Hypermastus tokunagai is a eulimid ectoparasitic on
Scaphechinus mirabilis,
a sand dollar mainly found in the Seto Inland Sea. This eulimid species is mostly found on its host but free-living individuals are often observed in the sediment. In this study, we investigated whether solitary individuals of
H. tokunagai re-infect
S. mirabilis following detachment in the field and tested the factors affecting re-infection in the laboratory. The field experiment demonstrated that
H. tokunagai is capable of re-infecting its host after detachment. The laboratory experiments suggested that
H. tokunagai were not apparently attracted by chemical stimuli from their hosts. They tended to prefer light over dark regions and white over black regions, but when placed on glass beads and exposed to light, many individuals submerged and moved under a black plate. More individuals of
H. tokunagai preferred dark regions when in the presence of chemical stimuli from their hosts. Our results suggest that the ability of
H. tokunagai to locate its host cannot be ascribed entirely to sensory receptor response to chemical stimuli, and
that
they also rely on vision to approach a host-resembling object in the process of re-infection. Once in close proximity, other factors, such as olfactory and/or tactile stimuli, are likely to play a role in host recognition.
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