We all know that giant clams of the Tridacna and Hippopus genera are fast-growing molluscs which are found in shallow water reef waters where they can bask in the sunlight, harvesting energy from their symbiotic zooxanthellae. Unlike corals which have Symbiodinium zooxanthellae which live inside the cells of their hosts, in giant clams the zooxanthellae live in specialized canals that are part of the gastric system, and can be regulated and harvested by the clam at will.
It’s easy to imagine the zooxanthellae of giant clams as being spread evenly across the surface of the clam’s mantle which points towards the life-giving sunlight. However, in giant clams the zoox are arranged vertically in three dimensions throughout the mantle and tissue of the clam, but how does it get light to penetrate its opaque tissues?
New research undertaken by Allison Sweeney of the University of Pennsylvania has demonstrated that giant clams have special adaptations that encourages light to penetrate deep into the flesh of the giant clam. Special living irridescent cells called iridocytes actually help to gather and concentrate light to reach all regions of the giant clams’ three dimensional, internal algae farm.
As you know there’s amazingly beautiful and brightly colored giant clam specimens, and then there are some duller specimens without color, and the iridocytes which help to concentrate light. By comparing giant clams with and without these living light-harvesting cells, Dr. Sweeney found a large effect from the presence of the iridocytes: “Clam tissue with iridocytes has about fivefold more particles of light, called photons, deep inside the tissue than clam tissue without iridocytes does, they found”.
On the surface it would seem that more colorful clams may need less light than their uglier, browner counterparts. But on the flip side we know that clams can become more colroful when placed in brighter lighting so perhaps there’s more dimension to this aspect of light harvesting in giant clams that we have still yet to learn. So the next time you see a giant clam, in your aquarium, at the LFS or in some other display, just remember that you’re looking at a truly solar powered bivalve that is way more evolved for its lifestyle than we could ever imagine. [LiveScience]