Host‐specific morphologies but no host races in the commensal bivalve<i><scp>N</scp>eaeromya rugifera</i>

<jats:title>Abstract</jats:title><jats:p>Speciation by host shift is one of the explicit models of ecological speciation. A prerequisite of this model is the formation of host races (sympatric populations that show host‐specific genetic structuring and phenotypes). Many members of the diverse marine bivalve superfamily<jats:styled-content style="fixed-case">G</jats:styled-content>aleommatoidea have obligate commensal relationships with invertebrate hosts. Some species have the ability to occupy multiple host species, thereby providing potential opportunities to test for the formation of host races. The<jats:styled-content style="fixed-case">N</jats:styled-content>ortheast<jats:styled-content style="fixed-case">P</jats:styled-content>acific galeommatoidean<jats:italic><jats:styled-content style="fixed-case">N</jats:styled-content>eaeromya rugifera</jats:italic>attaches to two strikingly different hosts: the blue mud shrimp<jats:italic><jats:styled-content style="fixed-case">U</jats:styled-content>pogebia pugettensis</jats:italic>and the polychaete sea mouse<jats:italic><jats:styled-content style="fixed-case">A</jats:styled-content>phrodita</jats:italic>spp. We tested if this host difference has resulted in the formation of host races using shell morphologies and genetic markers. We found that populations from different hosts differ significantly in shell morphology. However, based on mitochondrial makers,<jats:italic><jats:styled-content style="fixed-case">N</jats:styled-content>. rugifera</jats:italic>showed no distinct host‐specific genetic structuring, indicating the existence of a panmictic population. We conclude that the host‐specific morphologies these clams exhibit may reflect ecophenotypic plasticity rather than the existence of host races, but this needs to be corroborated with additional genetic data and larger sample sizes. The pronounced conchological variation within<jats:italic><jats:styled-content style="fixed-case">N</jats:styled-content>. rugifera</jats:italic>calls for further investigation of its taxonomic relationship with its poorly studied, but morphologically similar, sympatric congener<jats:italic><jats:styled-content style="fixed-case">N</jats:styled-content>eaeromya compressa</jats:italic>.</jats:p>