Embryogenesis and Development ofEpimenia babai(Mollusca Neomeniomorpha)

Abstract

Neomenioid aplacophorans (= Solenogastres) constitute one of the main lineages of molluscs. Developmental data of early embryogenesis and larval development of neomenioids are available for some species based on histological sections. I used other techniques to study the development of Epimenia babai Salvini-Plawen, 1997, and here I report new data on neomenioid development. The embryos of E. babai are lecithotrophic and cleavage is spiral, unequal, and holoblastic. Two polar lobes are formed, one at the first cleavage stage and one at the second cleavage stage. No evidence of external metameric iteration is visible through scanning electron microscopy or histology at any stage. A ciliated foot, a pedal pit, and aragonitic spicules develop from the definitive ectoderm. A spicule begins as a solid tip, continues to an open-ended hollow spicule, and finally becomes a closed-ended hollow spicule. The free-swimming trochophore larvae of E. babai have been considered unusual in lacking the characteristic neomenioid cellular test, an outer locomotory structure within which the entire definitive adult body develops. However, through the use of scanning electron and light microscopy, semithin sections, Hoechst nuclear staining, and programmed cell death staining to study the ontogeny and fate of the apical cells, I show that the entire pre-oral sphere (the apical cap) of the larvae is similar to the test of the other neomenioids. The results suggest that the test of the neomenioid larvae is an enlarged pre-oral sphere of a trochophore. The test morphologies of neomenioid larvae are compared to those of pericalymma larvae of protobranch bivalves, and the homology and evolution of molluscan larval tests is discussed.

Trochophora larvae: Cell‐lineages, ciliary bands, and body regions. 1. Annelida and Mollusca

“…The mantle edge can be recognized in scanning electron micrographs of aplacophoran larvae, and it can be seen that the mantle edges move close to the ventral midline in the solenogaster Epimenia (Okusu, 2002) and fuse in the caudofoveate Chaetoderma (personal observations).…”

“…The caudofoveate Chaetoderma (Nielsen, 2001) and the solenogasters Epimenia (Okusu, 2002) and Neomenia (Thompson,'60) show a prototroch with one ring of thick compound cilia, although Neomenia has pericalymma larvae (see below) where the prototroch zone is extended in a serosa with very large cells. In the polyplacophoran Ischnochiton, Heath (1899) found eight accessory, 16 primary, and six secondary trochoblasts.…”

“…Eernisse ('88) reported a variation among polyplacophoran larvae from planktonic larvae with a long apical tuft of several compound cilia to brooded larvae with a hardly discernible field of short cilia. Apical organs of varying types, ranging from two compound cilia to a group of compound cilia, to an apparently undifferentiated ciliated area, are known from solenogasters, gastropods, bivalves, and scaphopods (Conklin, 1897;Cragg, '96;Okusu, 2002;Wanninger and Haszprunar, 2001). In some species, the young larvae have a long apical tuft which is lost in older larvae (Cragg, '96;Page, 2002).…”

“…Okusu (2002) described how a pair of ciliated depressions on the episphere of Epimenia become internalized and form the cerebral ganglia. Similarly, it has been observed that in late gastropod embryos two small lateral groups of cells of the episphere near the apical organ divide rapidly and sink below the ectoderm to form the cerebral ganglia (Conklin, 1897;Delsman, '14;Jacob, '84;Page, '92 (Arnolds,'82;Verdonk and van den Biggelaar,'83).…”

“…Epimenia (and probably also Chaetoderma) show a gradual change, with the prototroch being discarded at the end of the planktonic phase (Okusu, 2002). Neomenia and Nematomenia, which have pericalymma larvae, show a more dramatic metamorphosis, where the serosa is rolled back and becomes internalized in the episphere (Pruvot, 1890;Thompson, '60); the fate of the apical organ and cerebral ganglia has not been followed.…”