Giant clams.
Giant clams.
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巨型蛤蜊。
DOI：
10.1038/scientificamerican0475-96
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发表时间：
1975
影响因子：
3
通讯作者：
C. Yonge
中科院分区：
文献类型：
--
作者：
C. Yonge
当潮水从澳大利亚大堡礁的上表面退去时，一幅非凡的景色出现了。在几英里外的珊瑚头中，偶尔可以看到巨大的砗磲蛤的圆形顶部。这些巨大的双壳类软体动物，长可达4英尺，宽可达2英尺，高可达2英尺，它们的出现立刻引发了两个生物学问题。一个是：这么大的双阀是怎么产生的？另一个原因是，热带珊瑚礁的海水中矿物质营养物质非常缺乏，因此支持海洋动物生命的浮游生物也非常少。当充足的食物供应对像巨鳄这样的双壳类动物至关重要时，它怎么能在如此贫瘠的环境中生存呢？这些问题的答案是相关的。早在三叉目动物出现之前，第一批现代珊瑚就出现了，大量的珊瑚礁出现在世界上的浅海中。据推测，造礁珊瑚就像今天一样，与单细胞褐藻共生：虫黄藻，一种光合生物，现在被确定为鞭毛藻晚期的一种，Gymnodinium microadriaticum，处于休眠阶段。在成千上万的软体动物物种中，这种共生只在7种情况下被发现：这一事实不仅表明，三齿蚌是在它们今天栖息的同一种珊瑚礁环境中进化的，而且表明，它们在这些营养贫乏的水域中生存，在很大程度上依赖于它们所栖息的光合藻类提供的营养。为了追溯双壳类的演化，回顾双壳类的一般解剖学是有益的。例如，人们可以更好地欣赏由c.m.y onge颠倒的奇特的三齿纲壳的位置，当它与其他双壳类的非正常方向的壳相比较时。在软体动物门中，双壳类形成双壳纲，因其壳分为两部分而得名。当双壳类仍处于幼虫期时，它就被包裹在分泌壳的肉质壳中。这个外层覆盖层由两个钙化的“瓣膜”和一个连接的弹性韧带组成，它可以通过动物内收肌的收缩紧密闭合。当肌肉放松时，壳会微微张开，使动物能够进入周围的水。然而，双壳类动物的嘴从来不与外界环境直接接触。这种动物所需要的氧气和氧气都是通过成对的、极大扩大的鳃来获得的。早期的解剖学家把这种独特的软体动物的鳃命名为栉鳃，因为它们有梳状的结构；ktenos是希腊语“梳子”的意思。器官具有显著的形态潜能，在双壳类动物中，它被改造成一个活的筛子，赋予动物在动物界已知的最有效的纤毛喂养方式。在最简单的形式中，双壳类动物的每一对鳃都由一条主轴和两侧的侧丝组成，这些丝具有特征的纤毛行。某些纤毛通过跳动产生强大的水流；其他的则是去除水中的食物颗粒和沉淀物。食物颗粒通常由浮游植物（浮游植物）组成，在进入动物的口中之前，主要根据大小由脊状和纤毛触须进行分类。在沙滩上；当其他的文明行为积累了伴随的沉积物进行驱逐。
Athe tide retreats from the upper surface of the Great Barrier Reef of Australia a remarkable vista materializes. Visible for miles among the coral heads are the occasional rounded tops of the giant clam Tridacna gigas. The presence of these huge bivalve mol lusks, which can be as much as four feet long, two feet wide and two feet high, immediately raises two biological ques tions. One is: How did such a large bi valve come into existence? The other has to do with the fact that the waters of tropical reefs are notoriously poor in mineral nutrients and therefore in the plankton that support the animal life of the sea. How, when an adequate food supply must be of vital importance to a bivalve as large as T. gigas, can it inhab it such an impoverished environment? The answers to these questions are re lated. Long before any tridacnids existed the first modern corals made their ap pearance, and massive coral reefs arose in the world's shallow h'opical seas. The reef-building corals presumably lived, as they do today, in symbiosis with unicel lular brown algae: the zooxanthellae, photosynthetic organisms that have now been identified as a species of dinoflagel late, Gymnodinium microadriaticum, in a resting stage, Among all the many thousands of molluscan species this kind of symbiosis is found in only seven in stances: in the six species of tridacnid clams and in one fairly close relative, This fact suggests not nly that the tri dacnids evolved in the same kind of coral-reef environment they inhabit to day but also that their survival in these nutrient-poor waters depends in no small part on the nourishment proVided by the photosynthetic algae they harbor. To trace the evolution of the tridac nids it will be useful to review the anat omy of bivalves in general. One can, for example, better appreciate the bizarre by C. M. Y onge upside-down position of the tridacnid shell when it is compared with the nor mal orientation of the shell among other bivalves. Within the phylum Mollusca the bivalves form the class Bivalvia, so called because of their two-part shell, While a bivalve is still in its larval stage it becomes enclosed within a fleshy man tle that secretes the shell. This outer cov ering consists of the two calcified" valves" and a connecting elastic liga ment, and it can be tightly closed by the contraction of the animal's adductor muscles. When the muscles relax, the shell gapes slightly, giving the animal access to the surrounding water. The mouth of a bivalve, however, is never in direct contact with the external environment. Both the oxygen and the nUh'ients the animal requires come to it through paired, enormously enlarged gills, The distinctive molluscan gills were named ctenidia hy early anatomists because of their comblike sh'ucture; ktenos is the Greek for" comb." The or gan has a remarkable morphological po tential, and in the bivalves it has been modified into a living sieve that endows the animals with the most efficient means of ciliary feeding known in the animal kingdom.In its simplest form each of the paired gills of a bivalve consists of a main axis with lateral filaments on both sides that bear characteristic rows of cilia. Certain of the cilia create a powerful water cur rent by their beating; others remove food particles and sediment from the passing water. The food particles, usu ally consisting of phytoplankton (plant plankton), are sorted, largely in terms of size, by ridged and ciliated palps before they enter the animal's mouth. At the san; e time other ciliated h'acts accumu late the accompanying sediments for ex pulsion.