The Decline of Order Unionoida Bivalves in Wisconsin Due to Dreissenid Introductions

by Hans Klopp
BIOL/WATER 361, Fall 2012

Key taxa: Mollusca, Bivalvia, Unionoida, Unionidae; Veneroida, Dreissenidae

Order Unionoida is the dominant bivalve taxon found in North America. There are many species of unionids that are threatened and endangered throughout the United States. Native bivalve populations have declined for many reasons including the introduction of exotic species. Dreissena polymorpha and Dreissena bugensis are two non-unionoid species (Family Dreissenidae) introduced to North American waters. Dreissenids have had negative impacts on the mussel fauna found in waters where they were introduced. D. polymorpha and D. bugensis have both caused native unionid bivalve populations to severely decline in areas where introductions occurred. Unionid bivalves are a unique native species, in which adaptive introduced bivalves in the Dreissena genus have caused large declines in the abundance of since they have been introduced to Wisconsin.

Unionid mussels are commonly found bivalves in the large rivers of Wisconsin. Unionid mussels are suspension feeders and use their muscular foot to stay attached to the bottom substrate. The unionids prefer to be in areas where current can deliver food to them, making them abundant in North American rivers (Bogan 1993). Unionid bivalves have effects on the sediment and nutrient cycles of lakes and rivers. Unionid bivalves have glochidia larvae, in which their larvae are parasitic on the gills of host fish (Bogan 1993). Many species of unionids rely on certain species of fish as a transport mechanism of their larvae (Bogan 1993). Unionid mussels are a unique order of mussels and the only bivalve order that has glochidia larvae.

More species of Unionoida mussles inhabit North America than any other continent in the world. There are 297 taxa that are found in North America and 72 taxa are listed as endangered (Williams et al. 1993). Around 50 species of Unionoida mussels are found in Wisconsin, most of which are found in the Mississippi River watershed (Stern 1990). Unionoida mussels are the most common bivalve taxon found throughout Wisconsin. There are 4 species of order Unionoida that are either endangered or extirpated from waters of Wisconsin (Stern 1990; Williams et al 1993). There are two species of threatened bivalves and 9 species of special concern found throughout Wisconsin in order Unionoida (Williams et al 1993). Unionid mussels are an abundant but imperiled group in the lakes and rivers of Wisconsin.

Unionid mussel populations have declined in recent years for various reasons. One factor thought to reduce bivalve populations is the clamming industry which harvests clams for their pearls (Williams et al 1993). Unionid bivalve populations also have declined due to pollution that has been added to the rivers they inhabit. Placing dams on rivers has also caused decreases in unionid bivalves by altering the flow regimes of rivers and the migrations of host fish (Bogan 1993). Many unionid species are intolerant of silt, so increased siltation of rivers has caused declines in unionid populations (Bogan 1993).

Dreissena polymorpha and Dreisseina bugensis are two introduced bivalves that are native to Eastern Europe. They reproduce using a veliger larval stage and are gonochronistic. Dreisseinids produce large numbers of veliger larvae that spread through habitats and become very abundant (Mills et al. 1996). They filter-feed and have substantial effects on the amount of plankton and suspended sediments found in lakes and rivers (Mackie 1991). Zebra mussel populations can completely change the nutrient cycles in the lakes moving nutrients from the pelagic to benthic zones (Nalepa and Fahnensteil 1995). Dreissenids attach to hard surfaces using their adhesive byssal threads (Mackie 1991). Dreissena species attach to hard surfaces such as rocky reefs and docks and also to soft surfaces such as sand and muck (Mills et al 1996). D. polymorpha were first found in Lake St. Clair in 1988 and have spread into waters across the United States since then (Nalepa and Fahnensteil 1995). D. bugensis were first found in the Erie Canal in 1991. Both dreissenid species have been introduced into more waters in the United States since they were first discovered in Lake St. Clair and the Erie Canal. Both species have been found in the Mississippi River drainage since their introductions and are continuing to spread into more waters including some that are located in Wisconsin.

Unionoida mussels have effects on the aquatic ecosystems in lakes and rivers by filtering the water (Strayer et al. 1994). Unionids filtering effects the amount of suspended organic matter and phytoplankton found in lakes and streams. Dreissennids have severe effects on the plankton community and the aquatic macrophyte community due to their filter-feeding (Nalepa and Fahnensteil 1995). Dreissenids have reduced phytoplankton communities in lakes which have had an effect on the amount of food available for native bivalves to eat. D. polymorpha can filter large volumes of water such as a whole lake in a few days when they reach high densities causing large decreases in phytoplankton communities (Nalepa and Fahnensteil 1995). Since D. polymorpha has been introduced the amount of food available for unionids has declined causing reductions in bivalve populations.

