Found in the temperate waters of the southern and northern hemispheres, but not in the tropics (anchovy, sardines, herring shark, crustaceans, molluscs).

Bipolar species, species with a range divided by a tropical belt. Southern and northern forms are considered distinct but related species; some bipolar species are represented by distinct subspecies. The term "bipolar species" does not refer primarily to subpolar forms (Arctic and Antarctic), but to temperate species in both hemispheres. The total number of bipolar species is large, especially among marine animals. 

Examples of bipolar species: the paddlefish Calanus helgolandicus is found off the coast of the British Isles and New Zealand; in the genus elephant seal, there are two closely related species, one in coastal Californian waters and the other in subtropical and Antarctic waters; the trigla fish, or sea cock, occurs both in the North Atlantic and in the Far East, and off the southern coast of Australia and Africa. 

There are several explanations for the origin of bipolar species. The most widely accepted is L. S. Berg's theory that bipolarity was caused by the end of the last Ice Age, when temperatures began to rise in tropical latitudes and cold-loving forms 'dispersed' north and south.

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Bipolar species

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