Distribution in relation to life history in the direct-developing gastropod Batillaria Cumingi (Batillariidae) on two shores of contrasting substrata

摘要

Population structure and spatial distribution with growth of the direct-developing gastropod, Batillaria cumingi, were investigated on two shores of differing substrata. Sand-mud shore and rocky shore populations differed in size structure: first-year snails were ca. 7 mm in shell length (SL) in both populations, whereas second-year snails, merging with older cohorts, measured 15-25 mm SL in the sand-mud shore population and ca. 15 mm SL in the rocky shore population. Egg distribution matched adult distribution in the sand-mud shore population, but was more restricted than that of adults in the rocky shore population. The distribution of newly-hatched juveniles (0-1 mm SL) was restricted in both populations, but the growth stage at which snails extended their distribution differed between the two populations: 1-2.5 mm SL on the sand-mud shore and > 5 mm SL on the rocky shore. Floating achieved by early juveniles (ca. 2 mm SL). was commonly observed in the sand-mud population, but rarely in the rocky shore population. The sudden expansion in distribution of the 1-2.5 mm SL growth stage in the sand-mud shore population is considered to have been caused by floating, while expansion of the distribution of older growth stages (> 5 mm SL) in the rocky shore population probably occurs by crawling.