Dreissena polymorpha are known to attach to surfaces using their byssal threads. D. polymorpha larvae are released into the water and settle onto hard surfaces (Schloesser et al 1996). Unionidae shells are a surface that the dreissenids like to attach to and are one of few hard surfaces found in sandy substrates. Unionid shells are above the sediments when the dreissenid veligers are released making them easy surfaces for the veliger to attach to (Schloesser et al. 1996). The veligers that are attached to Unionoida shells cause lots of effects on the functioning of the unionid. The zebra mussels attach to the upstream side of unionids where the siphon is located (Mackie 1991). The posterior siphons bring water into the bivalve to filter and carries away wastes. There also are shell deformities caused by the attached dreissenids. The attached dreissenid competes with the unionid it is attached to for food available to filter in the water. Unionids have a harder time filtering food when there are multiple D. polymorpha attached. The attached dreissenids interrupt the ability of the unionid to siphon in food to eat. Dreissenids also decrease the respiration ability of the unionid (Schloesser et al. 1996).

The locomotion of unionids is also affected by Dreissena polymorpha. The increased number of attached dreissenids decreases the ability of the unionid to burrow into sediments. The movement of valves is restricted when many dreissenids are attached to the unionid. The unionid has a more difficult time staying attached to the substrate when there are large colonies of D. polymorpha attached (Schloesser et al. 1996). The weight of the unionid also increases when more D. polymorpha are attached to it, making it more difficult for the unionid move. Unionids are more susceptible to predation when more D. polymorpha are attached to them. Sometimes the unionids are stranded below the mud-water interface and become buried by sediment due to their inability to move due to their increased weight from attached D. polymorpha (Schloesser et al. 1996).

Unionid mussels have been in decline since the early 1900s. They are sensitive to many factors including the introduction of exotic species. Introduction of two exotic species in the Dreissenidae family has been linked to decreases in Unionoida order mussels in United States lakes and rivers. As the dreissennid populations increase, the density of unionids has decreased. When Dreissena polymorpha were introduced to Lake St. Clair their population rapidly increased. Densities of dreissenids greater than 1000 per square meter have rapidly decreased the amount of unionid mussels (Ricciardi et al. 1995). Within 4 to 5 years after dreissenids become established in lakes and rivers, unionid populations will likely become extirpated from the water body the dreissenids were introduced to (Ricciardi et al 1995). Densities of unionids are low in the Great Lakes system and the Mississippi River where D. polymorpha and Dreissena bugensis were introduced. Unionid population declines and extirpations will likely occur as D. polymorpha is introduced throughout Wisconsin.

Some species of unionids are more sensitive to Dreissena polymorpha infestations than other species. Subfamilies Lampsilinae and Anodontinae are more susceptible to infestation than sub family Amblemlinae (Schloesser et al. 1996). There are all three of these bivalve subfamilies found in Wisconsin. There are 17 species of Lampsilinae found in Wisconsin along with 11 species of Anodontinae (Williams et al. 1996). There are 16 species of Ambleminae found in Wisconsin. Many species of the bivalves in Wisconsin are in the more sensitive subfamilies to the introduction of dreissenids. The unionids that brood their young over a longer period of time (9-12 months) are more sensitive to D. polymorpha infestations than short-term brooders (Schloessler et al. 1996). Lampsilinae and Anodontinae are both long-term brooders. The introduction of dreissenids to Wisconsin is likely to have an effect on the biologic community of bivalves in Wisconsin lakes and rivers as the range of D. polymorpha and D. bugensis expands the amount of lentic and lotic places it is found in Wisconsin.

Dreissenids have been found to cause rapid decreases in the native mussel fauna when they have been introduced into the Ohio, Mississippi and Illinois Rivers (Ricciardi et al. 1998). As their range spreads throughout Wisconsin they are going to cause large declines in the native Unionoida fauna when dreisseninds become established. The four endangered or extirpated species (Lampsilis higginsi, Potamilis capax, Quadrula fragosa and Leptodea leptodon) two threatened species (Epioblasma triquetra and Plethobasus cyphyus) and 9 species of special concern (Alasmidonta viridis, Alasmidonta marginata, Cyclonaias tuberculata, Ellipsaria lineolata, Lampsilis cardium, Lampsilus ovata, Ligumea recta, Simpsonaias ambigua and Venustaconcha ellipsiformis) that are likely to become extirpated or endangered in the Wisconsin waters they are found in (Williams et al. 1993). Dreissenid range expansion in the Wisconsin is likely to cause even more decimation to the struggling Unionoida order of bivalves throughout Wisconsin (Ricciardi et al. 1998).

Unionoida bivalves are the most commonly found bivalve taxon throughout Wisconsin. They have a unique life history and are adapted for living in a certain environment that fits their life history. Many species in the Unionoida order have been in decline since the early 1900s. Introduced species such as Dreissena polymorpha and Dreissena bugensis were introduced to North America. Their populations have flourished and their range has spread throughout North America. They have had major effects on North American aquatic ecosystems in many ways. One of the ways that they have changed our aquatic ecosystems is by competing with unionids for food and attaching to them. D. polymorpha have caused major declines in the number of unionids found in lakes and rivers where they were introduced. D. polymorpha and D. bugensis introductions are likely to cause increased rates of extinction and extirpation of the native unionid mussel fauna as their populations expand and increase in lakes and rivers throughout Wisconsin.

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