Muskrats To Lead Charge in Eradicating Zebra Mussels in Cazenovia Lake?
Recently the Lake Association recieved this letter from a Cazenovia Lake Resident.
'The following photos show an ice shelf inside a boathouse on Cazenovia Lake. They show two distinct piles of open zebra mussel shells and some yellow stains on the ice. Since the piles are so neat and distinct, I doubt if they were caused by any wave action. Something must be eating them! We have an animal that looks like a muskrat living inside the boathouse or just outside as I have seen it there. So if it is a muskrat and if they eat zebra mussels, how many muskrats will it take to clean up the zebra mussel infestation?
Could the Lake Assoc adopt the muskrat as out mascot?'
The following research was done by joint effort on the part of several Cazenovia Lake Association Members.
muskrat. [Photograph]. Retrieved December 14, 2007, from
Encyclopædia Britannica Online: http://www.britannica.com/eb/art-96131
Muskrat (Ondatra zibethicus) Predation on Zebra Mussels (Dreissena polymorpha) Attached to Unionid Bivalves
Muskrats (Ondatra zibethicus) commonly prey upon unionid bivalves, discarding the empty shells in discrete piles or middens. We found large numbers of live unionids and the shell remains of zebra mussels (Dreissena polymorpha) in muskrat middens on the upper Ohio River and upper Mississippi River. Muskrats apparently began preying on zebra mussels that were attached to unionids following an increase in zebra mussel abundance at the study sites. Significantly fewer zebra mussels were attached to live unionids at muskrat middens than to ones collected from reference sites away from middens. We also found a group of stones with numerous zebra mussel byssal threads at an Ohio River midden, suggesting muskrats retrieved and ate the attached zebra mussels. We view this, as well as the fact that muskrats discarded live unionids, as evidence that muskrats were selectively preying on zebra mussels rather than unionids.
Contact: John Tucker, Long Term Resource Monitoring Program Pool 26, Illinois Natural History Survey, 4134 Alby Street, Alton, IL 62002, USA
Zebra Mussel Infestation of Unionid Bivalves (Unionidae) in North America
SYNOPSIS
In 1989, zebra mussels received national attention in North America when they reached densities exceeding 750,000/m 2 in a water withdrawal facility along the shore of western Lake Erie of the Laurentian Great Lakes. Although water withdrawal problems caused by zebra mussels have been of immediate concern, ecological impacts attributed to mussels are likely to be the more important long-term issue for surface waters in North America. To date, the epizoic colonization (i.e., infestation) of unionid bivalve mollusks by zebra mussels has caused the most direct and severe ecological impact. Infestation of and resulting impacts caused by zebra mussels on unionids in the Great Lakes began in 1988. By 1990, mortality of unionids was occurring at some locations; by 1991, extant populations of unionids in western Lake Erie were nearly extirpated; by 1992, unionid populations in the southern half of Lake St. Clair were extirpated; by 1993, unionids in widely separated geographic areas of the Great Lakes and the Mississippi River showed high mortality due to mussel infestation. All infested unionid species in the Great Lakes (23) have become infested and exhibited mortality within two to four years after heavy infestation began. Data indicate that mean zebra mussel densities >5,000-6,000/m 2 and infestation intensities >100-200/unionid in the presence of heavy zebra mussel recruitment results in near total mortality of unionids. At present, all unionid species in rivers, streams, and lakes that sympatrically occur with zebra mussels have been infested and, in many locations, negatively impacted by zebra mussels. We do not know the potential consequences of infestation on the 297 unionid species found in North America, but believe zebra mussels pose an immediate threat to the abundance and diversity of unionids.
Contact: D.W. Schloesser, National Biological Service, Great Lakes Science Center, Ann Arbor, Michigan 48105
Zebra Mussel Impacts on Unionids
Naturally occurring substrates are also vulnerable to zebra mussel attachment. Native clams, such as unionids, provide hard substrates for zebra mussel settlement. The degree of fouling (i.e., number of zebra mussels per unionid) relates directly to both the local density of zebra mussels and the size of the unionid (reviewed in Strayer (1999)). In North America, zebra mussels have nearly extirpated native unionid from infested lakes and rivers by fouling their shells and outcompeting them for food. There is also evidence from Hudson River studies that zebra mussel can seriously affect unionid populations without directly fouling them, but rather by reducing food concentrations to levels too low to support their reproduction and survival (Strayer 1999).
Unionids can actively move to areas with good food and oxygen conditions and, by mixing the water while filtering, can improve the local food and oxygen conditions for attached zebra mussels (Karatayev et al. 1997). However, fouling by zebra mussels has negative effects on the host unionid bivalves (Schloesser et al. 1998, Karatayev et al. 1997). By attaching to their valves, Dreissena can make it more difficult for unionid bivalves to burrow and move through sediment, and the added mass of Dreissena can weigh down unionid bivalves, resulting in burial in very soft or unconsolidated sediments. Mussel attachment to unionid bivalve shells can increase drag and the likelihood of dislodgment by water motion for species living near shore. In addition, zebra mussel attachment can occlude the openings in unionid bivalve valves, inhibiting feeding and reproduction, or preventing the closing of valves. Dreissena may directly compete with unionid bivalves for food, occupy otherwise available space, and induce unionid bivalve shell deformities.
Thousands of zebra mussels per host unionid were reported in western Lake Erie in a 1989 study (Schloesser and Kovalak 1991). Such dense encrustations were killing native clams, eventually making it impossible for them to open their shell valves to adequately filter feed and burrow. The same unionid is shown before and after zebra mussel removal.
Unionid before zebra mussel removal.
Unionid after zebra mussel removal.
There are numerous examples of native mussels being impacted by zebra mussel encrustation.
The negative effect of zebra mussels on unionids can depend on a variety of factors, including: time since invasion; type of bottom sediment; unionid species, and unionid sex (reviewed in Karatayev et al. 1997). Short-term brooders appear to be less sensitive to zebra mussels than long-term brooders (Strayer 1999).
From European studies it is known that the extensive overgrowth by Dreissena of unionids, resulting in mass mortality, is characteristic of periods of rapid population growth of zebra mussels when they invade a new water body (Sebestyen 1937, Dussart 1966, Karatayev and Burlakova 1995a). Following this period, Dreissena tend 
to coexist with native bivalve populations. Although overgrowth can cause some host mortality, populations of unionids not only have survived, but also maintained high densities (Karatayev and Tishchikov 1983, Miroshnichenko et al. 1984, Miroshnichenko 1987). Will Dreissena have greater impacts on unionids in North America than in Europe? In pre-glaciation Europe, Dreissena and unionids coexisted. As North American aboriginal species have no evolutionary history of coexistence with zebra mussels, Dreissena may have a larger impact on North American than European species. The species composition of unionids in the North America is much different and more diverse than in Europe, and there may be species-specific differences in response to fouling. Recent North American studies report that areas of dreissenid/unionid coexistence can occur along Lake Erie in some wetlands (Nichols and Wilcox 1997) and wave-swept, nearshore areas (Schloesser et al. 1997).
