Giant Clam.

GIANT CLAMS! I know, a lot of pictures, but I really wanted to show you the variety of designs and colors these animals have. There are plenty more, too. They are many varieties of this genus. And they all live in the Pacific and Indian oceans. They can weigh in at 400 pounds and measure about 4 feet across when fully grown. And they live to about 100 years!

The coloration attracts all sorts of animals -- many animals confuse it with corals and think it is a good place to rest. Unlike most clams, this one keeps its shell open regularly, instead of closed. This is because it has a symbiotic relationship with algae -- their contribution is to keep the shell open during the day so that the algae living on the mantle's flesh get sunlight for photosynthesis.

It was thought that they could eat people (so much so that they originally named it killer clam). And while it could potentially hold someone underwater, it does not close its shells unless threatened, and even then they can't always shut their shells. And even if it can, it closes very slowly -- not the 'snap' you associate with clams. So you have to be really, really dumb to get caught by one. The US Navy Diving manual used to have instructions on how to escape them at one time, if caught. Strange.

They are listed by the IUCN as "vulnerable" -- though they are still in decent numbers, they live in fragile habitats, easily effected by pollution and human traffic. And their shells are sold on the black market for decoration. Their meat is sold as a delicacy in Japan and France.