Life history traits and ecological conditions influencing the symbiotic relationship between the flatworm Stylochoplana pusilla and host snail Monodonta labio

Abstract

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 snailMonodonta labio. This study elucidated the life-history traits ofS. pusillarelative to those ofM. labioin 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. pusillawas semelparous with a 1-year life cycle; it reproduced in midsummer and inhabited the shell ofM. labioexcept for the early juvenile stages, which were planktonic. More juvenileS. pusillanewly 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 juvenileS. pusillawas higher in the tide pool than on the boulder shore. The higher growth rate ofS. pusillaat the tide pool suggested that they were more actively feeding. Stable isotope analysis showed thatS. pusillaacquired food resources outside the host snail both in the tide pool and on the boulder shore. Thus, it was likely thatS. pusillaengaged in feeding outside the host snails but resulted in lower survival rates at the tide pool. These results suggest thatS. pusillais semi-free living and usesM. labioas a refuge in the eulittoral zone for protection against desiccation.