ORIGINAL PAPER Occurrence of Conchophthirus acuminatus (Protista: Ciliophora) in Dreissena polymorpha (Mollusca: Bivalvia) along the River Shannon, Ireland David Bruce Conn Æ Sarah E. Simpson Æ Dan Minchin Æ Frances E. Lucy Received: 27 December 2006 / Accepted: 30 April 2007 / Published online: 6 June 2007 Ó Springer Science+Business Media B.V. 2007 Abstract Samples of Dreissena polymorpha were collected at several sites along the River Shannon navigation in Ireland, to determine the occurrence and distribution of their obligate host-specific com- mensalistic ciliate, Conchophthirus acuminatus, in this newly invaded region. Mussels collected by various methods were fixed immediately in 75% ethanol, in which they were later dissected under a stereoscopic microscope, beginning with thorough flushing of the mantle cavity and removal of the gills. One ml of sediment flushed from the mantle cavity and dissection residue of each mussel was examined under a compound light microscope using brightfield, phase-contrast, and differential-interference-contrast optics. Of 180 mussels examined, 125 (69.44%) harbored C. acuminatus. The ciliates were invariably well fixed and easily identifiable in all preparations. Mean sampling intensity for infected mussels was 8.47 ciliates per ml of sediment. Both prevalence and sampling intensity varied between sites, but no pattern was discernible. The present results are consistent with other reports of C. acuminatus being the most widespread and abundant symbiont of D. polymorpha throughout Europe, often occurring where no other symbionts occur. Its occurrence in Ireland indicates introduction of the mussels as adults, since planktonic veliger larvae are not known to harbor ciliates. Following similar reasoning, it is possible that the earlier North American invasion by D. polymorpha included only veligers, since C. acuminatus has not been found on that continent. Using these simple and quick methods, the ciliates could be easily identified and counted to give general comparative data among sites regarding intensity and prevalence. Thus, this method has promise for future efforts to obtain basic information rapidly in newly invaded systems. Keywords Commensal Á Invasive Á Ciliate Á Mollusk Á Conchophthirus Á Dreissena Á Ireland Á Symbiotic Á Zebra mussel Introduction When species invade new geographical areas, they often have a significant ecological impact on the newly colonized environment. Among animal invad- ers of freshwater systems, few species have had as great an impact as the zebra mussel, Dreissena D. B. Conn (&) Á S. E. Simpson School of Mathematical & Natural Sciences, Berry College, Mount Berry, GA 30149-5036, USA e-mail: bconn@berry.edu D. Minchin Marine Organism Investigations, 3 Marina Village, Ballina, Killaloe, Co. Clare, Ireland F. E. Lucy School of Science, Institute of Technology, Sligo, Ireland 123 Biol Invasions (2008) 10:149–156 DOI 10.1007/s10530-007-9118-9