Diversity of photo-symbiotic bivalves and modeling of their diversification

自Yonge（1936年）的详细研究以来，已知双壳类与共生虫黄藻（光共生双壳类）。这些都发生在Tridacna（Yonge 1936），Hippopus（Yonge 1936）和Corculum（Kawaguti 1941）的属中，它们都是印度-太平洋热带浅水居民（用于说明代表性的光共生双壳类）。心虫总科的大多数双壳类动物都采取了动物群内的生活方式。因此，上述光共生双壳类的祖先很可能是底栖双壳类。这就提出了一个问题，这些双壳类的动物群祖先和需要光的光合虫黄藻之间的共生关系是如何起源的？我们的调查揭示了以下几点。光共生的脆杆藻和绿脆脆杆藻完全在沉积物中潜穴，并通过沉积物提供光照， ...更多信息 一个后壳张开嘴，使其内部柔软的部分变成虫黄色。这种新的双壳类光共生模式可称为sciaphilous（喜阴），而迄今已知的一种为heliophilous（喜阳），其中双壳类将外套膜或透明壳暴露在沉积物中以收获光。目前研究的Fragum unedo是喜阳的。藻黄藻的喜光共生是由于藻黄藻在远低于其栖息地环境光强度（50 m-Einstein m-2s-1）的光强度下开始光合作用，因此藻黄藻对低光强度的预适应可能在藻-双壳类共生的起源中起重要作用。喜光共生使双壳类能够在不被捕食或附着的情况下享受光共生的好处，因此可能在化石光共生双壳类中很流行。在两种虾形藻的生长过程中，后壳裂孔长度的不成比例的快速增长和后壳与腹壳之间的夹角的快速减小，保证了虫黄藻有效的光捕获，可以作为寻找虾形藻-双壳类光共生化石的依据。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