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.
Unionid populations in Lake St. Clair have been reduced to extremely low levels. Zebra mussels are rapidly reducing unionid populations in inland lakes such as Lake Wawasee, Indiana. It is estimated that the larger unionids in Lake Wawasee have more than 500 zebra mussels per unionid (Benbow et al. 1994). The impacts of unionid decline on local trophic structure are as yet unknown. However, many threatened and endangered unionid species are much more likely to decline in numbers if zebra mussels invade their watersheds. Recent studies (Schloesser et al. 1997, 1998) continue to document the drastic reductions in unionid populations due to zebra mussel colonization.
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).