the organism living in arctic and antarctic regions are called

1. megatherms
2. mesotherms
3. microtherms
4. hekistotherms

The correct option is 4.
One of Alphonse de Candolle's physiological groups in the geographical distribution of plants, denoting such as can subsist with the minimum of heat: commonly used in the plural. Hekistotherms are both boreal (arctic) and austral (antarctic).