Easy Fish Facts
Complete Review on Fishes

Fish Review

1. What are the two main classes into which fishes are divided?

Fishes are divided into two main classes: chondrichthyes, fishes with cartilaginous skeletons (sharks, rays, dog-fishes), and osteichthyes, bony fishes (tuna, sardines, salmons).

“Chondros” means cartilage, “ictis” means fish (both from the Greek); the name chondrichtians is for fishes with cartilaginous endoskeleton. The name osteichthyes comes from the existence of a bony endoskeleton in these fishes (“osteo” means bone, from the Greek too).

Fish Facts - Image Diversity: chondrichthyes osteichthyes

Fishes are all aquatic animals and thus they have a hydrodynamic elongated body suitable to move under water, without limbs and with fins. The habitat conditions the branchial respiration too.

Compared to cyclostomes, evolutionary novelties presented by fishes are: pectoral and pelvic fins, symmetric and paired; the presence of mandibles.

Fish Facts - Image Diversity: fish body structure

Bony fishes have a specialized organ called a gas bladder, or swim bladder, whose interior can be filled with gas liberated from gas glands. The swim bladder works as a hydrostatic organ since it varies the relative density of the body regulating buoyancy and the depth of the animal in water.

Chondrichthyes do not have swim bladders and thus they must continuously agitate their body to keep swimming and maintain their depth in water. As an additional swimming aid, in chondrichthyes the liver is big and oily; this feature helps to reduce their body density relative to water.

Fish Facts - Image Diversity: swim bladder

From Hydrostatics it is known that an object does not sink if its density is equal or less than the density of the fluid in which it is immersed.

When the swim bladder is filled by gas it reduces the density of the fish body and when it is emptied this density is increased. So this mechanism controls the fish depth under water.

Fishes “breath” through gills. Gills, or branchiae, are highly vascularized organs specialized in gas exchange under water and present in aquatic animals (marine annelids, crustaceans, fishes and tadpoles). Gills are a respiratory organ (analogous, for example, to lungs) containing very thin lamellae with many apparent blood vessels in direct contact with water.

In osteichthyes the gills are covered by a bony flap that protects them called operculum. In chondrichthyes there are no opercula.

Fish Facts - Image Diversity: gills

As in every vertebrate the circulatory system of fishes is closed, i.e., blood flows only within blood vessels.

The fish heart has only two consecutive chambers: a thin-walled atrium and a muscular ventricle. The arterial (oxygenated) blood comes from the gills and gains arteries towards tissues, then venous blood is collected by veins and reaches the atrium of the heart passing to the ventricle that pumps the venous blood towards the gills to be again oxygenated.

Fish Facts - Image Diversity: fish heart

Fishes have a pair of kidneys that filtrate the blood. Bony fishes excrete nitrogen as ammonia, NH3, (they are ammoniotelic) and cartilaginous fishes excrete urea as nitrogen waste (they are ureotelic, like adult amphibians and mammals).

The lateral lines of bony fishes are sense organs that extend along both sides of the animal body. They make contact with the environment by a series of specialized scales that transmit information about pressure variation and vibrations in the surrounding water.

Fish Facts - Image Diversity: lateral line

In chondrichthyes fecundation is internal by means of copulation. In osteichthyes fecundation generally is external and the gametes are released in the water, where they can fecundate their counterpart and form the zygote.

Examples of representing beings: sharks, rays, sardines, tuna, salmons. Basic morphology: hydrodynamic body, fins; cartilaginous skeleton in chondrichthyes, bony skeleton in osteichthyes. Skin: with scales in osteichthyes and placoid scales in chondrichthyes. Respiration: branchial. Circulation: closed, incomplete, heart with two chambers. Nitrogen waste: urea in chondrichthyes, ammonia in osteichthyes. Thermal control: heterothermic. Types of reproduction: sexual, internal fecundation in chondrichthyes, external in osteichthyes.

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