Freshwater Mussels

by Moneil Patel

Mussels are a species of marine life that inhabits many of the bodies of water in the United States. They serve as an important aspect of the ecological system of the water in which they occupy. However, the number of mussels have been declining such that many are now considered as endangered species. There are a few reasons for the decline in population, including domination by a single species of mussels called the zebra mussel. Conservation has begun on mussels because of their strong environmental influence. The two groups interested in mussels involve parties that use the lakes for recreation and consider mussels as a threat to their gaming as well as ecologists that understand the domination of zebra mussels and attempt to protect the native mussels from the zebra mussels.

Mussels are in a group of invertebrate animals called mollusks. Freshwater mussels are also referred to as clams, naiads, and unionids. Mussels spend its life anchored in rivers or lake bottom sediments. The vast majority of them are found in streams. Their movement is through either muscular feet or powerful flood currents. A mussel captures oxygen and microscopic food particles in flowing water through filtration (Vermont's Freshwater WWW). Mussels continuously pump water through their bodies. Water enters through the incurrent or branchial siphon and exits via the excurrent or anal siphon (Mussel Biology WWW). This pumping process allows the mussel to filter food from the water. The food consists of detritus, organic matter found on the stream, and plankton which are microscopic plants and animals suspended in water (Mussel Biology WWW). Mussels are long lived species with some living more than 10 years. Others have been recorded as over 100 years old. There are however pollutants that reduce this life expectancy.

Freshwater mussels have a high ecological value. They are an important food source for many other animals including muskrats, minks, otters, fishes, and some birds. More importantly, they serve as indicators in that they are sensitive to pollutants such as heavy metals, pesticides, agricultural nutrients, heavy loads of fine silts. These pollutants will kill mussels and thus provides a warning of the particular waters infested with these pollutants (Vermont's Freshwater WWW). It is inevitable that mollusks are going to be exposed to water born chemicals. Once exposed uptake is very rapid and very easy. Most of the chemical pollutants in the San Francisco Bay, for example, are organochlorine insecticides otherwise known as chlorinated hydrocarbons (West Coast Mollusc 1988). Examples of these compounds include benzene hexachloride, chlordane, DDT, dieldrin, and toxaphene. There are other unnoticeable pollutants that are found in commercial plastics such as Phthalate esters. Since mollusks are not very good at biodegrading these pollutants pose a threat to their lives.

North America hosts one-third of 300 mussel species. But of these, 70 percent are either extinct, endangered, or declining in population (Vermont's Freshwater WWW). In the US, there are 42 mussels that are listed as federally endangered or threatened and another 70 species have been proposed (Mussel Biology WWW). One of the more common is the dwarf wedge mussel (Alasmidonta heterodon) which is found in 9 states in the mid east portion of the United States (US Fish and Wildlife 1994). The state of Kentucky lists 11 species of mussels that are endangered, in danger of being extinct. Conservation efforts are starting to begin because of the various threats to native mussels.

Factors contributing to the decline of mussels are overharvest, siltation of their habitat, from agriculture, poor land management, channelization, and impoundments, pollution by herbicides, pesticides, and other chemicals (Mussel Biology WWW). Conservation has begun by methods of depuration by exposure of mussels to clean water (West Coast Mullusc 1988). The best chance for wildlife conservation and recovery is through habitat restoration and enhancement (Conch Net: What Can... WWW). But another reason to the declining population is by another dominant species of mussels that endanger the native mussels.

Zebra mussels originated in the Balkans, Poland, and the former Soviet Union. In 1769, Pallas first described populations of this species in the Caspian sea and Ural River. Zebra mussels are about 50 mm in size and their filter feeding involve inhalant and exhalant siphons. They can filter about 1 L of water per day and feed on algae. A typical reproductive cycle shows that 40, 000 eggs can be laid during this time (Dreissena Polymorpha WWW). Zebra mussels feed on phytoplankton and thus outcompeting zooplankton for food (Zebra Mussel WWW). But migration of the zebra mussels to North America is what has been the cause of concern.

Zebra Mussels were first discovered in North America in 1988, more specifically at Lake St. Clair in Canada. By 1990, the mussels were found in all of the Great Lakes. They then found their way to the Illinois and Hudson Rivers which then lead to the Mississippi River (Dreissena Polymorpha WWW). By 1992, 5 states were habited by the zebra mussel. That number increased to 23 by 1994 and slowly continue to increase (Dreissena Polymorpha WWW). The probable cause for the method of introduction of this species to North America is most likely due to ballast water from cargo ships from the Atlantic. Another form of introduction within the United States is due to private boats who move from lake to lake for leisure.

Zebra mussels have eliminated 90 percent of the native mussels in Lake Erie (Vermont's Freshwater WWW). Native mussels play an important role in nutrient cycline and sediment mixing. This could seriously affect the ecology of the Mississippi River system. The introduction of the zebra mussel could wipe out up to 140 species there(Zebra Mussel WWW). Information from Europe suggests that zebra mussels dominate over native mussels by interfering with feeding, growth, locomotion, respiration, and reproduction. Research shows that zebra mussels prefer to attach to live unionids (native mussels) rather than dead ones or rocks. An experiment conducted by Schloesser and Nalepa in 1994 showed a decrease in population of native mussels compared to zebra mussels after 2 years of introduction (Dreissena Polymorpha WWW).

The US Fish & Wildlife expects 5 billion dollars in damages by the year 2002, due to its large scale eco-system changes (Zebra Mussels WWW). Averages suggest that upon zebra mussel colonization, native mussels will deplete within four years (Zebra Mussels). Other parties have other reasons for removing or protecting against the zebra mussels. Zebra mussels colonize pipes, which constricts flow. Navigational and recreational boating can be affected by drag due to mussel attachment. They also damage engine cooling systems, fishing gear, dock pilings, and also cause the corrosion of steel.

Conservation against Zebra Mussels - Two institutions have been involved in conservation or rather protection of native mussels from zebra mussels. Their attempt is to minimize the amount of zebra mussel migration. The Great Lakes Sea Grant Network and National Biological Service have recommended precautions to the general public, specifically boat owners, to help prevent the spread of zebra mussels (Zebra Mussels WWW). Such recommendations include removing attached vegetation, washing boats or trailers, flushing engine cooling systems, not leaving unused bait behind, and inspection of hulls of boats prior to relocation. The hunting community suggest eliminating the zebra mussels by using chemical molluscicides, thermal methods (like steam injection), manual removal, dewatering, and electrical current. These methods of removal would greatly destroy other species as well as the zebra and is therefore not a technique used by restoration ecologists.

Placing native unionids in temporary refugia or transplantation to areas void of zebra mussels is being applied as a method of conservation (Dreissena Polymorpha WWW). Relocation research is being conducted at Elk River in West Virginia, Meramec River in Missouri, and St. Croix River in Wisconsin/Minnesota. Monitoring the relocation on the St. Croix River includes evaluation of overall mussel survival, growth and survival of special status species, handling methods, placement methods, and mussel condition (Conservation Conference WWW).

Due to chemical pollutants and zebra mussels, many of the unionids are declining in population or becoming endangered. Conservation efforts have begun by two main methods. Ecologists are focusing on mussel relocation to separate the native mussels from the zebra mussels, or polluted body of water. While those in the gaming industry are focused on killing the zebra mussels by a variety of different methods including electrical. Nonetheless many of the native mussels are endangered and must be protected, by mainly not polluting our freshwaters. In terms of species domination by the zebra mussel, perhaps that plays in the role of natural selection, and unless there are severe consequences, should be left alone.
 

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