ABSTRACT
A map of the study area is provided in Figure 1. On the Boso Peninsula, Kominato (N35°7′, E140°11′)
1 INTRODUCTION
Urchin barren or urchin-dominated barren ground is one of the most conspicuous habitats in and around seaweed beds around the world (Lawrence 1975, Harold & Pearse 1987). In Japan, urchin barren is a type of ‘Isoyake’, namely the prolonged decrease of seaweeds; sea urchins most commonly causing urchin barrens are those in the Strongylocentrotus (mostly S. nudus) in northern Japan, and Diadema (mostly D. setosum) and Anthocidaris crassispina in southern Japan (Fujita et al. 2008). Few studies (Yotsui & Maesako 1993, Dotsu et al. 2002) have reported the southern type of urchin barrens, while hundreds of papers (e.g. Agatsuma 1997, Fujita 1998) have documented the northern type of urchin barrens. Sea urchins in the genus Diadema are common in tropical to subtropical areas (Lessois et al. 2001). However, the earliest Japanese echinoderm taxonomist, Shigezo Yoshiwara (1874-1940), reported the occurrence of Diadema setosum from the temperate Pacific coasts of Boso and Izu peninsulas in central Honshu, as well as Kyushu, Japan, more than a century ago (Yoshiwara 1898). On the central Pacific coast of Honshu, urchin barrens have never been studied and understanding of the ecology of Diadema is limited to grazing behaviour on corals (Atobe and Ueno 2001). Recently, the authors found patchy and zoned Diadema barrens on the Boso and Izu peninsulas, respectively, and tried restoration of seaweeds there. In the present paper, we document
is the northern limit of D. setosum on the Pacific coast of Honshu (Yoshiwara 1898, Kawana 1988), but no urchin barrens have been reported there. Okinoshima (N34°59′, E139°49′) is a small tombolo island (1 km in circumstance) located at the southern mouth of Tokyo Bay. The southern coast of the island is composed of cuesta-like bedrock forming ledges (up to 1 m high) and boulders which have been formed when ledges collapsed. Okinoshima is unique because of the influence of the warm Kuroshio Current, so the biota here contains some tropical elements including Diadema and corals among temperate elements such as Eisenia, Ecklonia and Undaria kelps (Chiba Prefecture 1971, Miyata 1995).
