Comprehensive Description

The small giant clam is composed of two shells which are attached to the substrate by the byssus. The byssus is a fibrous, filamentous tissue used by a wide range of bivalve to stay attached to rocky substrate. Mussels, clams, and pen shell are some examples. They are commonly found in shallow reef habitat up to 20m depth, often encrusted in coral boulders, dead corals and rocks. When opened, the mantle goes out of the shell and covers the edges. This allows the dinoflagellates algae living within the mantle tissue to undertake photosynthesis.

Morphology of a clam. Adapted from Ellis (1998).

The giant clam has a bi-phasic lifecycle composed by a locomotory larval phase and a sessile phase, as most of marine invertebrates. Eggs become larvae after 10 to 15 hours spent in the water column. The larvae patterns are responsible for the wide distribution of the small giant clams. Larvae spend about 10 days as free-living in the water column, before they settle and metamorphose into a juvenile clam and acquire zooxanthellae symbiont (Ellis, 1998).

Originally, the small giant clam is a filter feeder. It uses the siphon to create a current and catch nutrients of the water. In addition, clams have developed an intricate symbiosis with zooxanthellae. Thus they use by-products of the photosynthesis of those micro algae as food sources.

Clams have a well developed sensory system for a bivalve. They have many eyes along the edges of the siphon (Wilkens, 1986). Those eyes are relatively primitive but allow them to detect rapid changes of luminosity. Communication between individuals (via pheromones) is not documented and other behaviors are not known yet.

The small giant clam can be subjected to predation because the mantle is constantly out of the shell. However, some defensive mechanisms minimise the predation. The first one is the retraction of the clam in its shell when eyes detect a potential predator, basically a shadow. The second one is the expulsion of water thanks to the excretory siphon. But both of those mechanisms are consuming energy and reduce the potential for photosynthesis. The last mechanism is a camouflage, clams are often colored as the immediate surrounding reefs or environments to avoid being seen by predators (Todd, 2009).

As the other bivalve, clams are using their siphon to move away their wastes, however some are caught by the dinoflagellates for photosynthetic purposes. On the other hand, the siphon is also used to bring oxygen to gills and nutrients for both the clam and its symbiont.

The locomotion of the clam is reduced to the 10 first days of its life, as a larvae.

Clams start their life as male, and become hermaphroditic after about 2 to 3 years (Ellis, 1998). They are broadcast spawner. However, they release sperm in the water column before, to avoid self-fertilization. Spawning events are thought to be regulated by lunar cycle, day-time and chemical cue from other clams in the water column.