Studies on an intertidal population ofRissoa parva(Gastropoda: Prosobranchia) in south-west Ireland

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Abstract
Populations of R. parva (da Costa) were investigated over a 2-yr period on the algae Lomentaria articulata Lyngb., Laurencia pinnatifida Lamour, and Gigartina stellata (Stackh.) between M.L.W.N. [mean low water neaps] and M.L.W.S. [mean low water springs] in southwest Ireland. Settlement was continuous throughout the year but with peaks in May-June resulting in late summer to early autumn maxima in abundance, with winter to late spring mimima. Seasonal settlement periods of R. parva were similar in artificial substrates and red algae, settlement being restricted to below M.L.W.N. Sediment deposition in artificial substrates exhibited a seasonal cycle of summer minima and winter maxima. The presence of recently settled R. parva on red algae is probably due to the physical trapping properties of the algae rather than to a chemotactic response by the veligers to algal secretions, organic properties of the sediment or presence of adults. Length-frequency analyses revealed a maximum of 5 generations/yr with life spans of approximately 10 mo. for overwintering populations. Variation in growth rate of R. parva caused population structures for the wintering population of 1978-1979 and 1979-1980 to be markedly dissimilar.