Bipolar Animal Distribution #Zoology 3rd Sem#Unit ll

The Bipolar Distribution of animals is the spread over of same species,genus or family in the  Arctic and Antarctic regions of Northern and Southern hemispheres with a gap between the two i.e. with no same members in the tropics .

e.g. (i) Coelenterates : Botrynema,Lampta, Grammaria, Myriothela

(ii) Molluscs: Limacina helicina , Clione

(iii) Ascadian : Didemnus albidum

(iv)Fishes:Oncorhynchus(Pacific Salmon), Lamma cornubica(Shark)

(v) Birds : Aptenodytes (Penguin)

(vi) Mammals : Balaenoptera musculus ( Blue whale) , Callorhinus (Seal), Caribou,

etc.

          There are several theories regarding bipolar distribution of animals , and three of them are:

Migration theory: According to this today's polar animals once migrated from tropical and intermediate zone .

Extinction theory: Polar animals once originated from cosmopolitan group , then distributed from one pole to another and at last disappeared from the tropical and sub-tropical region .

Ice-age theory :There are five significant ice-ages  (4th =Karoo=360--260 mya. and Quaternary=2.6 mya.--recent )in the earth's history .During that ice-age temperature dropped, earth surface reshaped , animal distribution pattern changed . Acoording to L.S.Berg during ice-age  animals from the Northern  hemisphere crossed tropics when the tropical temperature dropped and reached to the Southern hemisphere . Later when  the temperature in tropics rose , animals in the tropics died out but the polar animals survived .Thus, a discontinuous animal distribution pattern was established .