The Giants of the Ocean Bed

Some of the largest immobile molluscs of the world are the longest living creatures of the coral reef. They are the greatest treasures of the deep – the giant clams. Known to live between 60 to 200 years, they provide a rich ecological backdrop for marine life under the sea.


A Juvenile of Tridacna squamosa 

My first ever interaction with marine life was way back in 1970 at our family beachfront house in Sasvane, a small coastal village 120km away from Mumbai. I was just five, and my cousin and I would take strolls on the beach where I would be fascinated by the seashells. From then till today, my obsession for the oceans took me around the world, ultimately turning me into an ambassador for their conservation.  In the past two decades,  I managed to see every coastal state of India, including the islands of Lakshadweep and the Andamans and learnt a great deal by interacting with the coastal fishing community. From ‘non-page 3’ species like sea anemones and sea slugs to ‘page 3’ species like sharks and whales, coral reefs have everything to offer and each creature tells us a remarkable story of evolution. The unending wealth of our oceans makes us wonder how careless and abusive we are in our attitude towards it.  Even after 5,000 dive hours and photographing over 10,000 species, I know there is still a lot to be seen.


A close-up shot of Giant clam mantle showing the iridophores and gills

The story of the giant clam is just one piece of the big unresolved jigsaw puzzle in the larger picture of the coral reef.  So, to understand giant clams, you first need to understand coral reefs and to understand coral reefs, you need to go underwater.


Corals: a rich ecosystem

Coral reefs are the planet’s biologically richest ecosystems with its denizens so colourful and patterns so intricate that it could rival the skills of any professional fashion designer. Of course, some creatures are drab, dull, slow, sluggish, and go unnoticed, but if you follow a naturalist ’s instinct, these drab creatures will teach you remarkable lessons of survival in a realm that is full of predators and environmental uncertainties.

Coral reefs are formed by tiny marine animals called polyps, which extract calcium carbonate from seawater to form a remarkable ecosystem.  Millions of polyps form coral colonies and each colony can be several years old. Corals belong to the phylum Coelenterata (a group of animals with stings). This phylum has 9,200 species out of which corals constitute about 1,000 of which about 793 are reef-building corals. Corals fall under the class Anthozoa  (anthos means flower and zoios means animals;  thus they are flower-like animals). There are two sub-classes: Octocorallia – corals having eight tentacles – and Hexacorallia – corals having tentacles in multiples of five or six (usually, they have 12  tentacles).


Hexacorallia: A Stony coral (Acropora formosa)

The highly colourful coral gets its colours from the polyps and not from the skeleton, which is white.  Polyps are mostly pink, yellow,  brown, blue or green. The colour is primarily attributed to the Zooxanthellae (unicellular algae),  which the polyps harbour in their tissues. But you need artificial illumination to see the colours.  Red-orange colour spectrums get absorbed into the water below depths of   10m so without artificial illumination, a coral reef looks like a green-blue monochromatic seascape.


The  three  types of reefs  that  occur In various parts of the world


  • Fringing reef: This rocky reef piles up along the edges of the continent or around an island close to the Sometimes,   a shallow water lane called a lagoon separates the reef and the shore (eg: Gulf of Mannar).
  • Barrier reef: These develop along the edges of continental shelves or around islands that have partially submerged and are separated from the mainland or island by a wide and deep lagoon  (eg: The Andaman and Nicobar islands).
  • Atoll: In the beginning, fringing redeveloped around the coast of volcanic As the island subsided, either due to the sinking of the sea floor or due to a rise in the sea level,  the fringing reef forms a circular barrier reef and is separated from the island by a lagoon.  On the total submergence of the island, the original lagoon around the islands merges into a single expanse of water.  This whole formation is called an atoll (eg: Lakshadweep reefs).


The story of Giant Clams

Giant clams reside in the coral reefs.  They belong to a group of animals commonly called bivalves (animals having two hinged valves). These majestic boneless animals are a part of the phylum ‘mollusca’ (soft-bodied animals) and are commonly seen in tropical reefs in shallow waters. Giant clams are impressive in many ways. They can grow over a metre and are the largest living molluscs.  These are known to live between 60  to 200 years and are among the longest living animals. They remain fixed to the ocean floor and reach sexual maturity between  6  to 20 years, depending on the species. Their fluorescent-coloured mantle undoubtedly catches your attention when you dive or snorkel in shallow coral reefs. These animals have developed the ability to cultivate plants in their body tissue (usually called Zooxanthellae) demonstrating an unusual phenomenon of endosymbiosis, which can be seen in many animal species. Giant clams are however unique because they have improved this strategy significantly;  in contrast to other animals, they cultivate Zooxanthellae in a special circulatory system. Though these animals release millions of eggs and sperms each year, only two to 10 young clams (per hectare) ultimately manage to survive and grow. Of these,  probably only one to two clams will reach adulthood.  All these highly specialised characteristics make them highly vulnerable to any kind of stress.  Three species of giant clams are protected in India under schedule  I  of  Wildlife (protection) act, 1972. These are also protected in over 80 countries where they occur.  Giant clams are listed in CITES  Appendix  II since 1985.  Species listed in  Appendix  II are deemed not currently threatened with extinction but are at risk of becoming so unless trade is regulated.

