Facts, Characteristics, Anatomy and Pictures


What is a Fish

Fish Picture

All the gill-bearing aquatic craniates, belonging to the paraphyletic group, that lack limbs and digits are

collectively known as fish. Members of this group breathe through gills at all stages of their lives and

they have fins instead of limbs. Unlike biological groupings like mammals and birds, fish is not a clade. It

is one paraphyletic collection of various taxa including the extant lampreys, hagfish, lungfish along with

all the bony and cartilaginous fishes as well as a number of extinct related groups. In fact, coelacanths

and lungfish are more closely related to tetrapods than they are to other fishes like sharks and ray-

finned fish.

Evolution of Fish

The term “fish” does not represent one monophyletic group, so the evolution of fish is not a single


Evolution of Fish Image

According to fossil record, a group of small, armored and jawless creatures called the Ostracoderms

represent the earliest fishes. At present most jawless species have faced extinction. One extant species

named Lamprey bear resemblance to the ancient pre-jawed fishes. The Placodermi fossils first showed

evidence of jaws in fishes.

The earlier species are believed to have evolved from an aquatic creature resembling the coral-like Sea

squirt. This theory was drawn from the fact that the larva of the coral-like Sea squirt bears important

resemblance to primitive fishes. The earlier fishes may or may not have kept their larval form after

reaching adulthood. More research is necessary to understand the entire history of their evolution.

Fish Classification

The term “fish” is paraphyletic as all the animals included in it have the same ancestry and it excludes all the animals in the superclass tetrapod (the mammals, birds, reptiles and amphibians). Due to this reason, fish is not regarded as a proper group in systematic biology. Pisces (also ichthyes), the traditional term for these aquatic creatures is considered to be a typological rather than a phylogenetic classification. There are various systems used for classifying the animals belonging to this group. In the below mentioned system, fish is considered a superclass consisting of three extant classes and two extinct classes:

Class Agnatha Photo

Class Agnatha (includes the jawless fish)

  • Subclass Ostracodermi (the extinct armored jawless fish)
  • Subclass Cyclostomata (lampreys and hagfish)

Class Chondrichthyes Picture

Class Chondrichthyes (includes the cartilaginous fish)

  • Subclass Elasmobranchii (rays and sharks)
  • Subclass Holocephali (the chimaeras and their extinct relatives)

Class Osteichthyes (includes the bony fish)

  • Subclass Sarcopterygii (the fleshy finned fishes like lobefins and lungfishes, ancestors of tetrapods)
  • Subclass Actinopterygii (the ray finned fishes) _ This subclass is further classified into various orders:
  • Acipenseriformes
  • Amiiformes
  • Semionotiformes
  • Salmoniformes
  • Clupeiformes
  • Anguilliformes
  • Lophiiformes
  • Siluriformes
  • Class Osteichthyes Photo

  • Cypriniformes
  • Elopiformes
  • Atheriniformes
  • Percopsiformes
  • Batrachoidiformes
  • Lampridiformes
  • Myctophiformes
  • Gadiformes
  • Gasterosteiformes
  • Zeiformes
  • Pleuronectiformes
  • Tetraodontiformes
  • Perciformes

Class Acanthodii (the extinct spiny sharks, occasionally classified as bony fishes)

Class Placodermi (the extinct armored fishes)

Fish Anatomy

Fish Skeletal System Picture

Skeletal System

Their skeleton can be made of either bone (bony fishes) or cartilage (cartilaginous fishes). Bony fin rays

constitute the fins, a main characteristic feature of the fish skeletal system. Apart from the caudal fin,

the fins are not directly connected with the spine. Instead they are mainly supported by muscles. The

ribs of these animals are attached to the spine.

Respiratory System

Fish Respiratory System Image

Most fish species use the gills located on both sides of their pharynx for breathing by exchanging gases.

The gills are formed of numerous threadlike “filaments” with each filament containing of one capillary

network which provides wide surface area for carbon dioxide and oxygen exchange. These animals draw

oxygen-rich water through mouth and pump it over the gills to complete the gas exchange procedure.

