed that the animal buried in the sand can locate approaching prey in this way.

  The question of whether fish feel pain has long been discussed and cannot be answered clearly. The memory and the ability to learn in fish are poorly developed (a fish caught and thrown in again often goes back to the same line).

Ability of living organisms to actively generate and emit light.

Oxidation of certain phosphors (luciferin from Latin light carrier) under the control of the enzyme luciferase


Many animals emit light (bioluminescence). This ability is particularly widespread among marine animals, where it occurs in almost all groups, from protozoa to marine fish (10-15%). The glow takes place in the cells (intracellularly) in individual light granules and in light tissues (photophores) or organs. Complicated luminous organs therefore often resemble eyes in their structure. That means there is a reflector layer, a lens and even a color filter. Glands can expel a glowing mucus (which has also been observed in sea feathers).

Luminous organs produce cold light (chemiluminescence), which means that over 90% of the energy is converted into light, the rest into heat. The light is generated using chemical processes.

Luminescent bacteria

(Photobacterium, Vibrio)

 Many animals (fish, octopuses, sea squirts) have luminous organs (photophores) but no luminosity of their own. They cultivate luminous bacteria in their luminous cells and can regulate their luminosity by varying the supply of oxygen. Since these bacteria are constantly glowing, they are masked with eyelids or pulled into a black bag. This allows the duration of the glow to be controlled. The amount of bacteria that live in such a luminous organ is enormous. The luminous organs of the lantern fish contain 1010 bacteria per cm2!  The biological significance of glow is only partially known. In some fish the luminous organs are placed under the eyes. With this they illuminate their field of vision. Muscles align the organs so that they can be used like searchlights. In many cases, the luminous organs, for example on barbels and rods (deep sea angler fish) or even inside the mouth, serve to attract prey. Some animals secrete a glowing substance to deter predators, with others touching them triggers a chemical reaction and the whole body lights up.  Most animals, however, use their luminous organs to exchange signals with other members of their species. Luminous patterns in the dark are used to recognize and attract conspecifics, for example for mating or for swarming.

The lantern fish lives in caves during the day. His luminous organs are under his eyes. The organ generates flashes of light. Their frequency depends on the water temperature, time of day and possible predators. Flashes three times per minute when at rest and up to 50 times per minute in case of danger. To switch off the light, he pulls down a lid-like blind. With this flash, prey such as zooplankton and copepods are attracted. In the night he comes out and rises to hunt in higher water layers. Groups of up to 100 animals have been observed.

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Breathing and circulation

  The breathing of the fish involves opening and closing the mouth and putting on and spreading the gill cover. The gills are protected against foreign bodies in the incoming water by gill traps, a sieve system made of thorny appendages. In the case of plankton eaters, these are particularly long and dense because these animals filter their food out of the water at this point.


 In most bony fish, gills are divided into four gill arches. These carry regular, comb-like gill filaments on which the gill lamellae (gill lamellae) are tightly packed. This is where the gas exchange takes place. These lamellae have a huge surface area - for a fish weighing one kilo, they can be 18,000 cm2. This large surface is necessary because of the low oxygen content in the water. The oxygen is taken over by the fish blood and the carbon dioxide is transferred to the water. Fish can absorb 74 percent of the oxygen from the respiratory water through their gills.

This oxygen is now distributed throughout the body through the blood. The heart of the fish does most of the work, but the gill muscles also play an important role. Fish have high blood pressure and a relatively low blood volume.

The fish are cold-blooded animals whose body temperature depends on the surrounding water. Only some species like the tuna, the mako shark and the great white shark can maintain their body temperature a few degrees above the surrounding water.Coral fish can tolerate temperatures of 15-33 °. However, they are very sensitive if they are exposed to extreme temperatures or a rapid change in temperatures for long periods of time. Therefore, their distribution is limited to areas where the water temperatures are around 20 °.

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Swim and buoyancy

rounded - lanceolate

straight - sickle-shaped

forked - notched

ctenoid - cycloid - placoid

 Fish swim with such obvious ease that it is difficult to imagine how difficult it is to move forward through the water. The density and viscosity of the water require special adaptations: streamlined bodies, skin structures, fin and tail shapes, body movements (flicking the fins up and down or undulating movements).

But it is not enough just to swim, the body must not sink at the same time. Most bony fish have therefore developed a swim bladder. This is filled with gas and can be enlarged or reduced as required. This keeps the fish in hydrostatic equilibrium. Many bottom fish have regressed or given up their swim bladders (flatfish, bleached fish, frogfish, many gobies). Cartilaginous fish (sharks and rays) do not have a swim bladder, but instead have a lighter skeleton and an oily liver. The swim bladder also serves as a resonance space to create sounds.

Fish have found various solutions to help the water slide better over their skin surface. The shark has a skin made up of tiny teeth called placoid scales. The body of most bony fish is covered with small bone plates called scales. Along the whole body there are glands in the skin that cover the dandruff with a film of mucus. This mucus also protects against infections, so fish should not be touched underwater. The pattern of fish scales and whether they are pointed or flattened influence the flow resistance.

  In the case of human sharks, the connection between lifestyle and body shape can be seen very well. The gray reef shark has a caudal fin that is significantly longer at the top than at the bottom and pectoral fins that are shorter and more tightly fitting. So he can swim close to the ground without bumping into it.

The blue shark (and other deep sea sharks) has an almost symmetrical, moon-shaped caudal fin, large wing-like pectoral fins and a spindle-shaped body. The symmetrical fins generate more forward thrust than the asymmetrical ones.

Other fish that swim near the bottom, such as the moray eel or the eel, use undulating movements and have developed an elongated, almost eel-shaped body for this purpose. The ray also makes undulating movements, but by hammering the side fins.


Swimming movements eel - mackerel - boxfish - rays

  Fish that live in the corals, on the other hand, have to be able to move as agile as possible, if possible even swim backwards, turn and turn. Speed ​​becomes secondary. The boxfish (and also the puffer fish) is a very slow but maneuverable swimmer. While standing, it can turn its little fins like a "helicopter". The dorsal and anal fins are used to carry out wave-like movements, while the pectoral fins are moved like a propeller. The tail fins are only used for quick escape. Filefish can also stand in place or swim upside down and backwards, which helps them hide.

The longhorn boxfish can hover over a buried worm, blow away the sand and then suck it out with its tubular mouth.

The frogfish move on the reflex principle by ejecting water from its gill openings on the hind legs or by galloping if it moves on the bottom.

Garfish (Belonidae)

Half-beaked pike
Flying fishes
Flying half-beak

 Some fish even escape the water in case of danger and fly or glide up to 50m and at a speed of up to 55km over the water surface. The flying fish use their pectoral fins as wings. They dip their tail fin, which is extended downwards, into the water and with rapid blows (up to 50 times per second) they start running to take off again.


Individual chapters:

Anatomy - sense organs and communication - breathing and circulation - swimming and buoyancy


Individual keywords:

Ampoules - eyes - courtship - barbels - bioluminescence - blood - chemiluminescence - electrical currents - electrical organs - flying - noises - olfactory organ - skin - heart - gills - lateral line - luminous bacteria - Lorenzini - magnetism - maneuverability - pain - tail fin - swim bladder - lateral line organ - Temperature - tongues

End - top Index bony fish / cartilaginous fish

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