Article: The origin of algal-bivalve photo-symbiosis

The photo-symbiotic bivalves Fragum fragum and Fragum loochooanum burrow in sediments and supply light through a posterior shell gape to zooxanthellae within their internal soft parts. This newly discovered mode of photo-symbiosis in bivalves can be termed sciaphilous (shade loving), and the hitherto known one, in which bivalves expose mantles or transparent shells out of the sediment to harvest light, as heliophilous (sun loving). Fragum unedo, also examined here, is heliophilous. Sciaphilous photo-symbiosis in F. fragum is enabled by the zooxanthellae's low compensation point of photosynthesis (50 uEinstein 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 symbiosis. Sciaphilous photo-symbiosis allows bivalves to profit from photo-symbiosis without risking predation or epibiont attachment, and thus may have been common among fossil photo-symbiotic bivalves. The disproportionately rapid increase in the length of the posterior shell gape and the very rapid decrease of the angle between the posterior and ventral valve margins during the growth of two sciaphilous Fragum species, which ensure effective light harvesting by the zooxanthellae, can be used as criteria in searching for fossil sciaphilous microbial-bivalve photo-symbiosis.