Fish 101: Introduction to Fishes


Fish in all corners of the globe vary in size, colors, and species, and they are found in every ocean, lake, river, and stream.

Most fish prefer to consume plankton in the water (depending on size), insects, and smaller fishes.

Fish possess gills by the sides of their heads, allowing them to breathe underwater. The fish return to the water surface to get air at varying intervals.

Today, many people keep all kinds of fish in tanks and ponds due to the bright colors and the fact that fish are incredibly peaceful animals.

Scientific Classification



The word fish is from the Proto-Germanic language in English and other Germanic languages (German Fisch; Gothic fisks). It is also related to the Latin Piscis and Old Irish īasc, but the exact origin is unknown.


The fish, as a vertebrate, evolved as the sister of the Tunicata. Tetrapods are originated deep within the fish community.

They share the features of fish, which include possessing vertebrae and a skull. A small, jawless, armored fish known as ostracoderms is described as ‘early fish’ from the fossil record.

Jawless fish lineages are extinct for the most part. The lampreys will approximate ancient pre-jawed fish, which are existing clans.

The first jaws are located in Placodermi’s fossils, and they have to modify the oral surfaces of their jaw plates to serve the teeth’ definite purposes.

However, they lacked distinct teeth. The diversity of jawed vertebrates may indicate a jawed mouth’s evolutionary advantage.

The value of a hinged jaw that results in a greater biting force, better respiration, or a combination of factors is unclear.

Fish may have originated from a creature identical to a coral-like sea squirt whose larvae are similar to a primitive fish in many ways.

The larval type may have been preserved by the first descendants of fish into adulthood (as some sea squirts do today), but perhaps the opposite is the case.

Anatomy and Physiology


Most fish exchange gases using gills on either side of the pharynx. Gills consist of threadlike materials called filaments.

A capillary network is included in each filament, which provides a wide surface area to exchange oxygen and carbon dioxide.

Fish exchange gases by soaking up water rich in oxygen through their mouths and pumping it over their gills.

The blood flows opposite to the water in certain fish and induces countercurrent exchange.

The gills force oxygen-poor water out via holes on the sides of the pharynx. Some fish possess several gill openings, such as sharks and lampreys.

Nonetheless, bony fish on either side have a single gill opening, and it is hidden under a protective bony shield, called a speculum.

Juvenile bichirs have external gills, a very primitive trait they share with larval amphibians.


Fish from multiple groups may live outside the water for prolonged periods. Amphibious fish such as the mudskipper can live and travel on land for several days.

They can also survive in stagnant or otherwise oxygen-depleted water. Such fish can breathe air via several mechanisms. For example, Anguillid eels’ skin can directly absorb oxygen.

The electric eel’s buccal cavity can breathe air—catfish of the Loricariidae, Callichthyidae, and Scolopacidae families absorb air through their digestive tracts.

Lungfish have paired lungs close to tetrapods, except for the Australian lungfish and bichirs. They tend to go to the water’s surface to gulp fresh air through the mouth and pass them through the gills.

Gar and bowfin have a vascularized, equally working swim bladder. The loaches, trahiras, and catfish can breathe by moving air through the gut.

Mudskippers survive by consuming oxygen through their skin. Breathing air is mainly used for fish that inhabit shallow, seasonally variable waters where the concentration of oxygen in the water can decrease seasonally.

Fish that rely exclusively on dissolved oxygen, such as perch and cichlids, quickly suffocate, whereas air-breathes live for much longer in some water cases is nothing more than wet mud.

At the most extreme, some air-breathing fish will live without water for weeks in damp caves, entering a state of estivation (summer hibernation) before returning to the water.


Fish have a closed-loop circulatory system, and the heart pumps blood around the body in a continuous loop and consists of four parts in most fish: two chambers, an entrance, and an exit.

The first section is the sinus venosus, a thin-walled sac that collects blood from the fish’s veins before allowing the atrium, a large muscular space, to carry blood to the second part.

The atrium functions as an antechamber in one direction, sending blood to the third component, called the ventricle.

