A Giant Clam, Tridacna gigas (Photo by Stig Thormodsrud)

Today we celebrate the world’s largest bivalve mollusk, the magnificent and world-famous giant clam (Tridacna gigas).  Native to shallow coral reefs of the South Pacific and Indian oceans, giant clams can weigh up to 500 lbs and measure 50 inches across.  Huge specimens can be very ancient and some have lived for more than a century. Giant clams are hermaphrodites: every individual possesses both male and female sex organs–however a clam is incapable of mating with itself.   They are broadcast spawners producing vast numbers of gametes which they release in response to certain chemical transmitter substances. During these spawning events (which usually occur in conjunction with certain lunar phases) a single clam can release over 500 million eggs in one evening.  Giant clam larvae then swim free among the plankton.  They pass through several mobile transition phases before settling down in one favorite home (as can be seen in the comprehensive life cycle drawing below).

Giant Clam Life Cycle (After H.P. Calumpong, ed. 1992 "The Giant Clam: an Ocean Culture Manual")

As usual for sea creatures, the giant clam has a troubled relation with humankind.  Fabulists have asserted that the great bivalves chomp down on divers for food or out of spite (the clams do slowly shut when harassed, but the movement is a defense mechanism and happens gradually).  They are considered delicacies on many South Pacific islands and naturally the insatiable Japanese pay a premium to eat them as “Himejako”.  Their shells also command a premium from collectors.  Across the South Pacific, giant clams are dwindling away thanks to overfishing, reef destruction, and environmental factors.

Divers with a Giant Clam: Bikini Atoll in the Marshall Islands

It is sad that the gentle and lovely giant clam is suffering such a fate (although aquaculture is now bringing a measure of stability to some populations).  In addition to being beautiful and useful to ecosystems, they are remarkable symbiotic creatures.  A unique species of algae flourishes in the mantle of the giant clam and the clam gains much of its energy and sustenance from these photosynthetic partners. The clam possesses iridophores (light sensitive circles) on its flesh which allow it to gauge whether its symbiotic algae is getting enough sunlight–and perhaps watch for predators.  It can then alter the transparency of its mantle flesh accordingly. According to J. H. Norton, giant clams have a special circulatory system to keep their symbionts alive and happy. The happy and beneficial relationship between a clam and its algae allows the former to attain great size and the latter to remain alive in the ever-more competitive oceans.  I have concentrated on writing about T. Gigas, but there are many other members of the Tridacninae subfamily which lead similar lives (although they do not attain the same great size).  To my eye they are all remarkable for their loveliness.

Tridacna Maxima (in a home aquarium)

Tridacna Derasa (in a home aquarium)