| Fish (Latin: Pisces) the super class of water animals, the extensive group of gnathostomatous vertebrates, for which gill breathing is characteristic at all stages of post-embryo development of the organism.
Morpho-biological characteristics:
During individual development, chorda is replaced with a backbone. Chorda accomplishes the role of the basic skeleton only in larvae and embryos. Then, it is partially or completely reduced and its function is passed onto the spine chorda. In sturgeon fishes, the chorda is preserved for its entire life (but upper and lower cartilaginous vertebrae arches are formed around the chorda). Remnants of chorda are preserved even in osseous fishes in the form of beads between amphicoelous vertebrae.
Apart from unpaired fins (dorsal, anal and tail), mobile paired thoracic and belly fins develop in the fish. Movement of fish takes place due to a sideward twisting of the body or through movements of the tail. Paired fins serve the purpose of rudders and unpaired fins provide body stability.
The digestive system is differentiated: the majority of fish types have the stomach and intestines divided into thin and thick segments. Usually, the liver and pancreas are well developed.
Fish do not possess a constant body temperature and breathe with the help of gills. Additional respiratory organs, capable of acquiring atmospheric oxygen, are formed in fish living in oxygen insufficient conditions. Even some fish swallow air bubbles which are dissolved in water near the gills.
Fish have single blood circulation and a two-chambered heart; second pulmonary circulation formation is found in lungfishes.
Several mucous glands are found in the skin. The lateral line is well developed. The skin is covered with scales. Scales are protective formations which can sometimes have a difficult structure.
Fishes are dioecious. Fertilization, as a rule, occurs in water.
Sizes of modern fish vary from 7.9 mm (Paedocypris progenetica) to 13.7 m (whale shark).
The origin and evolution of fish:
The first fish-shaped animals originated no later than Silurian period and had no jaws. The first fish appeared with the development of jaws from one of the branchiate arches.
Classification:
From taxonomic point of view, fish belong to the paraphyletic group since the hypothetic ancestors of the terrestrial animals group – Tetrapoda, which obviously are not fish should be part of the paraphyletic group.
Fish are classified into two modern classes: cartilaginous and osseous. The relationship of basic subgroups of fishes is presented by the following:
Hyperoartia:
Petromyzontidae
Pteraspidomorphi
Thelodonti
Anaspida
Cephalaspidomorphi:
o Galeaspida
o Pituriaspida
o Osteostraci
Gnathostomata:
o Placodermi
o Chondrichthyes
o Acanthodii
o Osteichthyes
+ Actinopterygii
+ Sarcopterygii
# Actinistia
# Dipnoi
Some paleontologists consider that conodonts (group Conodonta) were chord animals, which allows them to be considered as the most primitive fishes.
The ecology of fish:
At present, fish are the dominating group of animals in biocenosis waters. Along with cetaceans, fishes complete the food chain.
With respect to habitats, sea, fresh-water and anadromous fishes are distinguished.
Littoral (plankton) – live in coastal zones (bullheads, blennies);
Pelagic fishes (nekton) – are found in deep waters;
Ground fishes (benthos) – for example, scates, flatfishes, catfishes.
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