Scuba Diving with a Glorious Giant Clam Closeup on the Great Barrier Reef near Port Douglas
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The giant clam, Tridacna gigas (known as pā'ua in Cook Islands Māori), is the largest living bivalve mollusk. T. gigas is one of the most endangered clam species. Antonio Pigafetta documented these in his journal as early as 1521. One of a number of large clam species native to the shallow coral reefs of the South Pacific and Indian oceans, they can weigh more than 200 kilograms (440 lb), measure as much as 120 cm (47 in) across, and have an average lifespan in the wild of 100 years or more.[2] They are also found off the shores of the Philippines, where they are called taklobo, and in the South China Sea in the coral reefs of Sabah (Malaysian Borneo). T. gigas lives in flat coral sand or broken coral and can be found at depth of as much as 20 m (66 ft).[3] Its range covers the Indo-Pacific, but populations are diminishing quickly and the giant clam has become extinct in many areas where it was once common. T. maxima has the largest geographical distribution among giant clam species; it can be found in high- or low-islands, lagoons, or fringing reefs.[4] Its rapid growth rate is likely due to its ability to cultivate algae in its body tissue.
The genus Symbiodinium encompasses the largest and most prevalent group of endosymbiotic dinoflagellates known to science. These unicellular algae commonly reside in the endoderm of tropical cnidarians such as corals, sea anemones, and jellyfish, where they translocate products of photosynthesis to the host and in turn receive inorganic nutrients (e.g. CO2, NH4+) (Fig. 1). They are also harbored by various species of sponges, flatworms, mollusks (e.g. giant clams), foraminifera (soritids), and some ciliates. Generally, these dinoflagellates enter the host cell through phagocytosis, persist as intracellular symbionts, reproduce, and disperse to the environment (note that in most mollusks, Symbiodinium are inter- rather than intra-cellular). Cnidarians that are associated with Symbiodinium occur mostly in warm oligotrophic (nutrient-poor) marine environments where they are often the dominant constituents of benthic communities. These dinoflagellates are therefore among the most abundant eukaryotic microbes found in coral reef ecosystems.
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