These southern elephant seals (Mirounga leonina) may look like beach bums, but when they are in the water hunting, they are anything but. Satellite tracking by tagging the animals has found that, during the 10 months they spend at sea, elephant seals spend most of their time underwater, hunting fish and squid at depths of 1,300 to 3,300 feet (400 to 1,000 meters).
In the deep water, it is very dark, and elephant seals don't have a great sense of smell, like penguins, or the ability to echolocate, like whales, to help them hunt. They can't even see particularly well in daylight -- but in the dark it's a different story. Elephant seals' eyes are specialized for dim light and, in particular, to light at the wavelength of 485 nm -- which is the same wavelength given off by the bioluminescent lanternfish that are the seals' main prey. And the seals preferentially spend more time foraging at the depth where most bioluminescent animals live.