In some species, the capillary blood flows to the water in opposite direction, leading to countercurrent

exchange. The oxygen-poor water is pushed by the gills through the openings on both sides of pharynx.

Some fishes, such as the lampreys and sharks have multiple gill openings. But, the bony fish species have

one gill opening on both sides. There is one protective bony cover known as operculum over their gill


Circulatory System

Fish Circulatory System Photo

All animals belonging to this group have closed-loop circulatory system. This means that the blood

is pumped by the heart through a single loop route. Their heart comprises of four parts _ two heart

chambers along with one entrance and one exit. The thin-walled sac named sinus venosus is the first

part of the heart that collects the blood from the veins of the creatures and then allows it to flow

into the large muscular atrium, which is the second heart chamber. The atrium functions as a one

way antechamber, directing the blood flow into the third part known as the ventricle. The ventricle,

another muscular thick-walled chamber pumps blood to the bulbus arteriosus (the fourth part) before

eliminating it from the heart. There is a connection between the bulbus arteriosus and the aorta for the

blood to flow to the gills and get oxygenated.

Fish Digestive System Picture

Digestive System

Fishes break down the food in their esophagus after ingesting it through the mouth. The digestive

procedure continues in their stomach. The digestive system of many species has the finger-shaped

pouches named pyloric caeca that help in digestion by absorbing various nutrients and secreting

digestive enzymes. Organs like the pancreas and liver assist in the digestion process by adding various

enzymes and chemicals to the food while it passes through the creature’s digestive tract. The process of

nutrient absorption and digestion completes in the intestine.

Excretory System

Like many other aquatic animals, majority of these species eliminate the nitrogenous wastes from their

bodies in the form of ammonia. Some wastes are diffused through their gills while the kidneys filter the

blood wastes.

The saltwater species often lose water due to osmosis. In these fishes, the kidneys are responsible for

returning the water to their bodies. The freshwater fishes undergo the reverse procedure as they gain

water osmotically. They generally excrete dilute urine. Some species show special adaptive feature

with their kidneys being able to change the function, allowing the animals to move from saltwater to


Central Nervous System

Fish Central Nervous System Image

The relative size of their brain to the body is much smaller compared to most other vertebrates. But,

certain species, such as the mormyrids and sharks, have large brains with the body-and-brain relative

size being as massive as marsupials and birds. Fish brains have several regions:

The Forebrain

The forebrain is divided into the two following parts:

Forebrain Photo

Olfactory lobes: The two olfactory lobes are located at the front side of the brain. These are responsible

for receiving and processing signals through the pair of olfactory nerves from the nostrils. A number of

fishes including the catfish, sharks and hagfish have very large olfactory lobes as they hunt mainly by


Telencephalon: This two-lobed structure is located at the back of the olfactory lobes. The telencephalon

in these animals is the structural equivalent of the cerebrum in advance vertebrates. In fishes, this part

is mainly concerned with olfaction.

The Midbrain

The midbrain (mesencephalon) consists of two optic lobes that allow fishes to see. Species like the

cichlids and rainbow trout have very large optic lobes as they hunt principally by sight.

The Diencephalon

The diencephalon connects the forebrain and the midbrain while performing the hormonal and

homeostatic functions. The pineal body is located above the diencephalon. Its functions include

detecting lights, maintaining circadian rhythms and controlling color changes.

The Hindbrain

The hindbrain (metencephalon) is mainly involved in maintaining balance and swimming. It contains

the single-lobed cerebellum which is generally the largest part of the fish brain. The cerebella are

considerably small in hagfish and lampreys while mormyrids have very large cerebella which are

believed to influence their electrical senses.

The Brain Stem

It is also known as the myelencephalon and constitutes the posterior part of the brain. It is primarily

concerned with governing the respiration and osmoregulation. The brain stem also controls certain body

organs and muscles, especially in the bony fishes.

