On a young crab found near the oral arms of the jellyfish, Rhopilema esculenta KISHINOUYE

Abstract

An interesting habit in some of the younger fish belonging to the Suborder Stromateina, that accompanying the jellyfish, is well known as an example of commensalism. In the young crab such habit as mentioned above, has not yet been known. On October 5, 1962, two individuals of jellyfish, Rhopilema esculenta KISHINOUYE, were obtained off the coast of Enoshima in Sagami Bay and were reared in an inspection tank of the Enoshima Aquarium in Kanagawa Prefecture. They were consecutively bred till October 22, and the attention of the author was attracted to one of the specimens which was accompanied with two small crabs. The small crab was identified as a juvenile male of Charybdis cruciata (HERBST), measuring 9.7 mm in carapace length and 12.9 mm in carapace width. The life history of Charybdis cruciata, is unknown at present, but it seems to be similar to that of the related species, Charybdis japonica A. M.-EDWARDS. In most of the Brachyuran carbs, the habitat and habit of the yonnger stage -Megalopa and early crab-stage- are gradually corresponding to those of the adult in the course ot the development. It is noteworthy that the young of Ch. Cruciata has been found living among the oral arms of the jellyfish, however, whether this fact is due to commensalism or to a mere accident is not certain to conclude at present. The author regards, however, the ecological terminology "exploitation" may be properly applied for the present case.