Department of Applied Aquabiology, National Fisheries University, Shimonoseki, Yamaguchi 759-6595, Japan
The sand dollars Scaphechinus mirabilis, Astriclypeus manni and Peronella japonica are distributed in the area of the Zostera bed of Mashima Island, eastern Yamaguchi Prefecture. A field survey in an area of 35 m×45 m in April 2006 showed that the highest density of S. mirabilis (30 inds./m2) was distributed in the area with low density of Z. marina (11 shoots/m2). On the contrary, 4 inds./m2 of S. mirabilis were observed even in the area with high density of Z. marina (200 shoots/m2). In an aquarium experiment, 5 shoots of Z. marina were transplanted into half the area of each of 18 containers of 940 cm2 with S. mirabilis or A. manni and were observed for 63 days. As a result, two species of sand dollars mainly stayed in the area without Z. marina. Z. marina grew well whether it cohabited with the sand dollar or not. From these results, it is considered that there is no interaction between sand dollars and Z. marina (neutralism).