OKUSU, A: Embryogenesis and Development of Epimenia babai (Mollusca Aplacophora)

Comparative studies of embryology and larval development within and among molluscan groups can inform evolutionary analyses of these groups. Aplacophora are small, shell-less, and vermiform molluscs. Given their hypothesized basal position among molluscs, development of aplacophorans may provide the key to answering major outstanding questions in the evolution of molluscan larvae. However, little is known concerning the developmental processes and features of aplacophorans. This study represents the first attempt to examine embryological development of neomenioid aplacophorans using scanning electron and light microscopy, semi-thin sections, and programmed cell death (PCD) staining. Embryos of a Japanese neomenioid aplacophoran Epimenia babai are lecithotrophic and cleavage is spiral, unequal, and holoblastic. Two polar lobes are formed, one each at the first and second cleavage stages. The ‘molluscan cross’ is apparent at the 64-cell stage. Any trace of metameric iteration is not visible at any stage. A ciliated foot, a pedal pit, and aragonitic spicules develop on the definitive ectoderm. The ontogeny of hollow spicules is described: growth begins as a solid tip, continues to an open-ended hollow spicule, and finally to a close-ended hollow spicule. The free-swimming larvae of E. babai have been considered unusual in lacking a test, or pericalymma, which is an outer locomotory structure within which the entire definitive adult body develops and is characteristic of neomenioid aplacophoran larvae and also of protobranch bivalve larvae. However, from the present detailed studies of the ontogeny and fate of apical cells of E. babai larvae it is shown that the entire preoral sphere, the apical cap, of E. babai larvae is homologous to the test of the other neomenioids.