EFFECTS OF FOULING BY THE HYDROID CORDYLOPHORA ON THE GROWTH AND
SURVIVORSHIP OF THE ZEBRA MUSSEL DREISSENA POLYMORPHA IN ADVERSE
ENVIRONMENTAL CONDITIONS.

Jason Polk1, Jim Stoeckel2, and Nadine C. Folino-Rorem3.  
1Defiance, Missouri 63341  2Illinois River Biological Station, Illinois Natural History Survey,
Havana, Illinois 62644  3Wheaton College, Biology Department, Wheaton, Illinois 60187.

Field observations of settling plates and natural benthic communities in the Illinois River during
the summer and fall of 1998 and 1999 suggest that two invasive species, the zebra mussel
Dreissena polymorpha and the freshwater colonial hydrozoan Cordylophora, frequently co-exist
in areas of suitable substrate.  Furthermore, Cordylophora colonies often foul the zebra mussels,
exhibiting rapid growth of dense colonies on the mussel shells.  This raises questions about
possible interspecific interactions, and the resulting effects on the growth and development of
these two organisms.  

Zebra mussels (5-10 mm) were collected from Lake Michigan and maintained in the laboratory
for several weeks prior to exposure to experimental conditions.  Eight randomly selected groups
of 50 mussels were attached to plexiglass plates which were vertically mounted in ten-gallon
aquaria with identical temperature, flow, aeration, and filtration conditions.  In four of the
aquaria, excised pieces of a Des Plaines River Cordylophora colony were tied to the plexiglass
plates next to each mussel and allowed to grow on and around the mussel shells.  Mussels in the
other four aquaria were kept free of fouling by Cordylophora.  All aquaria received identical
feeding regimes of brine shrimp nauplii and cryopreserved algae.  During an 11-week
experimental period, the experimental aquaria were exposed to a standard set of unfavorable
environmental conditions (e.g. high salinity, low pH, low calcium ion concentration, and low
flow) for weeks 1-6 and favorable environmental conditions for weeks 7-11.  The average shell
length, wet tissue mass, dry tissue mass, and gamete maturity of the zebra mussels were
measured by sacrificing a sub-sample of five mussels from each aquarium every other week.  The
average wet and dry tissue mass of Cordylophora was also measured by collecting all the hydroid
tissue within 1.5 cm2 surrounding each sacrificed mussel.  

Mussel growth was limited during the initial seven weeks of the experiment due to unfavorable
water quality conditions.  Experimental mussels exhibited high mortality, gamete immaturity,
and a decrease in average tissue mass.  There was no detectable difference between treatments
indicating that Cordylophora fouling did not affect mussel response to adverse environmental
conditions.  Following the remediation of the poor water quality during the seventh week of the
experiment, the mussels of both treatments showed signs of recovery in the form of increased
average shell length and tissue mass.  Zebra mussels not fouled by Cordylophora did show a
lower rate of mortality and a higher rate of recovery and growth than those fouled by the hydroid,
however this difference was not statistically significant at the ?=0.05 level due to the large degree
of variation among the mussel sub-samples of several aquaria.  The difference in mussel growth
rate between the two treatments during the final weeks of the experimental period suggests that
the presence of Cordylophora had a negative impact on the recovery and subsequent growth and
survival of experimental mussels, but additional experimentation using larger sample size, longer
experimental period, and stricter control of environmental conditions would be necessary to
confirm or deny its statistical significance.  Further investigations into the interspecific
relationships between invasive species in the Illinois and Mississippi River system should
provide valuable information concerning the expansion, impacts, and control of non-endemic
organisms.

Keywords:  zebra mussel, hydroid, invasive species, Cordylophora, Dreissena polymorpha