Spatio-temporal population dynamics of the sand snail Umbonium costatum: importance of ontogenetic migration and annual recruitment variability

The spatio-temporal population dynamics of the subtidal snail Umbonium costatum (Kiener) in Hakodate Bay, northern Japan, are described over a 9-yr period (1982 to 1988, 1992). Annual variations in recruitment success not only caused the highly variable age structure of the population, but also affected its distribution pattern. In heavy recruitment years (1982, 1984 and 1988), location of the densest population differed from the other years, and the areas with 0 yr old individuals were larger. Every year, the distribution of 0 yr olds was mostly restricted to inshore (within 320 m from shore), while the distribution of adults (>0 yr) was mostly restricted to offshore (>320 m from shore). Such patterns may be generated by ontogenetic migration of cohorts to offshore areas. Ontogenetic migration also could have buffered the large annual variation in spat distribution and maintained the small variation in the distribution of adults. (Prince et al. 1988). Density-related migration has also been reported in Acmaea digitalis (Frank 1965), Patella cochlear (Branch 1974) and Patelloida alticostata (Black 1977).Dense populations of the gastropod Umbonium costatum are found in some clean subtidal sandy-bottoms in Japan, e.g. inner part of Hakodate Bay. In the bay, the age structure of U. costatum revealed predominant year-classes. The 0 yr olds were restricted to the shallowest zone (<4 m depth) and most adult (<0 yr) cohorts were restricted to 6 to 7 m depths (Noda 1991a).This paper describes the temporal variability of population structure and distributional patterns of Umbonium costatum, and examines the distribution-regulating processes.