Coral Reef

A stony coral reef


Between myth and science

In the western world, giant clams remained a lore in adventure stories. Several horror movies and novels depicted these animals as dangerous beasts ambushing divers. The myth is mainly due to their large size. Though there is little truth in these stories, there are freak accidents, due to unintentional overstepping by divers. These large animals with powerful adductor muscles and heavy valves can seriously damage human fingers or feet. This is mainly due to the natural tendency of the clam to close its valves with the slightest touch to its mantle. The best way to avoid freak accidents is to keep a safe distance. These animals are purely plankton feeders (they feed on microscopic animals and plants by filter feeding) and have no taste for human flesh.

Ten species of giant clams are known the world over.  Of the ten species, five are known to occur in India. The most remarkable feature of the giant clam is its fluorescent and

colourful mantle, making it the most sought-after mollusc in the marine aquarium trade.  While close observation reveals tiny iridophores, which are nothing but eyes of the giant clam, there is no explanation as to why the mantle shows such a  variation in colour. Since giant clams are fixed to a solid substrate, they have adopted a special mode of reproduction called broadcast spawning, meaning they release eggs and sperms in open water. To compensate for the high mortality of larval life, clams produce an enormously large number of eggs and sperms, exceeding 500 million eggs per clam.

Like most wildlife, giant clams are also under severe stress globally. They are considered a delicacy, including sashimi,  across  South-east  Asia, causing populations to crash beyond recovery in many places while becoming extinct in some. The  Chinese pay a fortune for the adductor muscle, which is considered to be an aphrodisiac.  Empty shells of giant clams are traded in large numbers the world over as souvenirs due to their impressive look. empty shells of the largest species of giant clams  (Tridacna gigas) were even used as bathtubs for children.  Internationally, giant clams are used commercially as aquarium specimens, shells and shell-craft.  Besides these,  there are other biological and ecological stress factors, which affect giant clams like the fast deterioration of coral reefs, polluted waters, fishing in shallow reefs and death from boat anchors.  Giant clams spawn in a synchronised way;  both the male and female release eggs and sperms simultaneously and thus fertilisation occurs in water. Thus, if adult clams are dispersed far from each other, chances of fertilization are poor.  Larvae of giant clams are free floating and will drift with water currents for up to 70 hours. If a suitable habitat or substrate is not available during its settlement on the sea floor, these larvae will die. Giant clams cultivate Zooxanthellae in the mantle. Thus, they need two important environmental conditions:  a) shallow waters, which allow enough sunlight for Zooxanthellae for photosynthesis; and b) water temperature regimes between 18-25°C   for optimal growth of Zooxanthellae.   In the event of rising temperatures  (el nino), the Zooxanthellae will leave the giant clams, in turn killing them.  It is recorded that giant clams reach maturity very late (over 20  years). This, coupled with the harvesting of juveniles, is a critical factor for its large-scale decline in many parts of the world.  Juvenile clams are much more beautiful with intricate scales on the valves and thus most sought after as souvenirs. Thus, large populations of giant clams are harvested even before reaching maturity, which is also responsible for thinning adult population.


Hexacorallia: Sea anemone

I have been studying population ecology,  biology and conservation of giant clams in Lakshadweep since 2004. The project has now been extended to another fascinating destination,  the Andaman and Nicobar islands.  The programme  ‘Project  Giant Clam’ – being executed in collaboration with the Ministry of Environment and Forests, Government of India and the Whitley Fund for nature – aims to conserve giant clams through community participation.

For me, each year brings hope and discovery.   I am sure,  the next time you find your way to the edge of the sea, you will listen to the stories the waves have to tell you.


This article was originally published in Sep-Oct 2013 of Saevus Magazine

Read also: Revisiting the moments in the wild 

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About the Author /

Director at the Bombay Natural History Society and a Doctorate in Marine Ecology, Deepak has authored two books, The Book of Indian Shells and Field Guide to Marine Life of India and has 35 scientific publications to his credit like Nature, Evolution, Systematic and Biodiversity and JMBA (UK). Deepak is a member of Task Force on Marine Biodiversity Conservation set up by the Ministry of Environment and Forests. He has won prestigious awards like the Whitley Conservation Award, Whitley Continuation Award and is a Fellow of LEAD India and Smithsonian Environmental Leadership Programme and many other communities.

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