Seals

in June 2011 - Marine Scene

Seals on land are ungainly, smelly and faintly comical as they lumber about on rocks or splash clumsily into the waves. In Cape Town they also feature as victims of spectacular shark attacks, clutched helplessly in the jaws of a magnificently breaching great white. But seals are much more than this. Underwater, they are sleek and graceful.

These are animals superbly adapted for life in the ocean. The South African fur seal is the largest of the eared seals. It has a dense underfur and a layer of blubber to keep warm in chilly water. Their eyes have flattened corneas for better underwater vision and huge pupils to help sight in murky water. In darkness, they can use their whiskers to find prey, detecting the minute disturbances fish make in the water up to 30 seconds after the fish has passed.

Their breath-holding capabilities are extra-ordinary. Average hunting dives last 2.1 minutes  to 45m, with impressive maximums of 7.5 minutes and 204m.

They are physiologically well adapted for diving: they can tolerate high levels of carbon dioxide and lactic acid. Also, when diving their heart slows to a fraction of its above-water rate and blood is only supplied to vital organs, increasing their underwater endurance considerably.

And the sharks? Well, if they don’t get the seal on their first strike, they are unlikely to succeed on the second. Specially configured vertebrae mean that seals are particularly flexible, both for hunting agile prey and escaping hungry predators.