General Features Of A Fish

Updated: Jan 23, 2021

Fish Anatomy:

Fish are cold blooded animals. In this blog, we shall have a look at various features of fishes.

Body Shape: The fish’s body is a result of adaptation to survive in its environment which has water about 800 times denser than air. The body shape of fish could range from Torpediform or Fusiform, Ventrally flattened, Ribbon-like, Eel-like, Spheroid, Laterally flattened to Arrow-like. The torpedo shape is perceived as typical for fishes.


Most fishes breathe using gills. Gills are tissues which are thread like, called filaments. Each filament contains a capillary network that provides a large surface area for exchanging oxygen and carbon dioxide. Some fishes have mechanisms to breathe without gills. In some species, cutaneous respiration (cutaneous gas exchange) accounts for 5 to 40 percent of the total respiration and an important mechanism for breathing in species such as mudskippers. Mudskippers are amphibious and live and move about on land for upto several days or could live in stagnant water. There are a number of mechanisms to breathe in eels, such as via skin as seen in anguillid eels or using buccal cavity in electric eels. Catfish could breathe via digestive tract. A number of fish have evolved so-called accessory breathing organs that extract oxygen from air. This feature is specifically useful for fishes which inhabit shallow waters and high chances of low dissolved oxygen.

Operculum (Gill Cover): The operculum of a bony fish is the hard bony flap covering and protecting the gills. Apart from protection, opercula due to their back and forth movement enables water flowing over the gills, allowing the fish to breathe.


Scales refer to small plates or shield forming part of the outer skin of animals, found in fishes as well. There are numerous types of fish scales and they come in various sizes and shapes, to serve as an external layer of protection from environment and predators. However, some fishes do not have scales, rather are protected by a thick layer of mucous. Fish scales could be:

Placoid scales,

Cosmoid scales,

Ganoid scales,

Cycloid scales,

Ctenoid scales.

Placoid scales are bony, spiny projections with an enamel-like covering. Cosmoid scales are similar to placoid scales and consist of two basal layers of bone, a layer of dentine-like cosmine and an outer layer of vitrodentine. Ganoid scales are similar to placoid scales but are covered with a peculiar enamel-like substance called ganoin. Cycloid and Ctenoid scales constitute overlapping fish scales. Cycloid scales are large, thin, round or oval shape and exhibit growth rings. Ctenoid scales have comblike teeth on their overlapping edge and resemble cycloid scales.

Placoid scales are found in sharks and rays, Cosmoid scales are found in lungfishes and some fossil fishes, Ganoid scales found in Bowfish, Gars and the Cycloid, Ctenoid scales are found in most Bony fishes.


Fins are appendages used by the fish to maintain its position, move, steer and stop. Fishes could have different combination of fin types. The types of fins are as follows:

Dorsal Fin: It is an unpaired fin on the back of a fish or whale. Example is the dorsal fin of a shark or killer whale which is tall and triangular. This fin is useful for balancing in water and for sudden movements.

Anal Fin: This is an unpaired fin located on the underside of a fish, posterior to the anus. This is useful to attain stability while swimming.

Caudal Fin: Its the tail fin of the fishes used for propulsion.

Pectoral Fins: Could be defined as a pair of fins situated on either side of the body, right behind a fish’s head, to help fish control the direction of movement during locomotion. They correspond to forelimbs found in vertebrates.

Pelvic Fins: They are a pair of fins on the underside of a fish’s body, attached to the pelvic girdle and helping to control direction.

Adipose Fin: Its a small fleshy fin without rays found behind the dorsal fin and just in front of the caudal fin. Found in fishes such as catfishes, characins, salmon. Experiments conducted by Dr. Thomas E Reimchen and Dr. N.F. Temple suggest that these adipose fins could play a role in increasing swimming efficiency of fishes.

Caudal Keel: Its a lateral rifle found just forward of the tail fin on the caudal peduncle of some type of fast-swimming fish. It provides stability and support to the caudal fin.

Finlets: These are highly specialised fins located between dorsal fin and / or anal fin and the caudal fin. These are non-retractable fins and have been hypothesised to improve swimming performance. Found in some of fishes such as Scombrids, Bichirs.

Bony fishes could be classified based on their fins as ray-finned fishes and lobe-finned fishes. Ray-finned fishes refers to various bony fishes having fins supported by thin bony rays. Lobe-finned fishes are any of various fishes, having paired fins that are rounded and fleshy. Ray-finned fishes include most of the species of fishes as of today.

Caudal Peduncle: It is the narrow portion of the fish body to which the caudal or tail fin is attached. Its the region between the end of the anal fin and the base of the caudal fin. The depth of the caudal peduncle, which is measured at its narrowest point, gives some indication of the power of a fish and the speed at which it can swim.

Vision in Fishes: The eyes in fishes are rounder than mammals as an adaption to water environments. This is required due to refractive index of water and focus is achieved by moving the lens in and out and not reshaping the lens as in mammals.

Nostrils or Nares: The nares or nostrils are the olfactory organ which helps fishes to smell. It isn’t used for respiration. Their nares aren’t directly connected to their lungs, gills or even pharyngeal cavity. The fish have to move rapidly to force odourants past their olfactory bulb, in other words the flow of external water fluid into the nostrils determines the sense of smell.

Mouth and teeth: Fishes have mouths of various sizes, shapes and orientations. These are reflections about what the fishes eat, where they predominantly exist and their behaviour. For instance, predatory fishes generally have the largest mouths generally alongwith large, sharp teeth. The fish mouth could be classified into the following types:

Superior / Supra-terminal: Mouths are oriented upwards and lower jaw is longer than the upper jaw which functions more like a scoop. These fishes mostly feed on insects.

Terminal mouths: this is most common and mouth points straight forward: In this case, jaws are of the same length. Most of the fishes have this mouth type and are omnivores.

Inferior or sub-terminal mouths are turned downward: In this case, the lower jaw is shorter than the upper jaw. These fishes live on algae, invertebrates such as snails.

Protrusible Mouth: This feature seen in all fishes, enables them to extend their reach to the prey. It allows fishes to create a vacuum when they open their mouth to get hold of the prey. Sucker mouths common in fishes with inferior mouth as also protusible which allows them to extend their reach to prey. Some species use their sucker mouth type to combat water currents. Fishes like pleco use their sucker mouth to get algae off driftwood or rocks.

Elongated mouth: A greatly elongated mouth is an adaptation to poke into small crevices and holes to find food or access buried food.

Beak mouth: The mouth is sturdy enough to crush hard shells of invertebrates. Such a mouth is known as a rostrum. This kind of mouth is not very common. Examples of fishes possessing beak mouth are Pufferfish, Parrotfish, Octopus, Squid.

Lateral line: Its a system of sense organs and can detect vibrations, movements and water pressure. Its also known as laterals system. Lateral lines serve an important role in schooling behaivour, predation and orientation. The sensory ability is achieved via modified epithelial cells, known as hair cells which respond to displacement caused by motion and transduce these signals into electrical impulses via excitatory synapses. Lateral lines are usually visible as faint line of pores running lengthwise down each side, from the vicinity of gill covers to the base of the tail.

Vent: The vent is the external opening to digestive urinary and reproductive tracts. In most fish, it is immediately in front of the anal fish.

Internally, the fish comprises of spine, spinal cord, brain, swim / air bladder, kidney, stomach, intestines, pyloric caeca, liver, heart, reproductive organs and muscles.

Understanding various features of fish is a vast topic and this blog was just a basic introduction.

Recent Posts

See All