THE ORIGIN OF ALGAL-BIVALVE PHOTO-SYMBIOSIS

The photo-symbiotic bivalves Fragum fragum and Fragum loochooanum burr ow in sediments and supply light through a posterior shell gape to zoo xanthellae within their internal soft parts. This newly discovered mod e of photo-symbiosis in bivalves can be termed sciaphilous (shade lovi ng), and the hitherto known one, in which bivalves expose mantles or t ransparent shells out of the sediment to harvest light, as heliophilou s (sun loving). Fragum unedo, also examined here, is heliophilous. Sci aphilous photo-symbiosis in F. fragum is enabled by the zooxanthellae' s low compensation point of photosynthesis (50 mu Einstein m(-2) s(-1) ), a point far lower than the ambient light intensity of their habitat . The zooxanthellae's pre-adaptation to low light intensity might have played an important role in originating the zooxanthella-bivalve symb iosis. Sciaphilous photo-symbiosis allows bivalves to profit from phot o-symbiosis without risking predation or epibiont attachment, and thus may have been common among fossil photo-symbiotic bivalves. The dispr oportionately rapid increase in the length of the posterior shell gape and the very rapid decrease of the angle between the posterior and ve ntral valve margins during the growth of two sciaphilous Fragum specie s, which ensure effective light harvesting by the zooxanthellae, can b e used as criteria in searching for fossil sciaphilous microbial-bival ve photo-symbiosis.