フトヘナタリの分布の季節変化と繁殖行動

抄録

Seasonal changes in the distribution and mating behavior of the potamidid snail Cerithidea rhizophorarum A. Adams, 1855 were investigated on a tidal flat in the Atago River, Kyushu, the most northern mangrove forest in the world. Two potamidid species, Cerithideopsilla djadjariensis (K. Martin, 1899), and Cerithideopsilla cingulata (Gmelin, 1791), and one batillarid species Batillaria multiformis (Lischke, 1869) co-occurred with C. rhizophorarum in this area. Newly-recruited juveniles (3-6 mm shell width) of C. rhizophorarum appeared from November to March. Adults of C. rhizophorarum were found in higher tidal zones than the other potamidid and batillarid species. The distribution of juveniles of C. rhizophorarum, however, was limited to lower tidal zones. Copulating behavior was observed at daytime low tide from June to August. Courtship behavior of C. rhizophorarum follows a fixed pattern. First, one animal (the initiator) approaches another and mounts its shell. If the lower animal (the acceptor) accepts the courtship, the upper animal (the initiator) inserts its soft body into the body of the lower animal. In about 80 % of copulatory pairs, the upper animals were male and the lower ones female. However, male to male copulation accounted for about 20 % of copulatory pairs. Copulation lasted for 11 to 74 minutes (mean : 20 minutes). Mating of C. rhizophorarum was random with respect to shell size.