Observations of Degrowth in Zebra Mussels (Dreissena polymorpha) During Field Studies in Kentucky Lake, Kentucky

Jeffrey J. Herod
U.S. Geological Survey,
Florida Integrated Science Center, Gainesville, FL 32653 USA

James B. Sickel
Department of Biological Sciences
Murray State University, Murray, KY 42701 USA

Poster and abstract presented at the First Annual Symposium
of Freshwater Mollusk Conservation Society, Chattanooga, TN, 16-19 March 1999

Abstract

One hundred zebra mussels were collected from a site located near Kentucky Dam in the Kentucky portion of the Tennessee River on 20 April 1997 to study in situ growth rates. Four chambers, each containing 25 individual compartments, were secured to the substrate. A 100% survival rate was observed during this study. Initial measurements of shell length, shell height, shell width, and whole wet weight were compared with measurements obtained on 12 June 1997 when chambers were removed and zebra mussels were sacrificed. Chamber one had a change in mean shell length of +1.7 mm, in chamber two +1.8 mm, in chamber three +1.7 mm, and chamber four +2.5 mm. All four chambers contained individuals (%) that exhibited degrowth in shell height. Change in mean shell height in chamber one was 0.0 mm with 52% exhibiting degrowth, in chamber two 0.0 mm with 52%, in chamber three +0.6 mm with 20%, and in chamber four +0.9 mm with 8%. Chambers one and two contained individuals (%) that exhibited degrowth in shell width. Change in mean shell width in chamber one was +1.7 mm with 4%, in chamber two +1.9 mm with 8%, in chamber three +1.1 mm, and in chamber four +1.7 mm. Change in mean whole wet weight in chamber one was +0.541 g, in chamber two +0.555 g, in chamber three +0.556 g, and in chamber four +0.564 g. Zebra mussels do exhibit shell growth and increases in mass in Kentucky Lake. Negative values were observed for some shell parameters. Further studies need to evaluate the principle components involved in degrowth under various laboratory and field conditions.
 

Methods

Zebra mussels were collected from the Kentucky Dam area of Kentucky Lake. Divers using SCUBA removed individual mussels and placed them in dive bags. Mussels were brought to the surface, counted, and transported to Hancock Biological Station in coolers. Once in the laboratory, individual mussels were measured using digital calipers and then placed in growth chambers. Growth chambers consist of 25 individual compartments measuring 1 ½  inches each. This study used 4 chambers which were secured to the substratum with a hanger mechanism. Divers assisted in the placement of the hanger and the attachment of the growth chambers. The chambers were put in Anderson Bay TRM 45.4 on 20 April 1997. Chambers were not checked until they were removed on 12 June 1997. Divers removed the four chambers from the hanger, placed the chambers into coolers, and delivered the coolers to the laboratory. Upon arrival, the chambers were flushed with clean water and the screen was removed. All mussels were checked for survival, measured, and then sacrificed.
 

Results

We observed a 100% survival during our growth study under field conditions. Growth was observed as variable over the 53 total days. Preliminary results indicate that chamber one had a change in mean shell length of +1.7 mm (change ranged from +0.2 to +5.0 mm), in chamber two +1.8 mm (change ranged from +0.4 to +3.4 mm), in chamber three +1.7 mm (change ranged from +0.3 to +3.4 mm), and chamber four +2.5 mm (change ranged from +0.1 to +5.9 mm). All four chambers contained individuals (%) that exhibited degrowth in shell height. This study uses degrowth to indicate that there was a negative value associated with a particular shell morphometrics parameter. Change in mean shell height in chamber one was 0.0 mm with 52% exhibiting degrowth (change ranged from -2.5 to +2.1 mm), in chamber two 0.0 mm with 52% (change ranged from -3.1 to +2.1 mm), in chamber three +0.6 mm with 20% (change ranged from -0.4 to +2.1 mm), and in chamber four +0.9 mm with 8% (change ranged from -0.5 to +3.5 mm). Chambers one and two contained individuals (%) that exhibited degrowth in shell width. Change in mean shell width in chamber one was +1.7 mm with 4% (change ranged from -0.2 to +4.0 mm), in chamber two +1.9 mm with 8% (change ranged from -0.4 to +4.6 mm), in chamber three +1.1 mm (change ranged from +0.4 to +2.2 mm), and in chamber four +1.7 mm (change ranged from 0.0 to +3.9 mm). Change in mean whole wet weight in chamber one was +0.541 g (change ranged from +0.195 to +0.844 g), in chamber two +0.555 g (change ranged from +0.159 to +0.945 g), in chamber three +0.556 g (change ranged from +0.194 to +0.822 g), and in chamber four +0.564 g (change ranged from +0.087 to +0.866 g).

Table of Paired t-test Results for Shell Morphometric Parameters. * indicates significance at p value=.05

Discussion

Preliminary data support evidence of zebra mussel growth patterns in Kentucky Lake, Kentucky in relation to patterns in shell morphology. As expected, length had the greatest increase overall. It appears that smaller mussels had the greatest increase in length. Degrowth was observed for shell height and width for many zebra mussels. Zebra mussel weight increased and was similar between chambers. Chambers were not physically restrictive and were not believed to be directly responsible for degrowth based on our observations. We used mussels that were representative of the Kentucky Lake population. Future projects should attempt more quantitative growth studies which would reflect long term growth patterns and investigate properties involved in degrowth.