Another thick-walled muscular chamber is the ventricle, which pumps blood through the fourth part, bulbusarteriosus, a wide channel, and then out of the heart.

The bulbusarteriosus is bind to the aorta, from which blood flows for oxygenation to the gills.


Fish’s jaw permits them to consume various food, including plants and other organisms. The esophagus is where food consumed by fish is broken down and taken in through the mouth.

Food is digested further in the stomach and stored in finger-shaped pouches called pyloric caeca, which secrete digestive enzymes and absorb nutrients.

When the food passes through the digestive tract, organs such as the liver and pancreas add enzymes and complex chemicals. The bowel completes the digestion and nutrient absorption process.


Like many marine animals, fishes release their nitrogenous waste as ammonia, and some of the waste is diffused via the gills.

The kidneys are responsible for flushing out blood waste. Saltwater fish tend to lose water because of osmosis, and their kidneys reimburse water into the body.

The opposite occurs in freshwater fish, which appear to gain water osmotically, and diluted urine is excreted through their kidneys.

Some fish have kidneys that differ in function and are incredibly adaptive, allowing them to switch from freshwater to saltwater.

Sense organ

Most fish possess highly developed sense organs. Nearly all daytime fish have color vision that is at least as strong as a human’s.

There are also chemoreceptors in many fish responsible for the exceptional senses of taste and smell. Many fish can not hear very well even though they have ears.

Most fish have sensitive receptors that shape the lateral line system, which detects gentle currents and vibrations, and it also senses the motion of nearby fish and prey.

Some fish, such as catfish and sharks, have Lorenzini ampoules, electroreceptors that detect millivolt-ordered weak electric currents.

Other fish, such as Gymnotiformes, can produce weak electric currents used for navigation and communication.


Vision is an essential sensory system for most species of fish. Fish eyes resemble terrestrial vertebrates such as birds and mammals, but they have a more circular lens.

In general, their retinas have rods and cones (for scotopic and photopic vision), and most species’ color vision is present.

Some fish can see ultraviolet light, while others can see polarized light. The lamprey has well-developed eyes among jawless fish, whereas the hagfish has only rudimentary eyespots.

The fish’s vision indicates adaptation to their visual environment, including deep-sea fish, having eyes suitable for the dark environment.


New research has broadened preconceptions about fish’s cognitive capabilities. For instance, manta rays have exhibited actions linked to self-awareness in mirror test cases.

Individual rays are involved in contingency research, placing them in front of a mirror.

This features repetitive activities aimed at checking if the behavior of their reflection imitates their body movement.

Wrasses passed the mirror test in a scientific analysis conducted in 2018. Cases of tool use have also been found in archerfish and Atlantic cod, especially in the Choerodon family.

Muscular system

Many fish move by contracting the paired sets of muscles on either side of their backbone. These contractions form S-shaped curves that move down the body.

The backward force is applied to the water as each turn hits the back fin, which pushes the fish forward, following the fins.

The fish’s fins act like an airplane’s flaps and increase the tail’s surface area, thereby improving speed.

The amount of pressure from the water reduces the streamlined body of the fish. Fish must account for the difference or sink as their body tissue is denser than water.

Bony fish have an internal organ called a swimming bladder that adjusts their buoyancy by gas manipulation.


Testicles and ovaries are among the reproductive organs of fish. Gonads are paired organs of similar size in most animals, partially or entirely fused. Several secondary organs can also improve reproductive fitness.

The structure of teleosts testes reviews that there are two forms in terms of spermatogonia distribution.

The spermatogonia occur all along the seminiferous tubule, while they are restricted to the distal portion of these structures in theriomorphic fish.

Fish may exhibit cystic or semi-cystic spermatogenesis regarding germ cells’ release process in cysts to the seminiferous tubules lumen.

There are three types of fish ovaries: gyno-like, secondary gymno-like, and cyst ovarian. The oocytes are released directly into the coelomic cavity first and then enter the ostium.

They are then passed through the oviduct and extracted. Ova are shed into the coelom by secondary gymnovarian ovaries, from which they go directly into the oviduct.


