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Gametogenesis in Chitonida is highly unusual, giving rise to eggs with elaborate, often spiny hulls and sperm with needle-like nuclear extensions and tiny acrosomes. My studies of oogenesis in a variety of chitonid families have revealed unique mechanisms of egg hull formation. Three-dimensional reconstruction of serial sections of oogenesis in the chitonidAcanthochitona garnoti(Blainville) confirms that the egg hull is formed largely by microapocrine secretions, rather than by exocytosis of Golgi vesicles. This novel process, detailed for the first time in Mollusca, bypasses the endomembrane system and releases compounds formed in the cytoplasm directly from the cell surface via budding of ‘microaposomes’. Fertilization is no less unusual. In virtually all Metazoa, except chitonids, the entire sperm enters the egg, and sperm mitochondria are digested. Typically, the centrosome that derives from the sperm centriole acts as a microtubule organizing centre, moving the two pronuclei together to form the zygote nucleus and later initiating cleavage of the embryo. However, in chitonids the sperm digest a minute pore in the egg hull that allows injection of DNA but blocks entry of sperm organelles, including the nuclear membrane. Serial sections of fertilized eggs ofMopalia muscosa(Gould) confirm that a membrane bag, abandoned on the egg surface, contains the sperm organelles. Thus, not only is there maternal inheritance of mitochondria and centrioles in Chitonida but also other sperm proteins normally associated with fertilization may be blocked from entering the egg. [ABSTRACT FROM AUTHOR]

An extensive community of organisms inhabits the common periwinkle, Littorina littorea (L., 1758), along the wave-swept rocky shores of Canada's East coast. This community, which includes both facultative and obligate endosymbionts, comprises a diverse array of species from seven animal phyla, including Annelida, Arthropoda, Gnathostomulida, Nematoda, Nemertea, and Platyhelminthes, as well as ciliates and algae. The presence of larger numbers of endosymbionts was found to correlate with specific shell characteristics of the snail host, including a wider aperture and columella, suggesting that these individuals have a larger mantle cavity relative to snails housing a small community of endosymbionts. Snails with large communities of endosymbionts were usually encrusted with coralline algae and often had trematode infections. Although L. littorea has been extensively studied since the last century, the existence of this community of organisms has passed unnoticed. The large diversity of organisms in this community suggests that these snails may provide refugia for a wide range of smaller taxa. [ABSTRACT FROM AUTHOR]