Sensory Organs

Most species in this group have well-developed sensory organs. They have eyes, ears and nose. Their

retinas usually have both cone cell and rod cell for photopic and scotopic vision. Almost all the daylight

species have advanced color vision that is almost as strong as in humans. Some species are able to see

polarized light while others see ultraviolet light.

Fish Sensory Organs Picture

Chemoreceptors present in many fishes are responsible for their extraordinary senses of smell and

taste. But, many species have weak hearing abilities despite having ears. Majority of these creatures

have a lateral line system consisting of numerous sensory receptors. Some fishes like sharks and

catfish have special organs to help them in detecting very weak electric currents. The South American

electric fish from the Gymnotiforme group are capable of producing weak electric currents for social

communication and navigation.

Characteristics of Fish

All animals belonging to this group have some characteristic features that distinguish them from other


Fins Picture

Fish Habitat

They can adapt to a wide range of climatic conditions (tropical, temperate, polar) and can be found in

almost all types of water bodies.

Freshwater Ponds and Lakes Fish Picture

Fish Reproduction

More than 97% of all fishes are oviparous, which means the development of their eggs occurs outside

the female’s body. The oviparous species include goldfish, salmon, tuna, cichlids and eels. In most of

these fishes, the eggs are fertilized externally when both the female and male fish shed gametes into the

water after the eggs are laid.

Fish Reproduction Image

However, there are certain species that practice internal fertilization. In these species, the pelvic fins of

the males have been modified to form claspers that help in mating. The eggs of the ovoviviparous fishes

develop within the body of the females after being fertilized. However, they depend mainly on the egg

yolk for nourishment. The growth of each embryo occurs within its egg. Examples of some ovoviviparous

species include angel sharks, guppies and coelacanths.

Marine fishes are known to lay high number of eggs that are usually released in the open water. The

average diameter of these eggs is 1 mm (0.039 in).

Fish Life Cycle

The hatchlings of the oviparous fishes are known as larvae. They have poorly formed bodies and carry

large yolk sacs for nourishment. The appearance of the hatchlings is very different from that of the

juveniles and adults. The larval period is relatively short in oviparous fishes with the larvae developing

rapidly to change their structure and appearance to become juveniles. This process is known as

metamorphosis. During this phase, the larva starts feeding on zooplankton instead of getting their

necessary nutrition from the yolk sac. Many larvae starve and die during this stage due to an inadequate

zooplankton density.

Fish Diseases and Immune System

Like all other animals, fishes can suffer from various diseases and parasites. They have a number of

specific and non-specific defenses against these diseases. The non-specific defenses include their skin,

scales and the mucus layer on the skin secreted by their epidermis which traps harmful microorganisms

and prevents their growth. If some pathogen enters the fish’s body despite all these defenses, it triggers

an inflammatory response and the blood flow to the affected area increase so that the white blood cells

can destroy the pathogens.

Fish Diseases and Immune System Picture

Specific defenses refer to the specialized responses to some specific pathogens recognized by the

immune system of the fishes. At present, various vaccines are being used in ornamental fishes and

aquaculture. For example, vaccines are commonly used for preventing koi herpes in koi and furunculosis

(skin disease) in farmed salmon. Whirling disease, VHS and ich are some of the commercially important

diseases in these animals. Fishes living in the tropical regions often suffer from diseases like dropsy,

lymphocystis, internal fungus and various bacterial infections.

Importance of Fish in Human Lives

Fishes have great importance in the daily lives of humans. The edible fishes are one of the most

important sources of nutrition for humans while they are also used for various medicinal and other


Fish Edibility and Nutritional Facts

Numerous fresh water and saltwater species are edible and are an important part of human diet. All

edible fishes are rich in proteins, vitamins, minerals and many other essential nutrients. Recent research

shows that the minerals and other nutrients present in fishes, especially the omega-3 fatty acids, are

beneficial for human brain development and cardiac health.

Fish Edibility and Nutritional Facts Image

Salmon, tilapia, tuna, haddock, trout, mackerel, cod and sardine are examples of edible fishes and are

highly popular all over the world.


