Variation of growth rate with tidal level in the gastropod Monodonta labio on a boulder shore

Growth of Monodonta lablo (Linne) (Gastropoda: Trochidae) was investigated bimonthly for a period of 1 yr by quadrat sampling and mark-recapture on a moderately exposed boulder shore in Amakusa, South Japan. Linear regression was calculated for shell width at recapture on shell width at release. Growth in the high zone was slower than in the mid and low zones and decreased as shell width increased. Estimated growth curves fit well with the observed growth curves of individual snails. Seasonal growth patterns of juvenile (58 mm) snails may be affected by seasonal fluctuation of micro- algal abundance. Due to reproduction, growth of large snails decreased in summer. Tidal level varia- tion of growth rate caused large variation in growth curves in middle-sized snails (ca 10 rnm shell width), because of their wide range of distribution, which was related to vertical migration on the shore. Simulated growth curves showed that 8 mm snails in the high zone reached asymptotic size (ca 13 mm) 1 yr later than snails in the mid and low zones. In small (ca 6 mm) and large (ca 13 mm) snails, less van- ability in observed growth curves was explained due to their limited ranges of distribution. This study demonstrates that variation in growth rate is attributable to sue-specific distribution on a shore and indicates how migration expands size variation within a single age group.