Diversity of photo-symbiotic bivalves and modeling of their diversification

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Bivalves with symbiotic zooxanthella (photo-symbiotic bivalves) have been known since detailed study by Yonge (1936). These occur amongst genera of Tridacna (Yonge 1936), Hippopus (Yonge 1936) and Corculum (Kawaguti 1941) , all of which are Indo-Pacific tropical shallow water dwellers (for an illustration of representative photo-symbiotic bivalves.All the known photo-symbiotic bivalves belong to the superfamies Cardiacea and Tridacnacea which are closely related to each other. The majority of the bivaives of the superfamily cardiacea adopts an infaunal mode of life. Thus the ancestors of the photo-symbiotic bivalves mentioned above were most likely infaunal bivalves . This raises the question how the symbiotic relationship originated between the infaunal ancestors of these bivalves and the light-demanding photosynthetic zooxanthellae?Our invesitgation revealed the following points. Photo-symbiotic Fragum fragum and Fragum loochooanum burrow completely in sediments and supply light thro … More ugh a posterior shell gape to zooxanthellae of their internal soft parts. This new mode of bivalve's photo-symbiosis can be termed as sciaphilous (shade loving) , while the hitherto known one as heliophilous (sun loving), in which bivalves expose mantles or transparent shells out of sediments to harvest light. Presently examined Fragum unedo is heliophilous. Sciaphilous photo-symbiosis of F.fragum is enabled by the zooxanthellae's ability to begin photosynthesis at light intensity (50 m-Einstein m-2s-1) by far lower than the ambient light intensity of their habitat.Thus the zooxanthellae's preadaptation to low Hght intensity might have playd an important role in originating algal-bivalve symbiosis. Sciaphilous photo-symbiosis allows bivalves to enjoy profit of photo-symbiosis without risking predation or epibiont attachment, thus may have been popular among fossil photo-symbiotic bivalves. The disproportionately rapid increase in the length of posterior shell gape and the very rapid decrease of the angle between posterior and ventral valve margins during the growth of two sciaphilous Fragum species, which ensure zooxanthellae's effective light harvesting, can be used as criteria in searching for fossil sciaphilous algal-bivalve photo-symbiosis. Less