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Early events of fertilization are described in Chaetopleura apiculata and other selected Chitonida. C. apiculata egg hulls are elaborated into multi-branched spines with interlocking polygonal bases. Around the perimeter of each base are a series of open pores, ranging in size from 0.1-0.5 µm, which permit sperm direct access to the vitelline layer. In Callochitonidae (Chitonida) even larger pores occur in egg jelly coats, but this is considered to be the plesiomorphic condition, found also in Lepidopleurida such as Deshayesiella curvata. Other Chitonina, such as Rhyssoplax tulipa and Acanthopleura granulata, have a continuous outer dense layer that lacks pores and must be digested by penetrating sperm. Fertilization in Chitonida is unique and involves injection of chromatin into the egg via a narrow tubular nuclear extension that appears to exclude other sperm organelles, including mitochondria, centrioles, and flagellum. New evidence from studies of fertilization in Mopalia muscosa (Chitonida: Acanthochitonina) supports this hypothesis. This type of fertilization implies maternal inheritance of both mitochondria and centrioles, which is highly unusual, because in most animals one sperm centriole assists movements of pronuclei and regulates organization of the mitotic spindle. This mechanism of fertilization is defined by a series of apomorphic characters that unify the order Chitonida., Introduction Sirenko (1993) presented evidence for classifying chitons into two Orders, Lepidopleurida and Chitonida (which includes most extant chitons), and subdivided Chitonida into two Suborders, Chitonina and Acanthochitonina; this classification [...]

Spermiogenesis and fertilization in the limpet Tectura testudinalis (Mollusca: Archaeogastropoda: Patelloidea) were examined using scanning and transmission electron microscopy, as well as light microscopy. Spermiogenesis was similar to that described for other Lottiidae (Patelloidea), but some key differences were noted. We observed that the acrosomal vesicle forms on the plasma membrane from several Golgi-derived proacrosomal vesicles during the early spermatid. Evidence from anti-actin antibody staining of a related species, T. scutum, indicates that the acrosome probably contains G-actin in three different locations, but there are no preformed actin filaments. The flagellum of mature sperm tapers to a thin, filamentous end-piece. The mature egg of T. testudinalis is enclosed by a thin vitelline layer and a thick jelly coat bound by follicle cells. Where a mature sperm encounters the intact jelly coat, its acrosome tip may elongate, up to twice its original length. The acrosome reaction is not induced, but by this mechanism the acrosome bridges the jelly coat and extends down far enough to contact the vitelline layer. During the elongation of the acrosome, microfilaments are formed inside the tip. As well, new membrane required for this process is supplied by elimination of any slack in the plasma membrane and also perhaps by spontaneous formation of vesicles inside the acrosome. Once the tip of the sperm contacts and binds with the vitelline layer it undergoes the acrosome reaction, in which the plasma and acrosomal membranes fuse and roll back in a manner typical of scaphopods and some polychaetes. Sperm also may bind directly with the vitelline layer when the jelly coat is absent. Furthermore, the acrosome tip may undergo spontaneous fusion with a naked egg microvillus that formerly connected with a follicle cell process. Thus, the sperm either individually or collectively are equipped to deal with any one of several potential routes to successful fertilization of the egg. Current theories of gastropod phylogeny place the limpets as basal to the Gastropoda, but new evidence presented here supports the idea that they are a divergent group, with some unique innovations in sperm design., Introduction The process of fertilization in invertebrates typically involves intricate structural responses of both sperm and egg; these are often unique to a particular species and function to regulate sperm-egg [...]

IAN G. PATERSON [1],[*] VANESSA PARTRIDGE [2] JOHN BUCKLAND-NICKS[2] Abstract. Parental identity for juvenile Littorina obtusata was determined from three egg masses by means of micro- satellite DNA markers. Results [...]

JOHN BUCKLAND-NICKS [1][*] ALAN N. HODGSON [2] Abstract. A fine-structural study of fertilization in Callochiton castaneus has revealed that the mechanism of sperm penetration into the egg is intermediate between [...]