Maorithyas hadalis n. sp. and Parathyasira kaireiae n. sp. (Thyasiridae) are described. These species were both collected from hadal (ultra-abyssal) zone in the Japan Trench. The former species is characterized by large quadrate shell with a weak flexure, truncated or even concave posterior margin, and roughened surface by concentric threads. The latter species is small and typical shape for the genus with acutely prosogyrated beak, a slightly shorter anterior portion and a single strong flexure. The present discoveries may be occurrences of thyasirids from unusually great depths suggesting an association with chemosynthetic environment.

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Trapping behaviour against muricid whelks by a mytilid bivalve, Hormomya mutabilis, using byssal threads was investigated by field and laboratory observations at Shirahama, Japan. Byssal threads of H. mutabilis were attached to 10 of the 108 muricids occurring on the mussel beds in June and July 1997. Muricids with byssal threads attached to their shells and turned over remained trapped for over 30 hrs in the field. The actual processes in which byssal threads were attached to a muricid and pulled by H. mutabilis could be observed in the laboratory. This behaviour may result in trapping or turning over the muricids through the similar process to that for Mytilus edulis reported in previous studies. This is the first report on the immobilization of muricids by the Modiolinacean species using byssal threads. The trapping behaviour by byssal attachment may be a widespread anti-predator defense in many mytilid bivalves.

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A total of 341 Cellana radiata (Born 1778) limpets were randomly collected from the Gulf of Aqaba, Red Sea for age, growth and shell morphometric determinations. The results show that the life span of this limpet may extend to five years. Collected limpets ranged in length from 6.9 to 38.5 mm and their shell and dry body weights ranged from 10 to 3530 mg and from 0.9 to 487 mg, respectively. The relationships of the length to shell width, height, width/height ratio, the ratio of the anterior distance from the apex to the anterior margin to that of the posterior distance, and the shell width to shell height relationship are all negatively allometric. The relative coefficient of condition (Kn) was higher (1.01-1.05) for long limpets than small ones (0.61-0.74).

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To clarify sexual maturation in the land snail, Euhadra peliomphala, seasonal changes in some items of the albumen gland were examined. No clear differences were detected in the shell size and whole body weight. The weight of the albumen gland and the ratio of the albumen gland weight to the whole body weight showed relatively high values from spring to summer, and low values from autumn to winter in the semi-laboratory condition. Further, soluble proteins extracted from the albumen gland in each season were quantified, and were subjected to polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate (SDS-PAGE). Levels of soluble proteins in the snails in May were 3-fold larger than those in the snails in March. SDS-PAGE showed clear differences of the protein composition in each season. The 68k protein seemed to be synthesized from spring to summer. In conclusion, the present study clearly showed the seasonal changes appeared in the weight of the albumen gland, the ratio of the albumen gland weight to the whole body weight and the protein contents of the albumen gland in E. peliomphala.

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[in Japanese]

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