Here’s a video of one of our ORA Tridacna derasa clams moving its food in order to reposition itself in the sand. This behavior is very common for juvenile clams, and one I’ve seen countless times at fish stores and in my personal aquariums. However, this is the first time there has been a camera handy, so of course we had to capture it on video. The video is a two part-er, with the first half being zoomed out. The second half of the clip was shot with a macro lens, allowing us to view the details of the foot.
The clam uses its foot for locomotion, trying to find that perfect spot in the aquarium or on the reef where it can get enough light for photosynthesis while also staying out of strong currents. It obviously cannot move very fast, but Tridacna clams can move several feet on one day if they are determined. This species of clam grows quite large, up to 20 inches in fact, and once it gets to a certain size, it will no longer rely on this foot near as much. In fact, the byssal opening from which the foot emerges gradually closes up as the clam gets larger.
About halfway through the video, a small hermit crab can be seen wandering about the aquarium on the hunt for a new residence. These crustaceans use shells from snails and other reef invertebrates for protection, and as they grow, they need to replace their shells with a larger one. The hermit happens upon an abandoned shell near the clam, crawls over it and sizes it up, but apparently finds it to be unsuitable.