Timing and frequency of egg-collar production of the moonsnail Euspira fortunei

摘要

Predation on the Manila clam Ruditapes philippinarum by the non-indigenous moonsnail, Euspira fortunei, has had severe impacts on clam fisheries in some localities in Japan. This paper aims to reveal when and how many egg collars E. fortunei produce during the egg-laying season. To investigate the frequency of egg-collar production, 13 pairs of E. fortunei were reared in aquaria with natural seawater flowing from July to December 2009. Each pair produced only one or two egg collars during 1 day in October; thereafter, no egg collars were produced. The timing of egg collar production, estimated from observational information by fishermen in the field from 2004 to 2010, varied among years. The period between September 1 and the day of the initial observation of egg collars was positively correlated with the average water temperature in August, suggesting that water temperature can be used to predict the timing of egg collar occurrence.