Sunday,26 September 2021, 7 : 45 AM

Introduction to Fish

Channa Barca
Channa Barca
Image Credit: Hayath Mohammad


A fish can be defined as an aquatic creature, which is a cold blooded vertebrate and has gills though out life and limbs if any are in form of fins.

There are approximately 54,711 species of recognized living vertibrates and fish constitute slightly more than one-half of these.(Nelson 2006).
The number of species in fish is greater than the species of all other vertebrates like Mammals, birds reptiles put together.

There are 28400 species of fish spread in 515 families (Nelson 2006). Of the Indian region alone, there are 2500 species out of which 930 are freshwater and the rest are marine (Jayaram 1994).

The number of species is under constant revision as new species are being discovered and the existing ones are becoming extinct in many areas including the Amazon River Basin in South America and the Congo River Basin in Africa.
Unfortunately, habitat destruction, pollution and international trade, among other human impacts, have contributed to the endangerment of many fish species.

Red tail Shark

Epalzeorhynchos bicolor : Red Tailed Black Shark
Image Credit: Wikipedia

There are some species like Red tail black shark, Epalzeorhynchos bicolor, which have become extinct in wild due to destruction of its habitat in Thailand by construction of dams and drainage. It now exists only in aquariums and breeding farms.

Amongst various major groups of fish, the Actinopterygii or ray-finned fishes form a majority.
They comprise nearly 96% of the 28,400 species of fish.
Actinopterygii or ray finned fish are so called because their fins are made of webs of skin supported by bony or horny spines (the rays), as opposed to the fleshy, lobed fins that characterize the class Sarcopterygii (lobe-finned fish).

Fin bony

Fin Of a Bony Fish
Image Credit: Wikipedia

Fin carte

Fin of a Carteligous Fish
Image Credit: Wikipedia

The fin rays in fish belonging to Actinopterygii attach directly to the skelton.

Fish are found throughout fresh water and marine environments from the deep sea to the highest mountain streams.
Species can range in size from Paedocypris, at 8 millimeters to the massive Whale shark 14 meter.


Paedocypris_progenetica: Smallest Fish ar 11 mm
Image Credit: Wikipedia

Whale shark

Rhincodon typus: Whale Shark: Largest Fish at 45 ft and weighing 20 tonnes
Image Credit: Wikipedia

Fish or Fishes

When one is referring to one or more individuals of one species, the term fish is appropriate.
Hence it is correct to refer to 100 rosy barb as fish, when we refer to fish from different species like 2 types of barbs i.e. rosy barb and a clown barb they are referred to as fishes.

Some Interesting Facts

  • Lungfishes can live in a state of dry suspended animation for up to 4 years.
    They become dormant when their ponds dry up and reviving quickly when immersed in water.

    Lung Fish

    Neoceratodus forster : Lung Fish
    Image Credit: Wikipedia

  • Antarctic fishes live in water that is colder than the freezing point of their blood.
    These fishes keep from freezing by avoiding free ice and because their blood contains antifreeze proteins that depress their blood´s freezing point to -2°C.
    Some Antarctic fishes have no hemoglobin in their blood.

    Antartica fish

    Pagetopsis maculatus : Crocodile Icefish
    Image Credit: Wikipedia

  • Deep sea fishes include many forms that can swallow prey larger than themselves.
    Some deep sea anglerfishes are characterized by females that are 10 times larger than males, the males existing as small parasites permanently fused to the side of the female, living off her blood stream.

    Hunpback Angerfish

    Melanocetus johnsonii : Humpback Anglerfish
    Image Credit: Wikipedia

  • Fishes grow throughout their lives, changing their ecological role several times.
    In some fishes, differences between larvae and adults are so pronounced that many larvae were originally described as entirely different taxa .

  • Life spans of fish show a wide range, some have a maximum life spans of as little as 10 weeks (African Killifish and Great Barrier Reef pygmy gobies) and some as long as 150 years sturgeons and scorpaenid rockfishes).

    Some short-lived species are annuals, surviving drought as eggs, which hatch with the advent of rains. Longer lived species may not begin reproducing until they are 20 years old, and then only at 5+ year intervals.

  • Gender change is common among fishes.
    Some species are simultaneously male and female, whereas others change from male to female, or from female to male.

  • Fishes engage in parental care that ranges from simple nest guarding to mouth brooding to the production of external or internal body substances upon which young feed.
    Many sharks have a placental structure as complex as any found in mammals.
    Egg-laying fishes may construct nests by themselves, whereas some species deposit eggs in the siphon of living clams, on the undersides of leaves of terrestrial plants, or in the nests of other fishes .

  • Fishes are unique among organisms with respect to the use of bioelectricity.
    Many fishes can detect biologically meaningful, minute quantities of electricity, which they use to find prey, competitors, or predators, and for navigation.

    Some groups have converged on the ability to produce an electrical field and obtain information about their surroundings from disturbances to the field, whereas others produce large amounts of high-voltage electricity to deter predators or stun prey .

  • Fishes are unique among vertebrates in their ability to produce light; this ability has evolved independently in different lineages and can be either autogenic (produced by the fish itself) or symbiotic (produced by bacteria living on or in the fish).

  • Although classically thought of as cold-blooded, some pelagic (open water swimming) sharks and tunas maintain body temperatures warmer than their surroundings and have circulatory systems specifically designed for such temperature maintenance.

    Lampris guttatus

    Lampris guttatus: Warm blooded Fish
    Image Credit: Wikipedia

  • Fish deploy a range of tactics for catching food.
    Predatory tactics include attracting prey with modified body parts disguised as lures, or by feigning death.
    These are some specialists that feed on ectoparasites, faeces, blood, fins, scales, young, and eyes of other fishes.

  • Fishes can significantly change the depth of their bodies by erecting their fins or by filling themselves with water, an effective technique for deterring many predators.
    Also in some fish the ligament and levering arrangement of mouth bones allows them to increase mouth volume when open by as much as 40-fold.