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9.2. Fish

Fish Superclass

Fish are aquatic vertebrate animals whose body is made up of the head, trunk and tail.

Fish live in both salt and fresh water, but almost no species can live in both environments (salmon, for example).

Fish are generally ectothermic (cold-blooded) beings, and cannot regulate their body temperature, which varies depending on the temperature of the water in which they live.

The characteristics of the fish are what have allowed them to dominate the aquatic environment:

  • Its hydrodynamic shape, with a spindle-shaped body, allows it to move easily in the water.
  • They have a lateral line, an organ that allows them to pick up the vibrations of the water.
  • They are usually covered by scales.
  • The fins allow them to make a continuous movement in the water, and can be of two types:
  • Paired fins: pectoral and ventral.
  • Unpaired fins: dorsal and caudal (the only one that drives it).
  • The skeleton of fish can be of two types:
  • The way of feeding fish is varied. Most of them are carnivores, and prey on other organisms, but there are also scavengers, parasites, herbivores and microphages (they feed on planktonic beings).
  • They breathe through gills, with which they capture dissolved oxygen in the water.

Kinds of fishes

Although fish have many common features, there are others large enough that fish can be classified into three large groups (classes):

Cyclostome class

  • Cyclostomes are a type of agnathus, jawless fish, which emerged in the Paleozoic (about 500 my ago) of which only lampreys remain today.
  • There are species of agnathans in fresh waters and in salt waters, but they are not very abundant.
  • Agnates have the following characteristics:
    • Their mouths are circular, with several rows of teeth, but they have no jaws . 
    • They have a cartilaginous skeleton.
    • They have a cylindrical body, without scales.
    • They have two dorsal fins and one tail fin.
    • They have no stomach.
    • The digestive tube begins in a circular mouth that has suckers with which it grabs its prey. It has several rows of horny teeth that can tear apart, although they lack jaws. They are external parasites. They feed on the blood and body fluids of fish.
    • The respiratory system consists of gill bags that come out of the pharynx, as far as the water from which they take oxygen and then leaves through the gill slits.
    • The fins are odd. It has two dorsal fins and one caudal fin.
    • They have sexual reproduction, with external fertilization.
    • They are oviparous and ovoviviparous, like the lamprey.

Class Osteichthyos (bony fishes)

The main characteristics of osteichthyes are:

  • They have a skeleton made up of bone tissue.
  • Their gills are protected by a hard covering called the operculum. The water, loaded with oxygen, enters through the mouth, reaches the gills, where gas exchange takes place, and leaves through the operculum with less oxygen and more carbon dioxide.
  • They are fusiform in shape.
  • The caudal fin is homocercal, that is, symmetrical, with two equal halves.
  • The mouth is in a frontal position, at one end of the head.
  • The skin is covered with flat, round scales, one on top of the other, like roof tiles.
  • They have a swim bladder, a bag that they can fill or empty of gas, to help them float and be able to ascend or descend in the water.
  • Sexual reproduction. Bony fish are oviparous with external fertilization.
  • Some examples of bony fish are: sardines, troutbarbel, salmon, hake, etc.

Partes de un bacalao

By No machine-readable author provided. Salvor assumed (based on copyright claims). [CC BY-SA 2.5 ], via Wikimedia Commons

Class Chondrichthyans (cartilage fishes)

The main characteristics of chondrichthyans are:

  • They have a skeleton made up of cartilaginous tissue.
  • The gills are not protected by an operculum.
  • The mouth is located in a ventral position.
  • The caudal fin is heterocercal, that is, asymmetrical, with two halves of different sizes.
  • They are fusiform in shape.
  • Its skin is not covered with scales, but with thick, pointed dermal denticles, placed next to each other, without overlapping.
  • They do not have a swim bladder.
  • They have sexual reproduction. They are ovoviviparous and viviparous, with internal fertilization.
  • Examples of cartilaginous fish are sharks, rays, mantas, etc.

Partes de un tiburónDe Parts_of_a_shark.svg: Chris huh derivative work: mario modesto (talk) - Parts_of_a_shark.svg, Dominio público,

Difference Between Bony Fish and Cartilaginous Fish
Bony fishes (osteichthyes) Cartilaginous fish (chondrichthyans)
They have a bony skeleton They have a cartilaginous skeleton
Their gills are protected by an operculum. Their gills are not protected by an operculum
Caudal fin homocercal, symmetrical, with two equal halves Caudal fin heterocercal, asymmetrical, with two halves of different sizes
Flat, round scales, arranged like shingles on a roof Dermal denticles, thick and pointed
They have a swim bladder They don't have a swim bladder.
Mouth in frontal position, at one end of the head Mouth in ventral position
Oviparous, with external fertilization Oviparous and viviparous, with internal fertilization

Curiosity: puffer fish deflating

Surely you know the puffer fish, a fish that is capable of greatly increasing its size to deter predators. If you did not know, you can already imagine why it is called a balloon.

It is also the most toxic fish there is and can kill the person who eats it. However, in Japan,  fugu (blowfish) sashimi is considered a delicatessen that only a few expert chefs can cook, since they have to remove the liver, ovaries, and remove the skin in certain parts to prevent the fish is contaminated with toxic substances that are stored in the liver.

Do you want to see how much water a puffer fish stores inside?