Abstract
The grapsid crab Cyclograpsus intermedius Ortmann, 1894 in the Amakusa Islands, Japan, is parasitized by two sacculinid parasites: Sacculina oblonga Liitzen & Yamaguchi, 1999 and Sacculina confragosa Boschma, 1933. Average prevalences at four sites are 1.6 to 12.1 % in S. oblonga and 0.4 to 1.5 % in S. confragosa. The number of externae attached to crabs infected with S. oblonga is only one without exception. Most of the crabs parasitized by S. confragosa had one externa but a few crabs had two or three externae attached. A significant difference in infection rate between the sexes was found in S. oblonga. Female crabs seem to be more commonly infected by the parasite. The morphological modification is more significant in smaller males than in larger crabs.
