What are Fish adaptations?

In this post, we know a little about fish adaptations and how these animals were so successful during the evolution of animals.

What are Fish adaptations?

Fish are a group that have an immense variety of physiological, morphological and behavioural adaptations. These adaptations took place over thousands of years and helped several species survive in the face of environmental changes.

Fish are a group of vertebrate animals found in aquatic environments. To survive in these places, these animals need to be adapted to locomotion in the aquatic environment and, for that, it has ways to overcome the resistance of friction and the action of gravity, and ensure an efficient oxygen uptake from the environment. Below, we list some of the main adaptations of fish to the aquatic environment.


Most fish have a fusiform body, that is, the ends are more tapered than the centre. This hydrodynamic shape facilitates movement in the environment, as it reduces friction with the water. 


Fish are covered with skin and in a slimy secretion, called mucus. Their skin has two layers: epidermis, with an ectodermal origin, and the dermis, with mesodermal origin. In the dermis, scales form, and it is

where the venom glands, mucous membranes, electrical organs, bioluminescent structures (photophores), sensory receptors, and pigments are found.

Mucous (glandular) cells allow chemical communication between fish, reduce the body’s friction with water and also protect against fungi, parasites and in some cases, predation. Hagfish, for example, when threatened, secrete very viscous and thick mucus, which often increases in volume in contact with water. Some lungfish (Lepidosiren and Protopterus) build a cocoon with mucus and mud before the stream dries up to prevent dryness. In the reproductive period, when salmon return to fresh water and eel to the sea, they increase the production of mucus and the skin becomes thicker, this allows reducing energy expenditure with osmoregulation.  

Mouth and muzzle

The position, size, shape of the mouth and types of teeth give great clues about the habits, way of catching prey, and feeding of a fish. The presence of sensory barbels below the mouth is a strong indication that the fish “sniffs” their food by moving over the substrate.

A large mouth that opens while the fish swims is typical of filter feeders such as the whale shark. Many of these filter feeders have long and numerous gill tracks, acting as a sieve to retain food.

Regarding position, the mouth can be terminal, sub-terminal, inferior or superior. Fish with mouths in an inferior position usually feed on what exists below, near the bottom; they are often bentophagous; some have strong teeth and pharyngeal agents for crushing crustaceans and molluscs. Fish with a terminal mouth usually feed on the water column. 


Fish have a wide variety of adaptations that help optimize their vision. These adaptations include changes in size, shape, position, orientation,

eye protection mechanisms, and even the types and amounts of visual pigments and cells.

The absence of functional eyes also occurs in a converging way in hagfish (Myxinidae) and several fishes restricted to submerged caves. Some species of rays (Rajidae) have a pupillary operculum, a membrane that acts as a curtain, allowing to regulate the entry of light into the pupil. According to some authors, under low lighting conditions, they improve focus, visual resolution and focus. As many of these species have a habit of burying in the sand possibly protects the eyes against friction with the sediment.

Also functioning as a protection mechanism, many sharks (Carcharhiniformes) have nictitating membranes, which act as eyelids, covering the eyes when they are attacking prey or when impacted. 


Fish do not have arms and legs, but fins, which are responsible for locomotion (caudal fins, in most fish), orientation (pectoral and pelvic fins), and stability in movement (dorsal and anal fins). 

Lateral Line

This line is made up of several openings along the entire side of the fish’s body. The water penetrates through them and, through the sensory cells present there, the fish can detect movement and other signals. 


Fish can stay at different depths. In bony fish, flotation is assisted by the swim bladder, a gas bag that expands or retracts due to the pressure of water on the fish, making it less or more dense than the environment. As cartilaginous fish do not have a swim bladder, a reserve of oil in the liver of these animals plays this role in buoyancy. 


Osmoregulation is the control of salt concentrations in tissues to keep the body functioning regularly. It is important to prevent freshwater fish from continuously absorbing water and saltwater fish from losing it to the environment. In the case of marine fish, they drink seawater and eliminate excess salt through the gills and concentrated urine. 

Fish adaptations to temperature variations

When there are sudden increases in temperature in aquatic environments, some fish, such as common carp, can raise their respiratory and heart rate, which occurs due to the increase in their metabolism. The increase in heart rate is essential to offset the increase in metabolic rates in general.

Fish can also reorganize cell metabolism. When exposed to temperatures different from those they are used to, they use different metabolic pathways, such as changes in enzyme synthesis, the concentration of substrate and/or products of metabolic pathways and modulators of enzymatic reactions. Synaptic functions are extremely sensitive to changes in temperature. The synapse is considered the main adaptive area of ​​the nervous system.

When the water temperature decreases, fish tend to reduce their metabolic rates, allowing an economy of body reserves, an important characteristic since in places with low temperature there is potential reduced availability of food. American eels, for example, in environments with low temperatures (water below 8˚C) stop feeding and bury in the mud. Most fish species, however, do not bury themselves, just stay quiet to reduce energy expenditure.

The threat to life at low temperatures is not just the cold, but also the impossibility of preventing ice from forming in the tissues. The bodies of animals are composed of large amounts of water, and the formation of ice crystals can be lethal as they disturb the balance between the external and internal fluids of the cells, resulting in cellular shrinkage and irreversible damage to the cells. tissues affected.


In this post, we learnt a little about fish adaptations and how these animals were so successful during the evolution of animals.

If you have any thoughts or doubts, feel free to drop us in a comment below!

Frequently Asked Questions (FAQs): Fish adaptations 

What are the main characteristics of fish?

Fish are aquatic animals that have fins and respiration, in most cases, gills. Generally, in this group of animals, they are ectothermic, typically have a fusiform body, and their breathing occurs through the gills. 

How do fish move around in the water?

The locomotion of these vertebrate animals occurs using the fins and wave movements of the body caused by the contraction of muscles. The hydrodynamic shape and the presence of scales and mucus are also factors that favour the locomotion of these organisms. 

What is the fish type of circulation?

The circulatory system of fish is a simple system, that is, the blood passes only once per cycle in their heart, and it is also a closed system, as the blood only passes through the vessels and has no other cavities. Fish have a heart divided into two cavities, an atrium and a ventricle.

How is the fish body prepared for the aquatic environment? 

Most fish breathe through gills, move around using their fins, reproduce by laying eggs in the water, and their bodies are covered with protective scales.

How is the fish skin?

Fish have their bodies covered with scales. But there are two different layers of the skin under the scales. The surface of the skin is called the epidermis and the internal one is called the dermis, They also have glands that produce mucus with the function of protecting the body and facilitating movement in the water.

What are fish fins made of?

Bony fish fins are composed of spines or rays that protrude from their body. They are covered with dermis and epidermis forming a paddle-like structure.


Fernald, R. D. (1988). Aquatic adaptations in fish eyes. In Sensory biology of aquatic animals (pp. 435-466). Springer, New York, NY.

Wilkie, M. P., & Wood, C. M. (1996). The adaptations of fish to extremely alkaline environments. Comparative Biochemistry and Physiology Part B: Biochemistry and Molecular Biology, 113(4), 665-673.

Locket, N. A. (1977). Adaptations to the deep-sea environment. In The visual system in vertebrates (pp. 67-192). Springer, Berlin, Heidelberg.

Poulson, T. L. (2001). Adaptations of cave fishes with some comparisons to deep-sea fishes. Environmental Biology of Fishes, 62(1), 345-364.