203 THE REPRODUCTION AND EARLY LIFE HISTORY OP THE GASTROPOD BEMBICIUM NANUM (LAMARCK, 1822) (FAM. LITTORINIDAE). By D. T. Anderson, University of Sydney. (Plate viii; five Text-figures.) [Read 26th July, 1961.] Synopsis. Spawning in B. nanum talies place during-the spring-and summer. Eggs are laid in gelatinous egg masses attached to the rock surface and hatch after about 12 days as pelagic planktotrophic veligers. The relationship between habitat and breeding in New South Wales littorinids is similar to that of littorinid species in other parts of the world. Introduction. It is well known (Anderson, 1960) that the littorinids present an intriguing problem of development, some species of the mid-and upper reaches of the shore lacking a free-swimming stage (e.g., Littorina obtusata, L. saxatilis, Pelseneer, 1911; Delsman, 1914; Linke, 1934; Thorson, 1946), others of similar habitat retaining plankto-trophic veligers (e.g., L. neritoides, Lebour, 1935; L. angulifera, Lebour, 1945). The marked zonation of the New South Wales rock-platform littorinids Bembicium nanum (Lamarck), Melaraphe unifasciata (Gray) and Nodilittorina pyramidalis (Quoy and Gaimard) raises the question, whether corresponding anomalies exist among these species. A partial answer can now be given on the basis of the present study of the reproduction and life history of B. nanum. Spawning. Observations during 1960 and 1961 on-the breeding season and spawning of s population of B. nanum inhabiting the rock platform at Harbord, north of Sydney, showed that both males and females contained ripening gametes by the beginning of September, but the first egg masses were not found in this locality until the end of October. Thereafter, however, they were commonly found until the end of April. No female of B. nanum was observed in the act of spawning, but the egg masses of the species were identifiable on the more indirect criteria of correlation between the onset of seasonal sexual ripening and the occurrence of egg masses in the habitat, and of exact correspondence of dimensions and colour of ovarian eggs transferred to sea-water and eggs in the egg masses. The occasionally observed close association of females with newly deposited egg masses also supported the identification. Egg masses of various ages were maintained in dishes of aerated seawater and observed at frequent intervals and drawings of living embryos made using a camera lucida. The photographs of Plate viii were taken by the Department of Medical Artistry, University of Sydney. Development. The spawn (PI. viii) consists of an irregularly distributed, closely packed series of oval transparent jelly masses each containing about 100-200 creamy-white eggs lOOfi in diameter, surrounded by ovoid transparent envelopes 200-220/^ in length and 190-210//. in width. The group of jelly masses is firmly attached to the rock surface, generally in a sheltered position either in a fissure or under weed, in the habitat occupied by the adults. The eggs develop rapidly, passing in about two days through a simple yolKy trochophore stage (Text-fig. 1), then gaining in the succeeding two days the apical Proceedings of the Linnean Society-op New South Wales, 1961, Vol. Ixxxvi, Part 2.
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