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In macrobenthos with meroplanktonic larvae, larval settlement and recruitment are two crucial processes for population cohort formation, yet only a few studies have given explicit definitions for newly settled and recruited juveniles (settlers and recruits) and descriptions of those processes with relevant time scales. Using a comprehensive approach, our study demonstrated a clear pattern in cohort formation of the trochid gastropod population (Umbonium moniliferum) on an intertidal sandflat, southwestern Japan, in the breeding seasons of 2002 and 2007-2009. The study included (1) daily monitoring for spawned eggs and competent larvae in the field and the laboratory, (2) daily and fortnightly monitoring for juveniles, and (3) collection of larvae in the water column and in situ simulation of larval retention, releasing drift cards within an embayment embracing the sandflat. The population with shell widths of 5-14 mm occupied the lower shore in densities of about 1000 inds. m- 2, spawning gametes for lecithotrophic larval development with the shortest duration of 3 d during late September through October. The settlers occurring per day and the newly recruited cohort in the size-frequency distribution were defined as juveniles belonging to the smallest size class (0.20 and 0.21 mm) and to the most left-hand normal-distribution group, respectively. The growth rate was estimated at 0.02 mm d- 1. A synchronous mass egg spawning took place 2-4 d after neap tide, followed by the sequential occurrence of veligers and settlers in high densities, which was completed by the next neap tide, leading to the formation of one newly recruited cohort. Three distinct cohorts were produced consecutively, most probably due to this semilunar cycle. The iteroparous spawning was supposedly performed by the same females. By the following spring, the three cohorts had fused into a single young-of-the-year cohort. Veligers were present across all depth layers in the water column down to 20 m, with a quarter being present within the surface 1 m. Drift cards were driven ashore, with cumulative retrieval rates of 50 and 70% 3 and 9 d after release, respectively, suggesting that hydrodynamics allowed good retention of larvae in the water surface. Using an exponentially decreasing function describing competent larval numbers in a 2 to 9-d period after fertilization in the laboratory, the degree of self-recruitment in the population was assessed under the assumption of complete retention and no mortality of larvae due to predation. The results suggested the presence of some allochthonous subsidy., Journal of Experimental Marine Biology and Ecology, 389(1-2), pp.18-37; 2010