Data from: Phylogenetic history of the acquisition of molluscan hosts in acotylean flatworms

The question of how symbionts acquired hosts and diversified
phylogenetically during their evolutionary history is a focus of attention
in many symbiotic taxa. Marine polyclad flatworms are usually free-living,
but some are symbiotic, using animals as hosts. However, the history of
their acquisition of symbiotic systems is not well understood. Therefore,
we focused on mollusc symbiotic flatworms in the suborder Acotylea and
investigated the host specificity and phylogenetic history of symbiotic
acquisition. Field surveys revealed that symbiotic flatworms utilized
certain molluscs as hosts. In particular, Stylochoplana pusilla and
Stylochoplana parasitica utilized different molluscan species as hosts
sympatrically. Also, the phylogenetic analysis and the ancestral state
reconstruction indicate that the mollusc symbiotic flatworms formed a
monophyletic group and that their common ancestor shifted from free-living
to mollusc symbiosis. These results suggest that each of the flatworms did
not independently acquire a symbiotic system with molluscan hosts during
its phylogenetic history, but that their common ancestor acquired a
mollusc symbiotic system, which then underwent acquisition of host
specificity and speciation. This study emphasises that multiple host use
can be a driving force for niche advancement and speciation in the
symbionts.