Happy as a Clam

Another Lens on Life blog from Chuck Kopczak, PhD

11/9/2020
Kaleidoscopic patterns in black, gold and white cover the surface of this giant clam's flesh, called its mantle
Chuck Kopczak, PhD

Staring down the exhalant siphon of a fluted giant clam (Tridacna squamosa)

Today’s photo was taken on Rainbow Reef off Vanua Levu in the Republic of Fiji. It was taken with a Canon EF100 mm f/2.8 Macro USM lens on a Canon EOS 5D Mk. III in an Ikelite underwater housing. Lighting was provided by two Ikelite DS161 strobes in eTTL mode. The exposure was set at 1/60 sec. at f/16 and ISO 1600.

Are clams happy? It’s an age-old saying, but how would one determine the mental state of a clam? Needless to say, there probably isn’t a way to determine the happiness of a clam, or even if they have anything we’d call a mental state. But life for a clam may not be all that bad. Clams are classified in the same large group of animals with snails, slugs, and octopus, squid, and cuttlefish. Pretty diverse group, huh?

All these species belong to the group known as Mollusca, the mollusks, a group for which the major relationships between the various subgroups have yet to be worked out. About the only characteristic of mollusks upon which there is more, or less uniform agreement is that they all have soft, unsegmented bodies, usually with a “head” and “foot” region. Beyond that, some have hard exoskeletons like the shells of snails or clams, but other don’t.

Today’s photo is of the famous, or perhaps infamous, giant clam. This particular species is the fluted giant clam (Tridacna squamosa). The genus Tridacna includes the largest of the organisms known as bivalves, or those species who live within a pair for hinged shells, like clams and mussels. In spite of being thought of as deadly organisms capable of closing down on an unwary human snorkeler and holding them underwater until they drown, these clams actually close their twin shells quite slowly. You’d have to purposefully hold your foot or hand in place for quite a few seconds before the shells might begin to pinch closed on you.

Like all other clams and mussels, the giant clam is a filter feeder. They draw in seawater through one opening in the body, pass it along their gills where small organisms are strained out, and then pass it out through a tube-like siphon. In the fluted giant clam, all the patterned flesh that you see between the shells in the photo is part of the mantle. The patterns of sometimes bright colors on the giant clams is the result of the presence of single-celled marine algae in the tissue that photosynthesizes in the sunlight, passing along some of the material they create to the clam. In addition to the algae, known as zooxanthellae, the mantle contains other colored compounds that act as sunglasses, shading the algae from harmful UV radiation.

So, having the ability to filter their own food from the water, plus harboring a host of algae making food for them, it is probably safe to say that clams lead a pretty happy existence.

Sources

Humann, P., and DeLoach, N. (2010). Reef Creature Identification Tropical Pacific. Jacksonville, FL: New World Publication, Inc.

http://www.ucmp.berkeley.edu/taxa/inverts/mollusca/mollusca.php