LIFE CYCLE AND ENVIRONMENTALLY INDUCED SEMELPARITY IN THE SHORE ISOPOD LIGIA CINERASCENS (LIGIIDAE) ON A COBBLE SHORE ALONG TOKYO BAY, CENTRAL JAPAN

Abstract

A 3-year field study and 17-month rearing experiment were conducted on a population of the shore isopod Ligia cinerascens inhabiting a cobble shore in Tokyo Bay, central Japan. The research was designed to clarify aspects of life-cycle and reproductive traits in L. cinerascens. Most males, both in the field and in the rearing experiment, died before the onset of their second winter, but some field males overwintered twice. Most females overwintered once, bred from May through September, then died before the next winter. A small group of females, however, began breeding late in the season of the year of their birth, then most likely died without overwintering. Females in the field produced mostly I brood or at most 2 broods before dying, but those in the rearing experiment lived longer, producing up to 3 broods and possibly more. The results of this study indicate that the typical life cycle of L. cinerascens is annual with a 5-month breeding season, but including a shortened life cycle completed within a single breeding season. The data from the rearing experiment, however, suggest that the number of broods produced by a female may be limited by environmental factors rather than by genetic characteristics.