Corculum cardissa, better known as the heart cockle, doesn’t seem like it would have anything in common with our beloved giant clams. Except, maybe for the fact that it’s a bivalve. It may not be a direct ancestor, but the heart cockle is probably the closely living relative of giant clams in the Tridacna and Hippopus clams than any other creature that we know of.
Depending on which taxonomist you ask Corculum heart cockles are also placed in the family Tridacnidae and when you compare the shell of heart cockles and Hippopus clams it’s easy to see why. More importantly, and interestingly for us, is that heart cockles are also a photosynthetic bivalve, but how they go about it is inherently unique.
We are all familiar with the beautifully colored and abundant fleshy mantles of standard giant clams, and we know that they extend their mantle in order to expose the algae living inside their tissues to the sunlight they need. Where heart cockles differ is that they don’t open very much, instead having a specially adapted shell that allows light to penetrate through specially constructed opaque ‘windows’ in the shell to allow their symbiotic algae to photosynthesize.
We’ve known about this bizarre bivalve ever since we first fell in love with teh giant clams, and we thought they were mostly found lying around in shallow water like most cockles do. What was surprising about our first encounter with this curious creature in the wild is how and where we found them.
In a calm, shallow water environment in the Solomon Islands, very near some mangrove and rainforests, we spotted Corculum living embedded in large heads of bouldering Porites coral. The heart cockles had burrowed themselves into the coral and formed the kind of depression you would expect to see and find like Tridacna crocea clams create when they are embedded in coral or rock.
Interestingly enough the only other photo-clams we observed in this habitat was Tridacna crocea. Seeing an animal that we had read about many years before but never really encountered is one of the best perks of being proficient in invertebrate zoology thanks to having a good background in general aquariology. So next time you see a giant clam, or if you have one in your aquarium now, just remember; it’s just an overgrown cockle after