• Dorsal fin

The dorsal fin is found in such groups of animals as

Jawless: found in lampreys and most extinct bonypans and telodonts;

Most species of fish;

Reptiles (extinct ichthyosaurs have it);

mammals (present in dolphins and whales).

It is an unpaired fin that acts as a keel, sometimes transformed into a sucker (stickleback), a sail (sailfish), a rod (the first ray of the dorsal fin in the dodger), individual barbs (stickleback). In some fish the dorsal fin is used for propulsion. In fish with eel-shaped bodies, the dorsal and anal fins merge with the caudal fin to form a single fringing body fin. The number of dorsal fins varies from 1 (carp, salmon, whitefish, etc.) to 2 (perch) or 3 (cod). They are an important systematic feature.

Almost all species of fish have a dorsal fin, although there are some, such as the black-footed booby, that do not. Many fish have two or even three dorsal fins, or a single dorsal fin that is so long that it merges with the caudal fin. The most common variation, found in most well-known fishes, is an anterior fin consisting of sharp, rigid rays and a posterior fin that is soft.

Orcas have a very large dorsal fin compared to their body size. It can reach 1.8 metres in height. Many (anywhere from 30 to 100%) orcas in captivity have a drooping dorsal fin. This is probably due to a lack of physical activity; in captivity, the animals swim much less and there is no need to carefully monitor their orientation in space, which leads to atrophy of the muscles responsible for raising the fin. In nature, only 1% of wild killer whales show this phenomenon.

The bowhead whale has no dorsal fin at all as an evolutionary adaptation to life in the ice-covered polar seas. Over the course of a whale's life, the dorsal fin develops an individual pattern of nicks and lesions. Researchers use this to identify individuals.

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