Baltic sculpin

Freshwater mussels of the family Unionidae require an obligatory parasitic larval stage on a specific fish host species to complete their ontogeny. This relationship can be highly specific to co-evolving species and geographically variable, although information regarding host suitability remains scarce. Recent advances in molecular and phylogenetic studies of fish have uncovered cryptic host species that have yet to be tested for their role in mussel development. The genus Cottus, particularly European bullhead Cottus gobio, is known to be a primary host for the threatened thick-shelled river mussel Unio crassus complex, a group of cryptic species of large freshwater mussels endemic to Europe. However, the recent revision of the geographical distribution of another Cottus species, Baltic sculpin Cottus microstomus, raises questions about its suitability as a host for species in U. crassus complex. We report that the transformation of mussel larvae from mixed population of U. crassus sensu stricto and U. nanus into juvenile mussels is successful when C. microstomus serves as the host, with an effectiveness comparable to that of the well-established host Eurasian minnow Phoxinus phoxinus. The sympatric distribution of C. microstomus and U. crassus complex is confined to several river drainages in Central and Eastern Europe, areas that are affected by anthropogenic factors, which may pose challenges to their conservation.