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Zoological Science
Abstract
Ergaea walshi, a gastropod with a markedly flat shell, often lives inside empty snail shells occupied by hermit crabs. We investigated its lifestyle, shell growth pattern, and habitat preference for host hermit crabs and host snail shells. Four hundred sixteen snail shells, including 363 shells with hermit crabs and 53 empty shells, were collected from intertidal zones of sandy and muddy flats around Kii Peninsula, Japan. The specimens comprised seven hermit crab species occupying 24 shell species; E. walshi was harbored in 13.2% of snail shells with hermit crabs and 17.0% of those without hermit crabs. Although no preference was detected for particular species of hermit crab or snail shell, E. walshi preferred to live inside of snail shells with wider apertures used by comparatively bigger hermit crabs. This suggests that the occurrence of E. walshi was influenced by host size rather than host species. When looking at growth patterns, we found that the attached shell portion of E. walshi continued to be enlarged horizontally, while growth in shell height slowed at approximately 5.0 mm. The conspicuously flattened shell of E. walshi is considered as a growth pattern for adapting to the narrow space within the snail shell occupied by hermit crabs. Consistent with this idea, our comparison of shell growth patterns in 23 calyptraeid species showed that shell of E. walshi is the flattest in this family.
We gratefully thank the following persons who supported our sampling and field survey: Mr. Masanori Fukuoka and all of the staffs in Sugashima Marine Biological Laboratory, Nagoya University. Our special thanks are due to those who gave us opportunities to examine the calyptraeid specimens hold in their respective museums: Dr. Tetsuya Watanabe (Nishinomiya Shell Museum) and Dr. Taiji Kurozumi (Natural History Museum and Institute, Chiba). We thank those who improved our manuscript and supported our analysis: Ms. Faith Jessica M. Paran and Mr. Ryo Nakayama (Seto Marine Biological Laboratory, Field Science Education and Research Center, Kyoto University) and Dr. Katushi Kagaya (Hakubi Center for Advanced Research, Kyoto University). We also thank Dr. Geerat J. Vermeij (Department of Earth and Planetary Sciences, University of California) and the anonymous reviewer for their highly constructive comments
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