Clams are bivalves. Each half of a clam’s shell is properly called a valve, and the “bi” part is obviously added to it because there are two of them. Like coral skeletons, these valves are composed of calcium carbonate, and are produced by the living tissues of the clam. However, clams are about as distantly related to corals as an animal can be. While corals are members of the Phylum Cnidaria and are relatively simple animals, clams belong to the Phylum Mollusca. This phylum is also home to the snails, cephalopods, and a several other invertebrates that are far more complex than the corals and their kin.
Some of these molluscs swim around all the time while some crawl about. Some of them, such as the infaunal clams, live their lives buried in the substrate, too. However, I’ll be sticking to the epifaunal clams, which are the ones that live on the bottom rather than in it, or attach themselves to rocks, macroalgae, or other invertebrates, such as corals, etc. And with that said, I want to get back to complexities for a moment.
Clams don’t appear to be very complicated animals while just sitting around on the bottom of the sea or an aquarium. However, they actually have a full set of well-developed specialized organs. They have complex gills, a mouth, stomach, and intestines, a heart, kidneys, ovaries and/or testes, a well-developed (albeit brainless) nervous system, and more. Many of them even have eyes. So, they’re far more complicated than they might seem.