Photographic observations of the stalked crinoidMetacrinus rotundus carpenter in Suruga Bay, central Japan

Abstract

Photographic observations were carried out at depths of the shelf edge at the mouth of Uchiura Inlet, Suruga Bay, central Japan in order to clarify the life style of the isocrinid stalked crinoid,Metacrinus rotundus (Echinodermata). The distribution of the species was found to be restricted to a narrow area where boulders and rock outcrops were locally present. Mean density of the crinoid was 0.05 m−2, and this value was an order of magnitude smaller than that of the deeper isocrinid,Diplocrinus wyvillethomsoni, reported from the Bay of Biscay.Metacrinus rotundus formed a parabolic filtration fan with its arms recurved into the bottom current, and was thought to be a passive suspension feeder elevating the fan into the water column by its stalk. The distal half of the stalk lay along the hard substratum and about ten groups of cirri grasped the substratum. This mode of attachment was similar to that ofCenocrinus rather than that ofDiplocrinus. Metacrinus rotundus collect food at the layer between 10 and 50 cm above the sea floor, and do not utilize a higher layer even when this layer could be utilized by climbing over a larger boulder. Resuspended benthic materials are thought to be important as a food source forM. rotundus, and the crinoids seek not only locations of stronger currents but also the position where much resuspended matter is available.