Population Dynamics and Reproductive Traits of Phytal Gastropods in Seagrass Bed in Otsuchi Bay, North‐Eastern Japan

Abstract

Abstract. We studied the population dynamics and reproductive traits of two phytal gastropods, Lirularia iridescens and Hiloa tristis, inhabiting a seagrass bed consisting of two seagrass species, Zostera marina and Zostera caulescens, in Otsuchi Bay, north‐eastern Japan. The main objectives of the study were to (1) determine seasonal and between‐substrata variations in gastropod populations and (2) examine two major factors potentially responsible for the variations, namely space (seagrass) and food (epiphytes). The biomass of Z. marina reached a peak in summer and decreased through autumn, whereas the biomass of Z. caulescens showed little seasonal fluctuation. Epiphyte biomass was maximum in April and remained low from summer to winter on both seagrass substrata. The population densities of both gastropod species increased rapidly due to increases in their new recruits from May to July for L. iridescens, from June to October for H. tristis. Eggs of L. iridescens were found in almost all months of the study period, with a breeding peak in April, whereas oviposition of H. tristis occurred only from June to October. The new recruits of both gastropods grew rapidly, with their abundance decreasing from summer to autumn. The peak densities of new recruits of the two species coincide with that of seagrass biomass rather than epiphyte biomass, and the juveniles grew when the epiphyte biomass was fairly low. Hence, large postrecruitment declines in gastropods may be caused by food shortage. Egg density, size structure and growth rate of both gastropod species were not significantly different between seagrass substrata. Total abundance was not significantly different for L. iridescens. For H. tristis, however, abundance was lower on Z. caulescens than Z. marina during most of the year, mainly due to higher post‐recruitment decline on the former. The ability of L. iridescens to breed continuously may enable them to successfully inhabit two seagrass species with different phenology, whereas the much shorter breeding season of H. tristis, which coincides with the periods of higher Z. marina biomass, may be more adaptive for inhabiting Z. marina than Z. caulescens.