I’m trying very hard to think of a mollusk that is as impressive as the giant clam. It’s pretty difficult to do. These guys are just so massively amazing. There’s definitely something to be said for animals that are really, really, big. It’d be better if they had faces, but no animal is perfect.

Giant clams can be found in tropical areas of the Indo-Pacific Ocean. They like to hang out in coral reefs, especially shallow waters less than twenty meters from the surface. As far as what kind of substrate these jolly clams like, sandy places or coral rubble are the best.

You can’t tell, but it’s big.
Image by Nhobgood Nick Hobgood, CC BY-SA 3.0, via Wikimedia Commons

Giant clams are pretty big. One might even call them giant. Not me though. They do get to be up to considerable sizes, with some clams reaching 1.5 m in length. In addition to their size, giant clams can also be fairly colourful, with their mantle being green, yellow or brown and covered in iridescent blue, purple or green spots. Apparently giant clams are like those body builders who can’t touch their shoulders because their muscles are too big — once a giant clam reaches their full size, they are too big to close their shells completely.

Reproduction in giant clams is pretty simple: the clams squirt eggs and sperm into the water, and hope that the gametes meet the excretions from other nearby clams. The clams are hermaphroditic, meaning one individual can produce both eggs and sperm, but a clam cannot self-fertilize. To increase the chance that other clams will release sperm and eggs at the same time, giant clams use a chemical called the spawning induced substance that synchronizes fluid ejection from neighbouring clams.

Young larvae spend their time floating around the ocean searching for a suitable spot to settle down. By one week of age, the little clam starts to test out potential living spaces, moving around until the perfect location is found. I guess if you spend your whole life in one spot, you want to make sure it’s a good one.

Feeding for animals that don’t move can be a bit of a problem; waiting for enough plankton to drift by is not very efficient. Giant clams have a solution for this — they host algae inside their shells that provide the clams with the majority of their diet. The clam will open up during the day to provide the algae with enough sunlight to go properly. Younger clams have a large portion of plankton in their diet, but adult giant clams cannot survive without their algae friends.

Because they are large and interesting, people have hunted giant clams for both food and for the pet trade. This has severely reduced giant clam numbers, but aquaculture efforts are in place to preserve the species. Hopefully this can keep these big guys around for a while!

Cover image by Bernard DUPONT from FRANCE, CC BY-SA 2.0, via Wikimedia Commons