December 15 – Clam bake

Today’s factismal: An adult giant clam can get 75% of its food from algae living inside its skin.

One of the staples of Saturday afternoon movies is the deadly giant clam. As our hero swims fearlessly underwater through the coral reef surrounded by colorful fish and fierce pirates, suddenly his foot is caught by a giant clam. He struggles fiercely and is only able to free his foot and head for the surface after stabbing the clam to death with his handy bowie knife. But how realistic is that?

This man-eating clam is waiting for its next victim (My camera)

This man-eating clam is waiting for its next victim
(My camera)

It turns out that there is some truth in that scene, but there’s a lot more fiction. Let’s start with the true part: in coral reefs from the shores of Australia to the Philippines lives a giant bivalve that was once known as the man-eating clam; today, we call it by the less evocative name of giant clam. And the giant clam certainly deserves its name; this enormous bivalve can grow to be more than four feet across and can weigh as much as 700 pounds!

A three-foot long giant clam (My camera)

A three-foot long giant clam
(My camera)

But tales of men being captured and eaten by the clams are far more fiction than fact. The giant clam closes very slowly and would be unlikely to catch any but the least wary swimmer in its grasp. In addition, the larger clams can’t even close completely, allowing swimmers to wriggle free with no trouble. Finally, the giant clam is a filter feeder, with no way to digest any large prey that might accidentally get caught when it closes its shell.

However, that last statement isn’t completely true. The giant clam starts its life as a filter feeder, pumping water across its gills using a siphon and living off of the sediment and other goodies that get trapped inside. But by the time it has settled down for a life as a responsible adult clam (when it is about a tenth of an inch long), the clam has started to play host to a type of algae known as zooxanthellae (“little yellow critters”). These algae live inside the clam’s skin in special sacs surrounded by blood vessels. The giant clam will open up and spread its mantle to let the algae get the sunlight as it feeds them waste products that the algae use as food; the algae in turn will combine those waste products with sunlight to make food for the giant clam. A large giant clam will get as much as 75% of its energy from the algae and only about 25% from the goo it filters out of the water.

The zooxanthellae live in the colorful spots on the giant clam's mantle (My camera)

The zooxanthellae live in the colorful spots on the giant clam’s mantle
(My camera)

These giant clams are an amazing example of symbiosis and form an important part of the reef system where they live. Unfortunately, their reputation (and large closing muscle) have made them a popular target for poachers who can sell them for several thousand dollars each. And that’s why the Reef Environmental Education Foundation would like you to report any giant clams (or any other reef critters) that you’ve seen. If you’ve seen them alive, then they want to hear from you. And if you’ve seen them dead and being sold in the market, then they really want to hear from you. To learn more about their mission to save the giant clam and other reef animals, swim on over to:

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