The ocean’s giant clams

By Kirsty Melville 20 September 2017
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Don’t believe everything you hear about giant clams. They’re not the man-eating molluscs you think they are.

WHO REMEMBERS those old adventure movies where the hero narrowly escapes the clutches of a giant clam on the coral reefs of some exotic sea? It was certainly my first impression of what has been called by many divers the “killer clam”.

Whether or not the deadly tales are true, the giant clam deserves better press. It is, after all, the symbol of birth in Botticelli’s painting, The Birth of Venus, in which Venus stands on a giant clam shell.

The clam is also a very popular food among people of the Indo-Pacific region. Part of its flesh, the abductor muscle that stretches between the two valves of its shell, is a delicacy highly sought after in South-East Asia. Eaten either raw or as a dried flavouring in Chinese dishes, it wholesales at more than $30 a kilogram fresh weight in Taiwan. In South-East Asia conservative estimates of the demand for dried abductor muscle are 3000 tonnes a year.

Unfortunately, according to a 2010 study, as a result of habitat loss, technological advances in exploitation, growing trade networks, and demand by aquarists, giant clam numbers are in decline. 

So how does it achieve its elephantine proportions? 

It seems that this ability to be virtually self-sufficient in feeding habits is part of an amazing cycle of growth that sees the larvae, initially so small that they can only be seen under a microscope, grow to fully fledged adulthood. Giant clams are not, as previously believed, hundreds of years old, but are large because they grow fast. The most elderly giant clam you are likely to come across will have reached the sprightly age of 50 years old. On average, however, they live to be twenty.

Just how this mutually beneficial relationship works to produce such youthful but enormous clams is indeed fascinating.

Giant clams, like other members of the bivalved mollusc group such as oysters, scallops and cockles, are able to feed by filtering food particles into their system from the water passing over their gills. Unlike these other molluscs, giant clams are not dependent on filter feeding for their source of food. Instead they obtain virtually all their nutrients from the algae that grow in their mantle tissue. When these algae are exposed to sunlight they produce, by photosynthesis, food molecules that are released into the blood-stream of the clam. In turn, the waste products of the clam provide nutrients for these algae. So, the giant clam is ecologically a very sound creature.

The other interesting facet of this incredible clam is that it contains both male and female sex organs. Spawning initially involved the release of sperm and later hundreds of millions of microscopic eggs into the surrounding sea-water. When these eggs are fertilised they hatch into larvae and so the process of growth begins.