Clarification of the life-history traits of a symbiont and its host is necessary to evaluate the relationship between them. Stylochoplana pusilla, a polyclad, inhabits the mantle cavity of the eulittoral snail Monodonta labio. This study elucidated the life-history traits of S. pusilla relative to those of M. labio in a tide pool, which retained seawater, and a boulder shore, which was dry at low tide, in Mutsu Bay, northern Japan, from 2010 to 2012. In these habitats, S. pusilla was semelparous with a 1-year life cycle; it reproduced in midsummer and inhabited the shell of M. labio except for the early juvenile stages, which were planktonic. More juvenile S. pusilla newly settled on hosts in the tide pool than on the boulder shore, however, their survival rate was higher on the boulder shore compared with the tide pool. The growth rate of juvenile S. pusilla was higher in the tide pool than on the boulder shore. The higher growth rate of S. pusilla at the tide pool suggested that they were more actively feeding. Stable isotope analysis showed that S. pusilla acquired food resources outside the host snail both in the tide pool and on the boulder shore. Thus, it was likely that S. pusilla engaged in feeding outside the host snails but resulted in lower survival rates at the tide pool. These results suggest that S. pusilla is semi-free living and uses M. labio as a refuge in the eulittoral zone for protection against desiccation.
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