10 Characteristics of Fish

We explain what fish are and what their main characteristics are. Also, their classification and how they feed.

What are Fish?

The fishes are the first vertebrate animals that emerged throughout animal evolutionestimated to be about 400 million years ago. The development of gills, to be able to breathe in water, and the development of the swim bladder, to stay afloat, were crucial in the appearance of fish.

Fish have always had a strong bond with man: They have been and are a very important source of human food.and represent an interesting hobby for those who decide to raise fish as home pets and build and maintain their own aquariums.

Below we outline the main characteristics of fish.

See also: Whales.

Fish characteristics:

  1. Feeding

Within fish there are several ways of feeding:

  • Carnivores. They feed on other fish with a short digestive system and a fairly large stomach.
  • Herbivores. They feed on algae and other vegetables, and have a less voluminous stomach.
  • Limnivores. They feed on waste and various materials present in the bottoms of water courses.
  • Omnivores. They consume all types of food (vegetable or animal origin).

  1. gill respiration

In order to breathe in water, where air (and oxygen) are dissolved, fish They use special organs called gills. These are very fine and highly vascularized lamellar structures, grouped in bony arches.

In those gill lamellae gas exchange occurs, which is countercurrent, since it circulates there in the opposite direction to the water, this optimizes the capillary diffusion of dissolved oxygen. Some fish also have external gills, which emerge on each side of the neck.

  1. Classification

Broadly speaking, fish are divided into two main groups:

  • Cartilaginous or chondrichthyans. They have a cartilaginous skeleton; The skin is covered with placoid scales and they lack a swim bladder. The sharks and rays are derived from them.
  • Bony or osteichthyes. They have a bony skeleton and a swim bladder; however, in some groups the swim bladder has evolved and transformed into a kind of lung.

  1. Scales

The body of the fish It is covered by scales of a horny materialThese scales protect them from abrasive substances that may be present in the water and give the body a certain elasticity, which helps in swimming.

  1. Hydrodynamic body

Fish They have a shape that helps them move through the waterThey are usually flattened or spindle-shaped and their fins help them swim.

  1. Swim bladder

The swim bladder It is like a membranous sac filled with air that many fish have., this is located above the digestive tract. This organ allows floating without major effort; This is essential for bony fish, whose specific weight is somewhat higher than that of water.

  1. Heterothermia (“cold-blooded”)

This means that They cannot control their body temperature, which depends on external conditions. The same thing happens with amphibians and reptiles. That is why it is usually differentiated between cold water fish and tropical fish: the former thrive well at temperatures of 15 to 20 °C, the tropical ones at 24 to 28 ° C; That is why to raise tropical fish, often more attractive, it is necessary to add a heater to the fish tank.

  1. Fins

As extremities, fish have flat structures known as fins, which are very important in their mobility. The dorsal fin and the caudal fin (the tail fin) are the most characteristic. The ventral one is also noticeable and there may be others, usually smaller.

  1. Absence of external genitalia

Fish do not have external sexual organs, but in viviparous or ovoviviparous groups, males develop a copulatory organ from the modification of its ventral fins.

  1. External fertilization

Most fish are oviparous and carry out their fertilization externally: females spawn in some more or less protected placeand the males fertilize these eggs.

Some fish are viviparouswhich means that females give birth to their young as fully formed juveniles. There are also some families of ovoviviparous fish, in which the females retain the fertilized eggs inside them until they hatch. In these last two groups, fertilization is internal.

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