Ecological Studies on strategy of habitat selection of sea urchins.

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Verticlal distribution of the sea urchin Hemicentrotus pulcherrimus in relation to algal zonation was studied monthly from April 1998 to May 1999 at a permanent quadrat (100 m^2) in intertidal and subtidal zones in Onagawa of Miyagi Prefecture. From seasonal changes in size, gonadal development and food algae in gut contents, the factor leading the sea urchins to migrate seasonally was studied.H. pulcherrimus distributed densely under rocks at depths of 0-0.3 m in gentle slope where small algae grew throughout the year. The sea urchins migrated from deep waters where crustose coralline red algae grew dominatly to the community of small algae from October to March, then remained there. Strongylocentrotus nudus distributed densely in deep waters and migrated to the small algae in shallow waters from summer to autumn, then returned to deep waters in winter. Gonadal indices (gonad weight X 100/body weight) of H. pulcherrimus at crustose corallines were markedly lower than those at the community of small algae. From histological observation of gonads, from October to March when the sea urchins migrated, gonads developed from growing to mature and spawning successively. Therefore it is considered that the migration from deep waters to shallow waters was caused by foraging activity to succeed in maturation and spawning. From gut contents observation, the thallophyta Chondrus spp. Acrosorium polyneurum, and Sargassum spp. were supplied to the sea urchins as foods at the community of small algae. Contrary, poor food resouces at crustose coralline community reduced gonadal development of the sea urchin. It is concluded that the sea urchins migrated to the community of small algae to forage Acrosorium polyneurum as food.