Cameron Duke of MinuteEarth explains why the underwater creatures of the Antarctic are biologically different from their northern counterparts, growing larger and living longer. This has to do with the nature of the Southern Ocean itself and a natural phenomenon known as the Antarctic Circumpolar Current, which cuts off underwater contact from the rest the world.
It is the largest and fastest ocean current on the planet, and it’s also the deepest. It exists as a giant spiral of water that extends from the surface to the seafloor, acting as a barrier that has prevented most Southern Ocean life from interbreeding with life from other oceans for the last 30 million years.
The features of the current itself also help with the health of the local sea life.
But there’s a benefit to being trapped behind the current –as it spirals, it churns up nutrients from the depths, which means that the Southern Ocean is full of food for all of its strange life. …it’s very salty, allowing it to get super cold without totally freezing. And that cold water allows it to hold onto lots of dissolved oxygen – which might help animals to get bigger and age slower than they would elsewhere.