Larval Recruitment of the Tropical Mussel Modiolus philippinarum (Bivalvia: Mytilidae) in Seagrass Beds

Abstract

The tropical mussel Modiolus philippinarum is common and abundant in seagrass beds in Okinawa Island, southernmost Japan, and is commercially important to local fishermen. In order to elucidate the mechanisms that maintain the difference in density between benthic populations of the mussel within and outside seagrass beds, we monitored temporal variations in densities of the mussel at different developmental stages (planktonic larvae; new settlers; small and large individuals) within and outside seagrass beds in Okinawa Island over one year. Based on these data, the difference in larval density was not significant, but there were significant differences between densities of new settlers (shell length<250μm), small individuals (250μm≦shell length<1.0mm) and large individuals (shell length≧1.0mm), within and outside seagrass beds. Cohort separation was applied to data for shell length distributions of new settlers and small and large individuals, and revealed that larvae mainly settled in July to August. New settlers grew up to about 20 mm in shell length in their first year; their mortality was constant and/or low once individuals had attained shell lengths of about 300μm. These facts indicates that the much higher density of benthic populations of the mussel within seagrass beds may be determined at and/or shortly following larval settlement, though details of the mechanisms driving the above difference are not yet identified.