GIANT CLAMS TRANSFORM LIGHT FOR ALGAE
According to reports in ScienceDaily and Journal of The Royal Society Interface, giant clams live in shallow tropical seas, and are noted for their brilliantly colored mantles which contain algae (single celled plants). This is a symbiotic relationship that seemed to be more advantageous to the clam, as the algae supply the clam with nutrients, but the clam seemed simply to provide a place for algae to live. However, scientists have now discovered the clam provides more than just dwelling quarters – it provides ideal conditions for algal photosynthesis. Like all plants the algae do not use the entire spectrum of sunlight – they use the blue and red parts. Furthermore, strong sunlight can damage them and the light penetrating the shallow tropical seas can be too intense for them. Now we know that the clam filters the light so it is less intense, but still allows the right wavelengths for photosynthesis to get to the algae. How it does this is fascinating. The algae in the clam are stacked in vertical pillars, which sit under a layer of cells called iridocytes. The pillars seem to be orientated in the wrong direction for absorbing sunlight, but the iridocytes scatter the incoming light so that the red and blue parts of the sunlight are spread out in a cone below them. This means the light is directed to the sides of the algal pillars, making it available to all cells in the stack. The other wavelengths not used by the algae are reflected back and away from the algae by the iridocytes. This reflected light gives the clam mantles their bright iridescent appearance. One of the researchers, Daniel Morse of University of California, Santa Barbara, explained what an effective system this is for the algae and clam: “This provides a gentle, uniform illumination to the vertical pillars consisting of the millions of symbiotic algae that provide nutrients to their animal host by photosynthesis. The combined effect of the deeper penetration of sunlight – reaching more algae that grow densely in the 3-dimensional volume of tissue – and the ‘step-down’ reduction in light intensity – preventing the inhibition of photosynthesis from excessive irradiation, enables the host to support a much larger population of active algae producing food than possible without the reflective cells”. The researchers comment that the iridocytes work like a step-down electrical transformer and a Bragg mirror.
For those who don’t know the term, a Bragg mirror is a reflector made from multiple thin layers that selectively reflects only certain wavelengths of light. It only works if the layers are the right thickness to separate out the desired wavelengths of light. A step down transformer is a device that changes high voltage electricity from a generator or substation to a lower voltage appropriate for lighting and appliances in homes and buildings. Neither of these devices evolved by chance, but came about by clever engineers manipulating matter to achieve purpose that is not understood by the devices themselves. The brilliant functional system of clam and algae is a reminder that symbiosis is evidence for plan and purpose, not chance random processes. There is no need for clams to have cells that work like Bragg mirrors unless there is a purpose for modifying the incoming sunlight. Unless the algae in the pillars are bathed in the filtered light provided by iridocytes they could not photosynthesise properly. It is only when they are put together that they both benefit.
In the abstract (summary of their research) the scientists went to great pains to insist the giant clam and algal symbiosis evolved. However, this is a belief that has been imposed on the research, and not an actual scientific observation. The scientists were able only to observe how this system works now. They did not observe how it came into being. To see what we mean we suggest you read the abstract (click on Royal Society link above) and ignore the word “evolved” in the first and fourth sentences, as well as the words “highly evolved” in the last sentence. The description of what the scientists actually found is what’s left when you delete these words. Remember this when anyone claims that biology cannot be understood without belief in evolution.