The naticid snail Laguncula pulchella is an invasive species that preys on clams in tidal flats and has serious impacts on clam fisheries in Japan. Laguncula pulchella burrow in sand, but often crawl on sediment surfaces during low tide. We investigated seasonal changes in the abundance and sex ratio of crawling L. pulchella during the daytime at Matsukawaura Lagoon, Japan, from March to October from 2015 to 2019. The density of crawling individuals peaked in July. The sex ratio of crawling individuals varied with months and years but was significantly biased towards males during the main copulation period (July–August); males accounted for 77–98% of the mature crawling individuals (≥ 25 mm shell height). The somatic condition of mature males declined from June to August, whereas that of females was constant during this period. These results indicate that mature males actively come to the sand surface during low tide to search for females for copulation from July to August. Fishermen make efforts to remove crawling individuals in summer, but the male-biased sex ratio must also be considered for effective population control of this species.

This study was partly supported by JSPS KAKENHI Grant Number 19K06207.