Influence of shade and number of boulder layers on mobile organisms on a warm temperate boulder shore

The environment of the habitat is important in the organization of the intertidal community. In a boulder shore habitat, accumulation of boulder layers increases shade and the number of interstices. This study examined whether experimental shade increased the density of mobile animals and modified the community structure at 2 intertidal zones on a moderately sheltered boulder shore in Amakusa, Japan. Artificial shade reduced light intensity and surface temperature of boulders, and subsequently increased microalgal abundance. Mobile animals were sampled quantitatively at 2 zones (high and mid) for 4 treatments: single layer boulders without a roof as a control, double layer boulders without a roof, single layer boulders shaded by a roof for 6 wk and for 2 wk. Statistical analyses (MANOVA and ANOVA) showed strong effects of treatments on community diversity and the density of 15 abundant species in both tidal zones. Generally, the diversity was lower in the single layer boulders than the double layer boulders and increased with the experimental shade. Two carnivorous species (Hemigrapsus sanguineus and Japeuthria cingulata) and some herbivorous species (e.g. Nerita japonica and Nipponacmea nigrans) increased in density with the shade, while other herbivores (e.g. Monodonta labio and Littorina brevicula) were unaffected. In conclusion, a reduction of environmental stress produced by the shade increased community diversity on the boulder shore, but the responses of mobile animals were species specific. As these responses to a change in the environment occurred within a very short period (< 2 wk), migration may play an important role in the short-term variation of community structure on boulder shores.