Growth, reproduction and recruitment of the endangered brackish water snail Iravadia (Fairbankia) sakaguchii (Gastropoda : Iravadiidae)

Abstract

The population structure and reproductive activity of the brackish water snail Iravadia (Fairbankia) sakaguchii, which lives under partially buried stones on intertidal mudflats, were investigated monthly in the Waka River Estuary, Wakayama, central Japan. Observations of the reproductive organs indicated the main reproductive season as being from June to July. Maturation size was estimated to be approximately 4.0 mm in shell length (SL) in females and 3.9 mm SL in males, there being no significant sexual dimorphism in adult body size. The occurrence of snails smaller than 2 mm SL indicated that recruitment occurred between August and October, all the recruits co-occurring with adults under stones. The recruits grew to a large size (>3 mm SL) and reached maturity in August of the second year (1 year old), longevity being estimated as at least 37–38 months. Throughout the study period of 28 months, the population mostly comprised individuals between 3 and 5 mm SL, with recruits being fewer than older snails. The population density decreased each year, this being attributable to a low level of recruitment.