沖縄島産サカモトサワガニの生息場所および抱卵習性

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タイトル別名
  • On the habitat and ovigerous habit of the female of a freshwater crab, Potamon (Geothelphusa) sakamotoanus Rathbun from Okinawa-jima Island, the Ryukyu Islands
  • オキナワトウサン サカモトサワガニ ノ セイソク バショ オヨビ ホウラン シュウセイ
  • 沖繩島産サカモトサワガニの生息場所および抱卵習性

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説明

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).

琉球列島のサワガニ科(Potamonidae)のなかで沖縄島に生息するサカモトサワガニPotamon(Geothelphusa)sakamotoanus Rathbunの生息場所,抱卵,抱幼数,その腹肢などについて調べた.1)沖縄産サカモトサワガニは本州産サワガニPotamon(Geothelphusa)dehaani(White)とほぼ同様なhabitatをもち,山間部の川や小さな泉の石の下や岸辺に穴を掘つて住む.幼ガニは小石の下に穴を掘り,入口に土をもり上げているのが見られる(第1図).2)沖縄島首里付近ではサカモトサワガニの抱卵・抱幼期は7月中旬であつた.3)抱卵・抱幼個体のうち,最小個体の大きさは甲長26.2mm,甲幅33.Omm,最大個体の大きさは甲長35.0mm,甲幅44.0mmであつた(第1表).4)卵の大きさを2個体で測定したところ,それぞれ直径2.90-3.25mm(Sp.No.F),2.75-2.95mm(Sp.No.G)であり,抱卵数は前者で169個,後者で175個,1卵の重さは14.8mgであつた(第1表).5)抱幼数は95-170個体,幼ガニの大きさは甲長2.48-2.81mm,甲幅3.24-3.43mm,1幼ガニの重さは21.9-28.7mgであつた(第1,2表).6)各卵は直接腹肢の内枝に2-30糸をもつて剛毛に付着している.剛毛束は腹肢の内枝の上下側にあり,束数は第2,1,3,4腹肢の内枝の順に少なくなつており,第1,第3腹肢はほぼ同数である(第3表).抱卵数も第2腹肢の内枝が最も多く(55-67卵),第1,第3腹肢の内枝(31-45卵),第4腹肢の内枝(14-24卵)と少なくなる(第3図,第4,5表).剛毛束数は内枝の上下側を比べれば上側がわずかに多いか,あるいは等しいが,上側の剛毛が下側のものよりも長く,抱卵数も多い.内枝は第2,3,1,4,5節の順に抱卵数が少なくなつている(第5表).

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