Most aquatic animals are ectotherms, or poikilotherms, or what is often referred to as “cold-
Aquatic mammals are able to keep their body temperatures more or less constant regardless of water temperature. Marine mammals deposit most of their body fat into a thick layer of blubber that lies just underneath the skin.
This blubber layer not only insulates them but also streamlines the body and functions as an energy reserve. The fusiform body shape and reduced limb size of many marine mammals and organisms decreases the amount of surface area exposed to the external environment. This helps conserve body heat. An interesting example of this body form adaptation can be seen in dolphins: those adapted to cooler, deeper water generally have larger bodies and smaller flippers than coastal dolphins, further reducing the surface area of their skin.
Arteries in the flippers, flukes, and dorsal fins of marine mammals are surrounded by veins. Thus, some heat from the blood traveling through the arteries is transferred to the venous blood rather than the outside environment. This countercurrent heat exchange also helps to conserve body heat.