General characteristics of fish describe the General features of their structure and life:
fish live in fresh and salt waters; different species are adapted to different living conditions and feeding methods;
they breathe with gills; in bony fish, the gills are covered with Gill covers;
they have a bony, cartilaginous or cartilaginous skeleton;
fish have not only a skull, but also jaws with jaws;
paired limbs appear in the form of two pairs of fins (ventral and pectoral); in addition, the belts of these limbs develop;
the neural tube is differentiated into the spinal cord and brain; the brain, despite its small size, consists of five divisions, which is typical for all vertebrates;
well-developed sense organs (vision, hearing, smell, taste, touch); in addition, fish have a special sense organ (lateral line) that perceives water pressure;
the heart of fish is two-chamber; they have one circulatory circle; the circulatory system is closed;
fish are cold — blooded animals;
the skin of fish is covered with scales; in bony fish it is bony, in cartilaginous fish it is placoid; the scales are covered with mucus to reduce friction and protect against pathogens;
bony fish have a swim bladder;
fertilization in fishes the vast majority of the outer; the characteristic high fertility.
These characteristics are not a comprehensive General characteristic of fish. But they can be called key. So in the process of evolution in fish, the first appearance of jaws, paired limbs, and a two-chamber heart occurs. These innovations allowed fish to occupy different ecological niches of salt and fresh water, adapting to different conditions.
When giving a General description of fish, it should be borne in mind that there are some key differences between cartilaginous and bony fish. In the skeleton of cartilaginous fish, there are no bones, their entire skeleton consists of cartilage tissue. The skeleton of bony fish can completely ossify, but in a number of species, along with the bone tissue, cartilage is preserved. In bony fish, Gill slits with Gill lobes are protected by Gill covers. Cartilaginous fish do not have lids, and the Gill slits are often SAC-shaped.
The swim bladder is available only in bony fish and, changing in size, helps them stay in the water column without muscle effort. Cartilaginous fish, if they stop moving, then sink to the bottom.
The shape of the fish body is streamlined. However, in many bony fish, the body is flattened from the sides, while in cartilaginous fish it is fusiform (in sharks) or flattened in the dorsal-abdominal direction (in rays).
The jaws and teeth of fish serve them to capture and hold prey. However, the food is not chewed, but swallowed whole. Digestion takes place in the stomach and intestines.
A common characteristic of fish is the presence of not only paired (ventral and pectoral) fins, but also unpaired fins (dorsal, anal and caudal). Dorsal (and sometimes anal fins can be several). The caudal fin plays an important role in the forward movement of the fish. In bony fish, its upper and lower parts are usually symmetrical; in cartilaginous fish, the upper part is larger than the lower part.
Fish can only have sexual reproduction. In bony fish, females usually sweep out the eggs, and males water it with milk. Thus, external fertilization occurs. In this case, the number of eggs is usually very large, but not every fry develops, and not every fish fry survives. Consequently, the high fecundity of fish is an adaptation to the high mortality of young animals. Fish that take care of their offspring do not have a large number of eggs. Many cartilaginous fish have internal fertilization and live birth. At the same time, the number of offspring is also small.
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