沖縄

The author has studied the habitat and ovigerous habit of a freshwater crab, Potamon (Geothelphusa) sakamotoanus Rathbun from Okinawa-jima Island, the Ryukyu Islands. The results are as follows : 1) The adults are found either sheltering under rocks or burrowing into the sides of the streams in the hilly regions. The young crabs seem to dwell under small rocks or in burrows near the rocks (Fig. 1). 2) Many egg-laden females were found at the middle of July near Shuri in Okinawajima Island. 3) In the young-laden females, the minimum young crab has a carapace, 26.4×33.0 mm, and maximum one has a carapace, 35.0×44.0 mm (Table 1). 4) The size of eggs is 2.90-3.25 mm (Sp. No. F) and 2.75-2.95 mm (Sp. No. G) in diameter, and the number of eggs is 169 (Sp. No. F) and 175 (Sp. No. G) in two egg-laden females respectively. The weight of egg is 14.8 mg (Sp. No. F) (Table 1). 5) A mother crab has nursed 95-170 young crabs on the pleopods. The carapaces of the young crabs were 2.48-2.81 mm long and 3.24-3.34 mm broad, and their body weights were 21.9-28.7 mg (Tables 1, 2). 6) The egg is directly and firmly attached to the setae counted 2-30, on endopodites of pleopods, and the setae tufts are situated on both upper and lower parts of pleopods. The number of the setae tufts of upper parts is either same to, or a little more than that of the lower ones, and the former is longer than the latter in length. The eggs attached to each endopodite of pleopods are 31-45 on the first, 55-67 on the second, 31-45 on the third and 14-24 on the fourth (Tables 4, 5, Fig. 3). The eggs are attached from the first to sixth segments of the endopodites, and the maximum number is counted 58 on the second segments (Table 5).