Abstract
The symbiotic poecilostomatoid copepod Anthessius graciliunguis was redescribed from plankton samples collected in Ariake Bay, Japan. This species was created based on a single female in a blue mussel from Japan; the present study is the second record of the species and the first record of the male. Specimens described as A. graciliunguis from scallops in Korea belong to a different species because of differences in the caudal setae and ornamentation of the male maxilliped. Copepods were not rare in plankton (9 copepods in 200-liter water) as compared with limited previous information of Anthessius species from plankton samples and they occurred in the same season of the two years studied. This suggests that the free-swimming animals are not straying ones but result from usual behavior. The possible advantages of the behavior are releasing nauplii from egg sacs and changing the host.
