Hill R W, Dacey J W, Edward A
Department of Zoology, Michigan State University, East Lansing 48824, USA.
Biol Bull. 2000 Oct;199(2):108-15. doi: 10.2307/1542870.
The tridacnid clams maintain symbiotic associations with certain dinoflagellates (termed zooxanthellae). Tridacnids are thus candidates to have high tissue concentrations of dimethylsulfoniopropionate (DMSP), a tertiary sulfonium compound that is not synthesized by animals but is commonly produced by dinoflagellates. This study establishes that DMSP is about an order of magnitude more concentrated in the light-exposed and shaded mantle and gills of Tridacna maxima and T. squamosa than in any other known animal tissues. The DMSP concentration in the light-exposed, siphonal mantle--the location of most zooxanthellae--is an inverse function of body size, paralleling an inverse relation between apparent density of zooxanthellae (measured as pheophytin concentration) and body size. The shaded mantle and gills are high in DMSP despite having low densities of zooxanthellae, indicating that high DMSP concentrations occur in molluscan tissue, not just in algal cells. DMSP is almost an order of magnitude less concentrated in the adductor muscle than in other tissues. The high DMSP concentrations found in tridacnids, by providing abundant substrate for formation of volatile dimethylsulfide, probably explain the peculiar tendency of tridacnids to rapidly develop offensive odors and tastes after death: a serious problem for their exploitation as food. Tridacnids are the one group of animals in which DMSP concentrations are high enough in some tissues to be in the range capable of perturbing enzyme function at high physiological temperatures. Thus, tridacnids may require enzyme forms adapted to DMSP.
砗磲蛤与某些甲藻（称为虫黄藻）保持共生关系。因此，砗磲蛤有可能在组织中含有高浓度的二甲基巯基丙酸内盐（DMSP），这是一种叔锍化合物，动物自身并不合成，但通常由甲藻产生。本研究证实，在大砗磲和鳞砗磲暴露于光照和遮蔽的外套膜及鳃中，DMSP的浓度比其他任何已知动物组织中的浓度高约一个数量级。在暴露于光照的虹吸管外套膜（大多数虫黄藻所在位置）中，DMSP浓度是体型大小的反函数，这与虫黄藻的表观密度（以脱镁叶绿素浓度衡量）和体型大小之间的反比关系相似。尽管遮蔽的外套膜和鳃中的虫黄藻密度较低，但其中DMSP含量却很高，这表明高浓度的DMSP存在于软体动物组织中，而不仅仅是藻类细胞中。闭壳肌中的DMSP浓度几乎比其他组织低一个数量级。在砗磲蛤中发现的高浓度DMSP，通过为挥发性二甲基硫的形成提供丰富底物，可能解释了砗磲蛤死后迅速产生难闻气味和味道的特殊倾向：这对将其作为食物开发来说是个严重问题。砗磲蛤是一类动物，其某些组织中的DMSP浓度高到足以在生理温度较高时干扰酶功能。因此，砗磲蛤可能需要适应DMSP的酶形式。
