Hinge structures, their variations and changes during growth, of some Japanese deep-sea, giant white clams, Calyptogena, collected during the “Kaiko” Project

Abstract

Morphological characteristics, individual variations and morphological changes during growth of both the dental elements and the ligament of several species of deep-sea, giant white clams, Calyptogena, were studied on the materials obtained during the Franco-Japanese joint project “Kaiko”. The smaller and larger forms of clams with the subumbonal pit collected in one and the same locality, named C. laubieri and C. nautilei respectively, were found to be two distinct stages in growth, immature and adult, of a single species. Calyptogena laubieri Okutani et Métivier, 1986, was taken as the valid name of this taxonomic taxon. The shape of the growth line of the adult from corresponding to the size of the immature form is identical to the outline of the immature form. The morphological changes from the immature to the adult forms are as follows: the well demarcated subumbonal pit of the immature form becomes enlarged and expanded both ventralwards and posteriorwards and invested with ligamental membrane; some dental elements lose their primitive characteristics; the length of hinge ligament abruptly becomes much longer. The life habit also changes from the burrowing immature form to the half exposed mode of living of the adult stage. Such two distinct growth stages can be found also in a taxonomically related Japanese species, C. soyoae, and show similar changes of traits not only in the morphology but also in the mode of living. Two other species, C. kaikoi and C. phaseoliformis, collected during the Kaiko Project do not have the subumbonal pit, and belong to quite a different group of species within the “Calyptogena” group. In this group, the anterior lamellar layer of the ligament invests the internal shell surface very narrowly along the dorsal margin of hinge plate but rather widely invests the ventral surface of the hinge ligament. Along with the growth of the shell, this ligament layer invades the hinge area ventralwards rather parallel to the dorsal shell margin. As a result, the ligament layer abuts against the ventral surface of the hinge ligament at a point some distance posterior from its proximaty, leaving a vacant space between them. Consequently, the proximal portions of the dental elements become concealed. The posterior ramus of the subumbonal (dorsal) cardinal tooth is much broader and more trapezoid than that of the other group of species with the subumbonal pit. In these traits, these two species are rather closely related to the type species of the genus, C. pacifica. Several homologous characteristics which are peculiar to these two species were found in the morphology of dental elements, indicating closer taxonomical relationship with each other than any other species, notwithstanding their quite different external appearances.