Several fish species can create sounds by rubbing or grinding their bones together. These sounds produced by bone-on-bone interactions are called ‘stridulatory sounds.’An instance of this is seen in Haemulonflavolineatum.

A species generally referred to as the ‘French grunt fish’ makes a grunting noise by grinding its teeth together.

This action mainly occurs when ‘H.Flavolineatum’ is in situations of distress. The grunts produced by this fish species have about 700 Hz and last approximately 47 milliseconds.

The H. Flavolineatum does not emit sounds with frequencies above 1000 Hz and does not detect sounds with frequencies above 1050 Hz.

In a study conducted by Oliveira et al. (2014), the long snout seahorse was recorded, producing two sound categories; ‘clicks’ and ‘growls.’

Sounds emitted by the H. Reidi are achieved by rubbing their coronet bone over their neurocranium’s grooved section.

‘Clicking’ sounds were mainly created during courtship and feeding, and the click frequencies were within 50 Hz-800 Hz. Reidi experienced unpleasant conditions, such as researchers’ handling.

The ‘growl’ sounds consist of a sequence of sound bursts produced simultaneously as body movements.


Fish suffer from diseases and parasites, much like other species, and they have some protections to combat infections.

The skin, scales, and mucus layer secreted by the epidermis traps and inhibits microorganisms’ growth are non-specific defenses.

Fish may produce an inflammatory response that increases blood flow to the infected area, delivering white blood cells that try to kill pathogens if pathogens penetrate these defenses.

Specific defenses that respond to particular pathogens recognized by the fish’s body include an immune response.

Vaccines such as vaccines against furunculosis in farmed salmon and koi herpes virus in koi have been commonly used in aquaculture and ornamental fish in recent years. Some species use cleaner fish to eradicate external parasites.

Bluestreak cleaner wrasses of the Labroides genus found on coral reefs in the Indian and Pacific oceans are known for these.

Immune system

The immune organs of fish species differ. The proper lymphoid organs are absent in jawless fish (lampreys and hagfish).

These fish rely on regions of lymphoid tissue inside other organs to produce immune cells.

For example, erythrocytes, macrophages, and plasma cells are produced in the anterior kidney and some gut areas (where granulocytes mature). They all resemble primitive bone marrow in hagfish.

There is a more mature immune system in cartilaginous fish (sharks and rays).

These species of fish have three specialized organs peculiar to Chondrichthyes:

  • The epigonal organs covering the gonads (lymphoid tissue close to the mammalian bone)
  • The organ of Leydig inside their esophagus walls
  • A spiral valve in their intestine

Typical immune cells house each organ (granulocytes, lymphocytes, and plasma cells). Like chondrostean fish, the kidney, which houses several different immune cells, is the bony fish’s primary immune tissue (or Teleostei).

Also, teleost fish have a thymus, spleen, and scattered immune areas within mucosal tissues.

It is believed that teleost erythrocytes, neutrophils, and granulocytes reside in the spleen, similar to the mammalian immune system.

At the same time, lymphocytes are the primary type of cell found in the thymus.

In 2006, a lymphatic system similar to that in mammals was identified in one teleost fish species, the zebrafish.

This isn’t yet proven, but the system is possibly where naive (unstimulated) ‘T cells’ accumulate while waiting for an antigen to be detected.

Every jawed fish are found to have B and T lymphocytes bearing immunoglobulins and ‘T cell’ receptors, respectively—the adaptive immune system has grown into an ancestor of all jawed vertebrates.


The 2006 IUCN Red List outlines 1,173 fish species at risk of extinction. Species like Atlantic cod, Devil’s Hole pupfish, coelacanth, and great white sharks are included.

Fish are more challenging to study than terrestrial animals and plants because they live underwater, and the information on marine life is often lacking.

Nonetheless, freshwater fish seem particularly threatened because they often reside in relatively tiny bodies of water.

Types of fishes

Fish have immense diversity, with more than 34,000 species extending fresh and saltwater habitats.

A sampling of the various orders and families of fish can be found below:

1. Sharks


There are 535 species of sharks classified into today’s eight orders (23 of which remain undescribed). Whale sharks, which range a foot longer than other sharks, are over 40 feet in length.

