Under the waves of the reefs and lagoons off the Indo-Pacific sits a gentle giant . It opens its shell wide and open, extending its flap like iridescent flesh out of the shell. Here our species of the day enjoys the warmth of the tropical sun, photosynthesising , thriving and flourishing.
The species featured today is 𝘛𝘳𝘪𝘥𝘢𝘤𝘯𝘢 𝘨𝘪𝘨𝘢𝘴 (Linnaeus 1758), the giant clam. With a size of up to 1.5 m in length, the giant clam is the largest bivalve, and a member of the Bivalvia class , which is a group of laterally-compressed soft-bodied animals with a hinged, two-part shell, including several thousands of other organisms like oysters, mussels and clams. These giants utilize different methods to obtain food. They rely heavily on the sun , and like corals, giant clams farm algae (zooxanthellae) thanks to their huge extended flesh (the mantle). They spread it out of their shell to maximize the area for sunlight absorption for photosynthesis. In other words, they have a symbiotic relationship with the zooxanthellae algae, with this latter producing sugars (food) for the clam, while the clam offers protection for the algae, by letting them live within their body. Other than the sun, they also actively filter feeds and eat the microscopic plankton in the water column.
Giant clams (Genus 𝘛𝘳𝘪𝘥𝘢𝘤𝘯𝘢) are not dull coloured like most of the other clams, in fact their mantle extended out of the shell comes in a wide array of iridescent and shiny colours , namely green, blue and brown. The brown comes from the photosynthetic algae housed in the mantle, but the blues and greens comes from iridescent cells called iridocytes. The bright colours that clams exhibit are not just for show, but serve a vital purpose in the photosynthesis of their algal residents. The bright sunlight of the shallow reefs are often so bright that it stresses and damages the algae, lowering the amount of food they produce or even injuring them. Thus the iridocytes come into play, they reflect away the damaging ultraviolet radiation and redirect some useful blue and green wavelengths of light into the clam, making every algae in the clam’s flesh equally receiving the right “type” (wavelength) and amount of sunlight. Hence the algae can produce as much food as possible for the clam.
Even though adult giant clams are giant, being a big clam which extends its flesh out of the shell also means that you are a free meal for clam-loving fishes like pufferfish and triggerfish. Thus the giant clams have developed a few strategies to protect themselves against these predators. They developed defense mechanisms from a young age. Juvenile giant clams frequently face predation by fishes and crabs, thus they have a lot of plate-like outgrowth from their shells (scutes) , forcing the crabs and fishes to use more energy to crush the shell before accessing the meat . This extra shell-crushing effort also deters predators to hunt them as crushing their sharp shells risks damaging their own claws or teeth.
When looking at a giant clam, you are also being stared at by 3000 pinhole eyes . In fact a giant clam has very primitive light and colour detecting eyes (pinhole eyes) around the mantle. Even though the clam’s vision can only detect light and form a rough shape , they are still very useful for their self defence. As the mantle is extended for photosynthesis, hungry fishes will nibble on it . When they see a shadow passing by they would rapidly tuck the mantle back into the shell. The speed of this reflex action is so fast that they were mistaken to close their shell as fast as tucking back their meat in ! And this is how the tales of the “killer clam” originated , the fear of divers’ hands or feet being stuck in the clam was so intense research divers’ guide in the 1970s provided details on how to escape from such an incident . In reality adults are incapable of fully closing their shells, leaving a gap which shows the meat in between.
So a clam unable to fully shut its shell sounds defenseless, but the giant clam has a trick! These clams are also capable of shooting out powerful jets of water aiming at the things that irritated them . The strength of these currents are capable of pushing small fishes away and large fishes stunned , thus it can buy more time to slightly close its large shell .
But how do they grow such large shells? Their shells have 2 layers, the upper layer and the lower layer. The upper layer grows away from the umbo (hinge) increasing the length of the shell , but the inner layer grows from the “inner side” of the shell , gradually increasing the shell’s thickness and weight .
Now that you have known how this giant grows. Come to the Hong Kong Biodiversity Museum to find the giant clams in the cabinet of the phylum Mollusca . The giant clams are also home to other organisms , you can try to find the corals that grew on the shells of our specimens. And if you enjoy this post, please share it with friends and follow us for more amazing biodiversity stories weekly.
Text: Hugo
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