Fishing is a popular activity that involves catching fishes for recreational purposes or for commercial

and edible reasons. Many people in the less-developed countries earn their livelihood by fishing. It

is counted among the most popular activities in countries like USA and UK. Various techniques, like

netting, spearing, hand-gathering and angling are used for catching fishes. The tools commonly used for

this purpose include fishing, tackles, lures, rods and nets.

Fishing Photo

Game fishing involves catching large fishes with angles solely for recreational purposes. The game fishes

are sometimes eaten after they are caught, but at present, anglers are practicing catch and release for

improving the population of these creatures.

Fish Adaptations

All fishes do not have similar appearance and the same adaptive features. But all the species are

adapted to survive in their natural watery habitat:

Fish Adaptations Picture

General Adaptive Features

  • Their bodies are partly supported by the external up-thrust of water because water is

    approximately 770 to 1000 times denser compared to air. The structure and shape of these

    animals is internally supported by their skeleton (scull, backbone, ribs and fins).

  • They have special adaptive features to adjust to their visual environments. For example, the

    eyes of the fishes inhabiting deep sea habitats have adapted to suit their dark environment.

Fish Locomotion Adaptations Image

Locomotive Adaptations

  • Most fishes have the same basic body shape that helps them to locomote under water. Their

    front parts are pointed with a bulky middle and a tapered tail.

  • They move with the help of their fins. The tails help them to move forward rapidly while the

    dorsal and anal fins help to maintain the lateral stability. The pectoral fins are located behind

    the operculum to assist in slow turning and hovering. Deep water fishes often have larger pelvic

    fins compared to the open water species so that they can rest while swimming.

Respiratory Adaptations

Fish Respiratory Adaptations Photo

Water contains various gases, including oxygen and carbon dioxide, in their dissolved form. So, fish

respiratory system has some special adaptive features to allow them to absorb higher amounts of


  • Their gills have a wider surface area with many filaments lined along with the blood vessels near

    the surface.

  • The water enters their bodies from only a single direction with its movement being continuous.

    It reaches the gill filaments after entering the body through mouth and exits through the gill


  • The functioning of the gills is carried out on the “counter current” principle.

Adaptation of the Sensory Organs

  • They generally have a lateral line that runs along both sides of the body and helps them to

    detect gentle vibrations and currents as well as sense the motion and location of their prey or

    any other nearby animal.

  • All species have touch senses under their scales that help them in detecting water vibrations.
  • Their nostrils enable them of chemosensing.

What Do Fish Eat?

Food of Fish Picture

The diet generally varies with the type and species of the fish. These animals can be carnivorous,

herbivorous or scavengers. They are known to eat anything from plankton to decomposing animals as

well as smaller fishes and other aquatic creatures.

As Pets

The beautiful marine species and certain freshwater ones are very popular as pets throughout

the world. The coral reef species are most admired as aquarium fishes because of their attractive

colorations. They should be kept in large tanks where the water temperature and salinity should be

maintained to resemble their natural habitat.

As Pets Picture

Other Benefits

Fish is the source of various other edible and medicinal products are very useful in the daily lives of

humans. These include fish oil, fish sauce, fish glue and fish emulsion. Fish oil is extracted from the

tissue of various oily fish. It has useful medicinal properties that make it highly beneficial for the

management of diseases such as cancer, skin problems (such as acne) and different mental conditions.

The rich omega-3 fatty acid contents of this oil make it beneficial for weight loss. Fish sauce is produced

by fermenting certain fish and is used in various cuisines.

Fish Conservation

Fish Conservation Image

The IUCN Red List enlisted around 1,173 species as threatened with extinction in the year 2006. These

include the Devil’s Hole pupfish, Atlantic cod, the great white shark and coelacanths. Many more species

have become critically endangered or even extinct. The principal threats to their existence include over

fishing and habitat destruction. It is comparatively difficult to study the fish populations than other

animals and plants due to the fact that they live under water. As a result, there is a lack of information

regarding the populations of various fish species.


Sharing is caring!

Leave a Reply