Apex predators such as the great white shark, which plays an essential role in the ocean food chains, are also included.

2. Skate fish

Skate fish

More than 200 species of skate fish are found worldwide. Skate fish is flat, having a close resemblance to rays. However, they usually live in deep ocean waters. This family’s species can grow over 8 feet in length and over 200 pounds in weight.

3. Salmon


There are 140 salmon species distributed across the 33 genera and six families. Whitefish, trout, pikes, and charts are among the species of salmon.

Salmon reproduction occurs in freshwater, and young fishes return to the sea for the rest of their lives.

Salmon is an essential food source worldwide, with more than 70% of salmon being a vital food source worldwide. Salmons are huge — the chinook salmon weighs about 105 pounds and is the heaviest salmon known.

4. Dragonfish


Dragonfish are mostly Deepwater fish with at least 250 species distributed across 51 genera and four families. Dragonfish are rampant throughout the mid-water or deepwater oceans.

Viperfish, loose jaws, and hatchet fish are unique kinds of dragonfish. Dragonfish differ widely in size, having the largest species exceeding 20 inches in length, while the smallest dragonfish is just a bit of an inch.

5. Lizardfish


Lizardfish is an order of fish that came to be during the Upper Cretaceous. It consists of 14 families, 43 genera, and 220 species.

Lizardfish are hermaphrodites that live in and migrate from shallow coastal waters to the deep ocean, and they can also self-fertilize. The largest lizardfish that can grow to 7 feet is known as the lancetfish.

6. Codfish


Varieties of codfish include cods of freshwater, polar ads, hakes, and morbid cods. Codfish are incredibly varied, comprising nine families and over 500 species.

Codfish are essential food sources that are regularly captured with the Alaska Pollock and gadids. The biggest codfish is the Atlantic cod and can reach more than six ½-feet in total length!

7. Flatfish


Approximately 540 flatfish are distributed across 117 genera and seven families. Flatfish lie close to the bottom of the ocean and possess a tiny body size.

Although their jaw contorts into a sideway position, their eyes are both on one side of their body.

The giant flatfish that weighs up to 697 pounds is the Atlantic halibut. Fluke fish (or summer flounder), which can be found from Florida to Canada’s Maritime provinces, is famous for commercial and recreational purposes.

8. Suckerfish


Suckerfish is a family of fish with 79 species, and they are abundant in streams and freshwater throughout America. The fish are usually small, but some species can weigh up to 80 pounds when fully grown.

The bigmouthed buffalo, a suckerfish species confirmed to live for 112 years, is the longest-living bony freshwater fish globally.

9. Angler fish

Angler fish

Anglerfish live close to the bottom of the deep sea and the ocean. Anglerfish have dorsal fins that evolved to look like “lures.”

It is used can attract fishes closer rather than using energy to hunt. Anglerfish quickly jump forward to ambush their prey when fish come close to examine the lure.

About 300 separate species of anglerfish are distributed across 18 families. One common anglerfish is the monkfish, valued for its meat called “lobster of the poor man.”

Interesting facts


Photo by jacekwicek from PxHere

  • There are more than 34,000 species of fish known. They live in almost every aquatic habitat. The ocean’s lowest depths is the only place you won’t find fish, as no fish have been found below 8,400 meters down the sea.
  • According to a 2009 study published from the Science discovery, fish biomass is estimated to be between 800 and 2,000 million tonnes. In other words, that’s 2 to 5 times the biomass of every human on earth. Furthermore, a recent report from CSIC, which studies the deeper profundity of oceans, estimated that fish biomass might be up to 10 billion tonnes, which number is at least five times greater than previous estimates. This assumes that we know only a little about the ecology of sea fish.
  • A fish species known as coelacanth was thought to have evolved 400 million years ago for the first time and went extinct at the end of the dinosaurs. Nonetheless, scientists made an extraordinary discovery in 1938, saying the coelacanth had survived. The second species of coelacanth was even found in 1999. Once again, discovering a species considered extinct after the dinosaur era reveals how much we still have to find out about fish and oceans.
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