57 巻 (2002) p. 75-78
The reproductive strategy of the parasitic isopod Onisocryptus ovalis in the luminescent ostracod Vargula hilgendorfii was studied at a beach in Kumamoto Prefecture, Japan, and in the laboratory, mainly from April 2000 to May 2001. Onisocryptus ovalis is a protandrous hermaphroditic species and changes from a mobile male to an immobile female that loses most of its appendages and has a sack-like body. Male parasites O. ovalis invade female hosts V. hilgendorfii, eat the host eggs, change sex, produce eggs, and die after the eggs have hatched. Some parasites invade immature or male hosts, but move to female hosts immediately afterwards. The percentage of hosts parasitized by O. ovalis was low (0-18%) from summer to autumn, but increased greatly (28-91%) from winter to spring mainly because of a shortage of hosts. Usually, one or two male parasites were found in a host, and only one male of them changed sex. From winter to spring, however, up to 8 male parasites were found in a host and a maximum of five changed sex. The most adaptive parasitic strategy of O. ovalis seems to fertilize eggs as a male as many times as possible, and then to produce more eggs as the only female in a host, although these parameters are not available at present.