Two types of symbioses between grapsid crabs and a host thalassinidean shrimp

Abstract

Two species of grapsid crabs, both of the genus Acmaeopleura, were collected from burrows of a thalassinidean shrimp, Upogebia major, in a tidal flat in Japan. One was Acmaeopleura toriumii, co-habiting in the burrows, and the other was an undescribed species, Acmaeopleura sp., clinging to the abdomen of U. major. Several specimens of A. toriumii were also collected from egg masses of U. major. The field experiment and samplings suggested that A. toriumii is not a burrowing species and depends on Upogebia burrows. Behavioral observations were also made on crabs of the two species maintained in aquaria together with the shrimp. Acmaeopleura toriumii never clung to U. major and seemed to be an omnivorous feeder. In contrast, Acmaeopleura sp. always clung, and almost specifically to the ventral side of the first and second abdominal segments of the host; moreover this crab sometimes migrated to another host individual. A few days after Acmaeopleura sp. was observed clinging to a shrimp, wounds were observed on the abdomen of the host. This ectosymbiotic crab probably feeds on host tissue. In the field, about 13% of the collected U. major had abdominal wounds attributable to Acmaeopleura sp. Symbiotic relationship between Acmaeopleura sp. and U. major will be the first case reported of a crab living ectoparasitically on another decapod.