Family Unionidae (Mollusca: Bivalvia) native to Wisconsin and the Ecology of Non-Threatened or Endangered Species within this Family

by Andrew Teal
BIOL/WATER 361, Fall 2012

Key taxa: Mollusca, Bivalvia, Unionoida, Unionidae

At the present time there are 48-51 species from the family Unionidae in Wisconsin (some may have been extirpated from the Upper Mississippi River System), of which only 26 are not imperiled in some way (Havlik and Sauer, 2004). These species have survived this long due to their inherent ability to adapt well, and quickly, to their dynamic surroundings. The ecology of these macro-invertebrates of the family Unionidae has been studied by many, and so this article will bring many of those sources together. Unionidae also happens to have similar ecology and morphology to that of family Margaritiferidae, which is in the same order as Unionidae (Graf and Cummings, 2007). Be aware that while one looks like the other, Margaritiferidae will not be discussed more than to acknowledge it as a look-alike in the same niche. Mussels native to Wisconsin which are not endangered can be found in many locations in the state, but these bivalves have and continue to face many challenges in their sensitive aquatic ecosystem.

Family Unionidae exists in three different aquatic habitat types, and these can all be found in areas with moderately flowing water. The first of these habitats is lotic-erosional, which means the area is a large river cutting into the bed and banks. The second is known as lotic-depositional, wherein sediment from the suspended load in the river water falls out of suspension and drops onto the bed, adding a new, soft layer to the substrate. The opposite of lotic is lentic, which are places where water usually stays relatively still and does not flow (backwaters, lakes, ponds, etc.). The last of these three habitat types is lentic-littoral, meaning that the unionids would live near shore in shallow water where water laps against the bank or beach (Voshell, Jr., 2002). While there are populations in lentic-littoral areas, they are not high in number and are considerably lower than populations in rivers (Thorp and Rogers, 2011). Bivalves in family Unionidae native to Wisconsin, which are not threatened or endangered, are successful in places like impoundments, large streams and rivers and the shallows of natural lakes; however they do not tend to live in small streams, springs or ponds. Bivalves of this family inhabit areas two to six meters deep (sometimes up to ten meters) in harder water, as it increases the diversity and raises the population number of the mussels (Voshell, Jr., 2003).

Furthermore, the situation of living in a river provides them with an advantage. As benthic filter feeders, they can burrow in to the substrate and simply wait for food to float past. The need to find organisms small enough to consume and in a large quantity has led to an extraordinary ability exhibited by native bivalves. Unionids can filter one to two liters of water per hour per gram of dried mussel tissue to catch zooplankton and algae. Combined with the invasive dreissenids, these two groups can filter 1.5%-5.3% of Lake St. Clair‘s total water volume in a day (Thorp and Rogers, 2011). All of Lake St. Clair would be filtered in roughly three to nine weeks. While Lake St. Clair is not in Wisconsin, it provides a perspective of what these bivalves are capable of doing every day.

Thorp and Rogers (2011) explain the choice of substrate is also connected to the thickness of the shell, depending on the species in question. Generally, the bivalves will spend their time hiding among thick mud, sand or gravel because these substrates are stable. They will typically avoid dense silt and shifting sand though, because these wash away easily and can become a source of smothering. While the preferred locations of the unionids are available and abundant, the places they live in are susceptible to invasion by exotic mollusks such as Corbiculidae and Dreissenidae, which out-compete them (Thorp and Rogers, 2011). What is worse is dreissenids use byssal threads (proteinaceous threads) to attach to any hard substrate, including native mussels. Non-native mollusks binding themselves to the native population puts extra stress on the native (Thorp and Rogers, 2011). This forces it to work harder to respire and feed, until it eventually can no longer open itself and dies. These pests also take away food which otherwise would have gone to the native species. Abiotic factors can cause the same problems in similar ways, many of which are human induced.

An example of a human-induced adverse effect on native Wisconsin unionids is dam removal. The removal of dams which have been decommissioned or fallen into disrepair has been linked to large die-offs of several mollusk species. These mass die-offs include the native unionid Quadrula pustulosa (pimpleback). It has been extirpated from the sample sites in Koshkonong Creek, but was still present in other areas of the state (Sethi et. al., 2004). Results of the study were that no live mussels were found upstream in the former impoundment (though shells were collected and confirmed at least that many individuals had lived there prior to removal). Downstream from where the dam used to stand, eight species of bivalves were discovered, and all of the specimens were collected post mortem. Among the dead were Quadrula pustulosa, Lampsilis siliquoidea, and Lasmigona complanata, all of which still had bits of tissue in the shells and suggested recent death. The specimens collected were gathered from beneath a silt layer 10-20 cm thick, a thickness that followed the banks downstream for 1.7 kilometers (Sethi et. al., 2004). While the dam was in place, sediment movement was stable and minimal. The dam‘s removal reversed the amount of water and silt contained both above and below the spillway, stranding mussels upstream from the dam, and smothering them downstream.

Another of these abiotic issues was temperature ranges and how it affected the unionids‘ ability to cope with being relatively nonmotile. Waller et al. (1999) found that different temperature regimes affected four different species in vastly different ways. The four species being tested, Amblema plicata, Fusconaia flava, Potamilus alatus, and Lampsilis cardium, were put through experiments to determine how well they could move, burrow, and right themselves if flipped over, in the course of one week. The scientists found for every one degree Celsius increased, righting events happened eight percent faster and movement occurred ten percent faster, within the 7-21 degree Celsius range. Amblema and Fusconaia were the slowest in righting themselves compared to Potamilus and Lampsilis. The results suggested water temperature mattered greatly for different genera to survive, and for conservation and harvesting to be successful (Waller et al. 1999). There are many reasons why water temperature matters so much for mussel survival. Higher water temperature increases speed and function, allowing shell repair and growth to happen faster. Hard water is a key component to unionid success is because it contains calcium carbonate, a substance used by many organisms to create protective shells for themselves (Voshell, Jr., 2002).

