Checa Antonio G
Departamento de Estratigrafía y Paleontología, Facultad de Ciencias, Universidad de Granada, 18071 Spain.
J Morphol. 2002 Nov;254(2):195-209. doi: 10.1002/jmor.10028.
The formation of oblique ribs of bivalve shells usually has been attributed to processes of reaction-diffusion of morphogens from cell to cell at the mantle margin or neural activation and lateral inhibition in the mantle. In particular, such ribs appear with high rates of lateral diffusion. Nevertheless, theoretical models fail to explain either partially or wholly some varieties of oblique ribs. After surveying the modes of formation of the shell and oblique ribs by the bivalve mantle and associated fabricational defects, I have determined that the mantle is able to develop an elaborate behavior in order to displace the rib in a particular direction during growth. The mantle margin is, therefore, not only the shell-secreting organ, but also the main morphogenetic unit. In particular, there are two main fabricational strategies. In forms with strict contact guidance (SCG) the mantle is able to project far enough beyond the shell margins so as to feel the already formed reliefs and to align new growth increments of the ribs in the appropriate directions. The shell margin is always strongly reflected. In bivalves with reduced contact guidance plus constant lateral shift (RCG), the margin is usually acute and the information about ribs available to the mantle is reduced. During rib construction the mantle extrudes slightly from the shell edge and then pushes laterally by muscular action; in this way, the new growth increment of the rib is displaced laterally on a small scale. The contact-guidance model is supported also by the homogeneous structure of the shell-secreting mantle. From the morphogenetic standpoint, oblique ribs are related to commarginal ones and both differ completely from other ribbing patterns of bivalves.
双壳贝类贝壳斜肋的形成通常归因于形态发生素在壳套边缘细胞间的反应扩散过程，或者壳套中的神经激活和侧向抑制作用。特别是，此类斜肋在侧向扩散率较高时出现。然而，理论模型无法部分或完全解释某些种类的斜肋。在研究了双壳贝类壳套形成贝壳和斜肋的方式以及相关的构造缺陷后，我确定壳套能够发展出一种精细的行为，以便在生长过程中将肋条向特定方向移动。因此，壳套边缘不仅是分泌贝壳的器官，也是主要的形态发生单位。特别是，有两种主要的构造策略。在具有严格接触导向（SCG）的形态中，壳套能够伸出到贝壳边缘之外足够远的距离，从而感知已形成的凸起，并将肋条的新生长增量沿适当方向排列。贝壳边缘总是有强烈的反映。在接触导向减弱且伴有恒定侧向位移（RCG）的双壳贝类中，边缘通常很尖锐，壳套可获得的关于肋条的信息减少。在肋条构建过程中，壳套从贝壳边缘略微挤出，然后通过肌肉作用向侧面推动；通过这种方式，肋条的新生长增量在小范围内向侧面移动。壳套分泌贝壳的均匀结构也支持接触导向模型。从形态发生的角度来看，斜肋与共边缘肋条相关，并且两者与双壳贝类的其他肋条模式完全不同。
