Spatial Distribution Patterns of the Parasitic Gastropod Hypermastus tokunagai (Eulimidae) in Populations of its Echinoid Host Scaphechinus mirabilis (Scutellidae)

Abstract

The eulimid gastropod Hypermastus tokunagai is an ectoparasite that utilizes the sand dollar Scaphechinus mirabilis as a host on the sandy bottom in Japan. The distribution patterns of H. tokunagai in populations of S. mirabilis were studied in the western part of the Seto Inland Sea, Japan. The density (= number of individuals per m²) of H. tokunagai was significantly higher in the central region where there is a higher host abundance. On the other hand, a higher prevalence (= percentage of infected hosts) and mean abundance (= number of parasites per host) were recorded in the marginal regions where there is a lower host abundance. These results suggest that eulimid larvae are widely dispersed in a host population but settle largely in a host-abundant area. The infection also showed higher levels in inshore regions than offshore.