水生動物の寄生・共生関係 短尾類に寄生するヤドリムシ類（等脚目：ヤドリムシ亜目）の寄生状況と宿主に与える影響

Abstract

The Bopyridae and Entoniscidae are two of six families that comprise the Epicaridea (Crustacea: Isopoda). The free-swimming epicaridean larva first infests a copepod and metamorphoses into a microniscus larva. The microniscus larva remains with the copepod until it metamorphoses into a free-swimming cryptoniscus larva that searches for the final host. Upon settling on the final host, the cryptoniscus metamorphoses into a female, or into a male if a newly metamorphosed female is already present. The parasitic status of Grapsicepon rotundum (Bopyridae) and Xanthion spadix (Entoniscidae) and their effects on their host crab, Leptodius exaratus, were studied on two pebble beaches (Tsujishima and Nogamajima) in Kumamoto, Japan. Crabs were collected monthly at Tsujishima from November 1999 to November 2000, and once at Nogamajima in February 2000. A similar study was examined of Entionella okayamaensis (Entoniscidae) in the Asahikawa River in Okayama from July to September 2001, which parasitizes Chiromantes dehaani. The prevalence of parasitic epicaridean isopods differed greatly with place and parasite species: G. rotundum (9.4-23.0%) and X spadix (3.9-13.1%) parasitizing L. exaratus at Tsujishima, G. rotundum (3.2%) and X spadix (0.5%) parasitizing L. exaratus at Nogamajima, E. okayamaensis (9.6-10.7%) parasitizing C. dehaani 8 km upstream from the mouth of the Asahikawa River, and E. okayamaensis (2.0-2.9%) parasitizing C. dehaani 3 km upstream from the river mouth. In most cases, one dwarf male accompanied a single mature female. In three crab species, no ovigerous host infested by the isopod was collected, so host reproduction may be greatly inhibited by the parasites.