KIRKENDALE, Lisa; University of Florida: Evolution of photosymbiosis in the cardiid subfamily Fraginae

In the marine clam family Cardiidae, two closely related subfamilies (Tridacninae and Fraginae) exhibit divergent morphological responses to a similar selective pressure- light capture for photosymbiosis. All giant clams maximize light capture via a similar suite of morphological characters, including mantle hypertrophy, pronounced valve gaping, and expansion of the posterior area. In contrast, the more diverse but poorly characterized Fraginae have travelled a much different route. Photosymbiotic taxa have responded to the same selective pressure of maximizing light capture by assuming a wide diversity of morphologies, including modifications to shell shape, shell size, shell microstructure and behavior. Three mitochondrial (16S, COI, CytB) and one nuclear (28S) gene regions were assembled to understand the relationships among over thirty photosymbiotic and non-photosymbiotic Fraginae taxa and key characters were then mapped onto this phylogeny. Briefly, I find evidence that 1) the Fraginae genera presently circumscribed are not monophyletic, 2) there has been a single origin of photosymbiosis and 3) there has been a single origin of certain novel characters important in photosymbiosis, including window shell microstructure.