76.1 Friday, Jan. 6 Giant clam iridocytes optimize photosynthetic symbiosis SWEENEY, A.M.*; HOLT, A.L.; GAGNON, Y; MORSE, D.E.; Univ. of California, Santa Barbara; Univ. of California, Santa Barbara; Duke University; Univ. of California, Santa Barbara email@example.com
Symbioses between metazoans and the brown alga Symbiodinium are a major component of the coral reef ecosystem. While the algal symbiosis with corals is the most famous, giant clams in the genus Tridacna also exchange nitrogen and carbon with Symbiodinium in their tissues, allowing the clams to grow to their storied size. These clams are also valuable in the aquarium trade for the bright, varied color patterns caused by iridocytes in clam tissue, which are closely spatially associated with actively metabolizing brown algae. Although a few hypotheses about the function of these iridocytes have been advanced in the literature, their role in clam physiology is not well understood. Based on detailed histological and optical measurements of these clams, we developed a monte-carlo based optical model to investigate the function of iridocytes in giant clams. Our results show that the clam’s iridocytes create a deep layer within the clam tissue where irradiance is optimized for the photophysiology of Symbiodinium. Without the iridocytes, only a thin layer of algae deep within the clam could function at an irradiance level that would not cause algal bleaching, thereby not providing enough photosynthate to the clam to support it. On the other hand, our models show that other plausible but non-observed optical arrangements in the clam cause too much shading. Our results show that the distribution and makeup of iridocytes in clam tissue is a precisely optimized optical solution allowing these symbiotic organisms to efficiently harvest photons in some of the most intense light environments on earth without bleaching symbiotic algae.