INTERACTIONS BETWEEN THE HYDROID CORDYLOPHORA SP. AND THE ZEBRA MUSSEL DREISSENA 
POLYMORPHA IN TWO MIDWEST RIVERS.

Emily Thorn1, Laura Page1, Nadine C. Folino-Rorem1, Jim Stoeckel 2, Chris 
Flinn1.
1Biology Department, Wheaton College, Wheaton, IL 60187, 2Illinois River 
Biological Station, INHS, Havana, IL 62644.

The freshwater-brackish hydroid Cordylophora sp. (Phylum Cnidaria) and the zebra 
mussel Dreissena polymorpha (Phylum Mollusca) are invasive species found to 
coexist in many waterways worldwide. Both are epibenthic organisms that foul 
ship hulls, dock pilings, and power plant intake pipes, and disrupt native 
ecosystems. It has been proposed that Cordylophora may enhance zebra mussel 
larval settlement by providing an appropriate filamentous substrate or may 
inhibit larval settlement by eating zebra mussel larvae.  The purpose of this 
study was to clarify the relationship between hydroid filaments and mussel 
larvae as these two Ponto-Caspian organisms co-exist within the Mississippi 
River system. Field experiments in the Des Plaines River, IL, where both zebra 
mussels and hydroids co-exist, were conducted to determine the effects of the 
hydroid on mussel settlement.  Settlement plates with live, dead, and no 
Cordylophora were deployed.  In addition, settling plates with hydroid “mimic” 
polypropylene filaments attached were placed in the Bark River, Wisconsin, in a 
location where Cordylophora has not yet been found but Dreissena is abundant.  
Larval samples were taken weekly at both sites to document the supply of zebra 
mussel larvae of settlement size. Preliminary results indicate that artificial 
filaments enhance mussel settlement while live and dead colonies at the Des 
Plaines location enhance settlement of a variety of invertebrate fauna. Zebra 
mussel larvae presented to Cordylophora in the laboratory were consistently 
rejected or failed to be ingested, suggesting that Cordylophora rarely consumes 
zebra mussel larvae. These data will help clarify the effects of the filamentous 
Cordylophora sp. on larval settlement of Dreissena polymorpha and their combined 
effects on native communities.

Keywords: Invasive species, Cordylophora sp., Dreissena polymorpha, fouling 
organisms, Ponto-Caspian