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 event.

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 openings.

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 freshwater.

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 smell.

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 tetrapods:

Fins Picture
  • Fins: They have various small and large fins located both on their dorsal and ventral sides. The principal fins include the caudal fin (tail), dorsal fins, anal fins, pectoral fins and pelvic fins.
  • Fish Scales Image Scales: Their bodies are covered in tiny to large scales that originate from the skin or mesoderm. The structure of the scales may be similar to teeth. But, there are various species that do not have scales.
  • Body Shape: fishes have a fusiform body shape that resembles a torpedo. This unique body shape is an important characteristic of these creatures.
  • Gills and Gill Covers: They are the only animals, apart from certain lizards, that breathe through gills at all stages of their lives. The gills have gill covers that protect them from any outside damage.
  • Fish Lateral Line Photo
  • Lateral Line: This important sensory organ of these animals is another unique feature.

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
  • Freshwater Ponds and Lakes: The freshwater habitats are inhabited by numerous small and large species. These water bodies hold a major portion of the total fish population.
  • Brackish Water Fish Image
  • Brackish Water: This is a type of aquatic habitat where the water is more saline than fresh water but the salinity is not as high as saltwater. Various fishes such as the guppy, sailfin molly, common toadfish and pufferfish are known to live in brackish waters.
  • The Open Ocean: There are countless saltwater fish species that live in the open oceans. It is one of the most significant types of habitat for the animals belonging to this group.
  • Water Surface Fish Photo
  • The Water Surface: Many small species live at the water surface. Larger fishes like bass and bluegill often come to the surface to feed as they can easily find small fishes and insects.
  • Coastal Waters: They can be found at all depths of the coastal water, but generally dwell at the bottom. Majority of the saltwater angler fish live in coastal waters while many marine species seasonally migrate to these areas.
  • Coral Reef: Despite the fact that coral reefs make up less that 1% of the total surface area of world oceans, they are inhabited by around 25% of the marine fish species.
  • Coral Reef Fish Picture
  • Deep Waters: Members of certain genera live very deep in the oceans where there is little or no sunlight. The water temperature remains relatively constant in the deep waters while there is also little current.

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 purposes.

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 oxygen.

  • 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 covers.
  • 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.

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