Distribution, size structure, and relative growth of Epigrapsus politus (Brachyura: Gecarcinidae) in a subtropical bay in Japan

Abstract

The distribution and population characteristics of the land crab Epigrapsus politus were studied in the Amitori Bay, Iriomote-jima Island, Japan. The crabs were most abundant on the western coast of the bay, particularly at the stations near the tip of the peninsula, where stranded carrion or algae often accumulate. E. politus was found only above the extreme high water of spring tide, where more cobbles and pebbles were piled by the waves. The carapace width (CW) of the individuals ranged from 6.94 to 21.94 mm for males (n＝98) and from 9.83 to 19.98 mm for females (n＝75). Because the population showed a polymodal length frequency or multiple modes, the lifespan was estimated to be at least two years. Adult males showed progressive polymorphism in chelae, and the proportion of heterochelous males and the size difference between major and minor chelae increased with increase in CW. The puberty molt brought about changes in the relative growth rate of the major chela in the males and the abdomen in the females. The size at 50% morphometric maturity was estimated to be about 14 mm CW and corresponded to the CW at about one year of age.