Size structure and distribution of Ovassiminea brevicula (Gastropoda) in a Thai mangrove swamp

Ovassiminea brevicula (Pfeiffer) (Assimineidae: Gastropoda) was one of the most abundant and representative benthic invertebrates in the Samut Songkhram mangrove swamp, Thailand. It lived mainly in the mangrove forest with high average densities ranging from 44 to 340 snails·m^-2 and abundant in the sites where much litter, dead branches and seedlings were present. Average shell length in the mature forest ranged from 4.73 to 5.74 mm (range: 2.26-8.30 mm) and size structures showed different patterns among the stations. In the seaward mudflat, O. brevicula appeared immediately after the saplings of Avicennia alba Blume 1826 were planted. Thereafter, its densities increased with the growth of A. alba trees, probably due to provision of favourable microhabitats under the canopies. In the mudflat with only small saplings and outside the canopy of the young forest 2 years after planting, mean shell lengths were 3.45 and 3.19 mm respectively. Under the canopy of the young forest, however, mean shell length was 4.34 mm with a few larger snails. Three cohorts were separated for snail populations in the mature forest and larval recruitment seemed to occur from February to May.