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The buds of the sponge Tethya seychellensis, collected in May 2006 from South Male Atoll (Maldives), have been processed for histological and ultrastructural investigations. Buds show three stages, different in morphology and size, reflecting different steps of their differentiation: round-shaped buds (stage I), cylindrical-shaped buds (II) and thin-stalked spear-shaped buds (III). Cells with inclusions, belonging to the category of spherulous cells, are the main components of these buds. At stage I, some irregularly shaped cells, probably precursors of the choanocytes, are able to engulf and remove portions of the collagen matrix, a feature which could be interpreted as an initial step in choanocyte differentiation. Tethya seychellensis stands out among the other congeneric species owing to the occurrence of choanocyte chambers in the unreleased buds, as observed at stages II and III. The precocious differentiation of the choanocyte chambers allows the detached bud to act as a young functional sponge, thus enhancing the survival chances of these filter-feeding organisms. The structure of the buds of T. seychellensis stresses once more that in the genus Tethya the organisation of the buds represents a species-specific process, which varies according to the plasticity and the morphogenetic potentiality expressed by a single species. [ABSTRACT FROM AUTHOR]

Although the New Zealand mudsnail Potamopyrgus antipodarum has been observed in Italy since 1961, no investigations have gone beyond recording its presence. In this study, populations from three Italian watercourses with different environmental features were comparatively analysed with a view to shedding light on the relationships between habitat type and reproduction. At the study sites, populations were exclusively composed of females. Ultra-structural details showed that egg growth takes place through the accumulation of stored materials, which are essentially constituted by translucent vesicles (lipids) and by electron-dense granules with a darker rim (proteins), the latter deriving from the confluence of smaller granules of the same type. The egg surface displays numerous microvilli, which interdigitate with those emerging from both adjacent eggs and bordering cells. Our comparative analysis shows that the populations differ slightly in fecundity, whereas the number of brooded embryos/female markedly decreases, in parallel with the mean density, in running-water habitats lacking aquatic vegetation. Statistical analysis confirms that P. antipodarum positively selects biological substrates, such as beds of Vaucheria sp. and even, on occasion, dead leaves and wood floating on the water surface. The results suggest that biological substrates constitute one of the key factors favouring the massive spread of P. antipodarum in watercourses. [ABSTRACT FROM AUTHOR]

Spermatogenesis of the marine sponge Halichondria semitubulosa (formerly Pellina semitubulosa) has been investigated at the ultrastructural level. This process can be observed in March when, among the choanocyte chambers of the aquiferous system, spermatic cysts are visible. They are delimited by pinacocyte-like cells and include elements in progressive development: spermatocytes of the first and second order, spermatids and spermatozoa. The early phase of spermatogenesis was not detected. Spermatocytes of the first order show an elongated shape, several small mitochondria and patched chromatin; spermatocytes of the second order, frequently connected by bridges, show denser chromatin, a single large mitochondrion with numerous tightly adherent cristae, glycogen, and round-shaped inclusions with a central electron-dense core. In the spermatids the chromatin tends to be packed in the central region. Spermatozoa have a uniformly dense nucleus in close association with the large mitochondrion. Sperm maturation takes place synchronously within the same cyst but asynchronously within the same specimen. [ABSTRACT FROM AUTHOR]