Diversity of photo-symbiotic bivalves and modeling of their diversification

具有共生虫黄藻的双壳类（光共生双壳类）是由Yonge（1936）详细研究后发现的。这些发生在砗磲属（Yonge 1936）、河马属（Yonge 1936）和圆螺属（Kawaguti 1941）中，所有这些都是印度太平洋热带浅水居民（用于代表性光共生双壳类的例证）。所有已知的光共生双壳类都属于心壳类和砗磲类超科，这两个超科彼此关系密切。心脏超科的大多数动物都以动物的方式生活。因此，上述光共生双壳类的祖先极有可能是水生双壳类。这就提出了这样一个问题：这些双壳类动物的动物祖先与需要光的光合作用的虫黄藻之间的共生关系是如何产生的？我们的调查揭示了以下几点。光共生的Fragum Fragum和Fragum loochooanum完全在沉积物中挖洞，并通过其内部柔软部分的后壳开口为虫黄藻提供光。这种新的双壳类光共生模式可以被称为喜阴共生（sciaphilous），而迄今为止已知的是喜日光共生（heliophilous），在这种模式下，双壳类从沉积物中暴露出斗篷或透明壳来收获阳光。目前研究的Fragum unedo是嗜日生的。由于虫黄藻能够在远低于其栖息地环境光强度的光强（50 m-爱因斯坦m-2s-1）下开始光合作用，fragum的坐骨光共生得以实现。因此，虫黄藻对低强度的预适应可能在藻类与双壳类共生的起源中起了重要作用。坐骨光共生使双壳类既能享受光共生的好处，又不会有被捕食或附着于附生体的风险，因此可能在光共生双壳类化石中很受欢迎。在两种坐骨藻的生长过程中，后壳间隙长度的快速增加和后瓣缘与前瓣缘之间的角度的快速减小，保证了虫黄藻的有效光收集，可以作为寻找化石坐骨藻-双壳光共生的标准。少

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