Contrasting characteristics in increasing and decreasing phases of the Nerita japonica (Mollusca: Gastropoda) population on a boulder shore

The present study demonstrated the seasonal and annual fluctuations in population characteristics of Nerita japonica on a boulder shore at Amakusa, Japan, and examined key characteristics in the increasing and decreasing phases for population monitoring. Monthly quantitative quadrat sampling and subsequent mark-recapture experiments were carried out during two periods: from April 1989 to July 1990 (1st period), and from August 1991 to November 1992 (2nd period). The density of the population was in a decreasing phase during the 1st period and in an increasing phase during the 2nd period. The density increase coincided with the extension of the vertical distribution range to a lower tide level, increase in the number of recruitment events, increase in the proportion of juveniles within the population, and increase in the survival of juveniles and adults. The growth rate was slightly increased but the average shell length of adult cohorts did not show any change. Additional observations showed a continued density increase and range extension until 1994. On the other hand, the density decrease coincided with recruitment failure and a low rate of survival with an unchanged range of the vertical distribution. Thus, for monitoring the population of N. japonica, recruitment success and changes in density and distribution range are key characteristics, but the shell size is not. These findings show the potential usefulness of N. japonica as an indicator of the shore environment.