Seasonal migration of the sea urchin Strongylocentrotus nudus in Oshoro Bay of southwestern Hokkaido, Japan

Seasonal migration of the sea urchin Strongylocentrotus nudus between intertidal and subtidal zone were investigated from May 1994 through December 1995 in Oshoro Bay of southwestern Hokkaido, Japan. The over 1-year-old sea urchins over 20 min in test diameter remarkably showed seasonal migration in compliance with the request of algal foods. The sea urchins with the growing stage of gonads inhabit in crustose corallines dominated subtidal zone from April to July, because drift algae for foods were abundantly provided from intertidal zone where Laminaria religiosa dominated. The sea urchins migrated to search for algal foods to intertidal zone from July to October when their gonads were growing, pre-mature and/or mature stage, and to subtidal zone from November to March when their gonads were recovering stage due to run out of algal foods and to be affected by high wave action in an intertidal zone. The yearling sea urchins inhabited in subtidal zone throughout the year.