2010 年 28 巻 p. 45-54
The biology of Nymphonella tapetis Ohshima, 1927, a pycnogonid endoparasitic on some bivalves, is reviewed. After the first discovery of this species from Hakata Bay, Fukuoka in 1926, there were scattered records of N. tapetis from several places in Japan before 2007, but they were on a small scale. In April 2007, N. tapetis appeared suddenly in the commercial bivalve, Ruditapes philippinarum and several other bivalves on the Banzu Tidal Flat in Tokyo Bay. The spread of the parasite was explosive, and caused a mass mortality of the bivalves in the area. Adults of the pycnogonid live freely on or just under the surface of sandy bottoms, and show nocturnal activity. The hatching larva is a typical protonymphon larva. The larva enters the host bivalve, attaches to various soft parts, and feeds on the body fluid of the host. The number of parasites in one host ranges from one to over 60. At least eight different developmental stages can be distinguished in the parasitic larvae and juveniles. Adults leave the host probably just after the maturation molt. The adult male receives one egg-mass onto his ovigers after each mating, and one male can bear up to seven egg-masses. Several experiments were undertaken to attempt to eradicate or reduce the number of N. tapetis, but no effective method has yet been found. Three species of the genus Nymphonella have been described from Japan, the Mediterranean, and southern Africa. They are very similar in morphology, which leads to potential taxonomic confusion.