Human harvest for commercial use is detrimental to native populations; however, laws were put in place to keep the take sustainable (obviously only the bivalves which are not endangered would be garnered). A study conducted by Rosenberg and Henschen (1986) showed in the course of harvesting these mussels, Midwestern suppliers have noticed an increase in nacre staining (used for seed in the cultured pearl trade). Since stained nacre is of no use in the pearl industry, and it can be used as a bioindicator of water quality, Rosenberg and Henschen conducted an experiment.

What Rosenberg and Henschen found was that the threeridge (Amblema plicata) was the most heavily stained of all the specimens they collected, which also happened to be the focus species. In this species, they found that sediment containing especially high amounts of aluminum, sulfur, silica, phosphorus and iron was getting into the shells of the mussels and decreasing the amount of calcium by almost half. High amounts of sulfur and phosphorus in the habitat of Amblema plicata, coupled with magnesium (which is a crystal poison), are disrupting calcium metabolism and calcium carbonate crystallization (Rosenberg and Henschen, 1986). Defects like this are damaging because the natural defenses of the mussel will weaken, preventing it from hiding in firmer substrates. Disruption of calcium metabolism and calcium carbonate crystallization also create thin areas for predators and parasites to gain access to the inside of the shell and the soft tissues within.

Predators in the same patch of habitat as unionids are a concern, as the ones that consume mollusks are usually opportunistic generalists, and mussels represent an easy meal if located. Animals such as turbellarian flatworms will use their muscular pharynx to grab onto the mussel and feed on its soft tissues, and crayfish can effectively break into any bivalve with a shell thin enough to crack (Thorp and Rogers, 2011). Freshwater drum, native catfishes, whitefish, sturgeon, the invasive black carp and many types of birds and terrestrial mammals also prey on these shelled creatures (Thorp and Rogers, 2011).

Freshwater mussels do not really make for ferocious or clever opponents in a fight against the aforementioned predators. Hence, they have evolved a few behaviors and traits to keep them alive long enough for predators to lose interest in eating them, like burrowing and maintaining thick shells. According to Trueman (1968), unionid mussels in his experiment would continue to burrow into the substrate until they were completely covered, even after their cerebral ganglia had been removed. Behavior and nerve action like that provides concealment and forces predators to actively search for this food resource, with the distinct possibility of never finding it.

Molluscan interactions with the environment may appear minimal and difficult to see, but given the proper knowledge and tools, they are all but obvious. Unionid mussels filter water at rapid rates when they are healthy and unimpeded by pests and pollution, making them valuable in several ways. The ability to filter feed removes large amounts of algae, plankton and toxins from water bodies, preventing potentially deadly blooms of any one species. Depending on the system or water body in question, rare, specialized or sensitive mussels may occur there. These are great indicators of excellent water quality, and the higher the population, the better the habitat conditions are compared to their counterparts in other regions of the state. Rosenberg and Henschen (1986) found compelling results to support this idea when they investigated the source of nacre staining. Decreased nacre quality seemed to indicate the beginning of the end for individual threeridge mussels, and given enough time, the whole species. Losing even one species would have a huge ripple effect both within and outside their habitat. Cultured pearls would decrease in supply, natural bioindicators would disappear and biodiversity in the local ecosystem would go down. Conserving these animals is important if future problems are to be avoided.

References Cited

• Graf, D.L. & Cummings, K.S. 2007. Review of the systematics and global diversity of freshwater mussel species (Bivalvia:Unionidae). Journal of Molluscan Studies. 73 (4): 291-314.
• Havlik, M.E. & Sauer, J.S. 2000-2004. Native Freshwater Mussels of the Upper Mississippi River System. Upper Midwest Environmental Sciences Center.
• Rosenberg, G.D. & Henschen, M.T. 1986. Sediment particles as a cause of nacre staining in the freshwater mussel, Amblema plicata (Say) (Bivalvia: Unionidae). Hydrobiologia. 135: 167-178.
• Sethi, S.A. , Selle A.R., Doyle, M.W., Stanley, E.H., Kitchel, H.E. 2004. Response of unionid mussels to dam removal in Koshkonong Creek, Wisconsin (USA). Hydrobiologia. 525: 157-165.
• Thorp, J. H. & Rogers, D.C. Mussels and Clams: Phylum Mollusca, Class Bivalvia. Field Guide to Freshwater Invertebrates of North America. Academic Press. Burlington: 2011.
• Trueman, E.R. 1968. The Burrowing Activities of Bivalves. Zoological Society of London Symposia. 22: 167-186.
• Voshell, Jr., J.R. Information about Different Kinds: Mussels and Clams. A Guide to Common Freshwater Invertebrates of North America. MacDonald and Woodward. Blacksburg: 2003.
• Waller, D.L., Gutreuter, S., & Rach, J.J. 1999. Behavioral Responses to Disturbance in Freshwater Mussels with Implications for Conservation and Management. Journal of the North American Benthological Society. 18 (3): 381-390.