Bipolar Distribution of Organisms

the interrupted distribution of plants and animals, whereby the same species, genus, or family lives in the temperate latitudes of the northern and southern hemispheres but is absent in the tropics. A bipolar distribution is particularly characteristic of marine organisms: several marine invertebrates (for example, mussels), marine fish (the anchovy, Pacific saury, basking shark, and porbeagle), some mammals (southern whale, gray dolphin), and many species of marine algae (kelp and fucus). The occurrence of bipolar distribution is caused by the conditions that prevailed in the ice age, when the temperature in the tropics dropped and northern organisms were able to spread to the tropics and reach the southern hemisphere. Later when the temperature rose, these organisms died out in the tropics, thereby giving rise to an interrupted (or noncontinuous) range. This explanation of bipolar distribution was suggested by the Soviet scientist L. S. Berg.