The Higgins eye pearlymussel (Lampsilis higgins) is a freshwater mussel that only occurs in parts of the upper Mississippi River north of Lock and Dam 9 at Keokuk, Iowa. It’s also found in three tributaries of the Mississippi: the St. Croix River between Minnesota and Wisconsin, the Wisconsin River in Wisconsin and the lower Rock River between Illinois and Iowa. Primarily a sedentary species, the Higgins buries itself at the bottom of large rivers. It has a soft body enclosed by the shell and consists of gills for breathing, a digestive tract for processing food, and a large, muscled foot for moving and for anchoring on the stream bottom.
The Higgins eye pearlymussel was listed as an endangered species on June 14, 1976. It was among the first freshwater mussels to receive federal protection. Habitat loss and degradation and exotic species are major threats to the survival of the Higgins eye. Impounded river systems have altered the water flow patterns, substrate characteristics and host fish habitat, which has severely affected how the mussels feed, live and reproduce. This is a species that depends on deep, free-flowing rivers and clean water. Municipal, industrial and farm run-off further degrade water quality. As filter-feeders, mussels concentrate chemicals and toxic metals in body tissues and can be poisoned by chemicals in their water. Dredging and waterway traffic produce siltation which can cover river substrate and mussel beds.
Zebra mussels (Dreissena polumorpha) is the greatest known threat to the Higgins eye. They are a freshwater mussel native to the Black and Caspian Seas that were introduced into Lake Erie in the late 1980s from ship ballast water discharge. These small mussels are less than 2 inches long, but tens of thousands can colonize a square meter area. Zebra mussels attach to any hard surface, including shells of other mussels, preventing them from normal travel, burrowing, and opening and closing their shells.
Several Higgins eye populations in the Mississippi River have been hit hard by zebra mussel colonization. The population in Prairie du Chien, Wisconsin, has been reduced from one of the most numerous to one of the most threatened. Technology to control zebra mussels is being studied, but no successful measures have yet to be developed that can reliably limit zebra mussel colonization and not harm native mussels.
The Higgins eye pearlymussel is usually found in deep water with moderate currents and stable substrate that vary from sand to boulders. The animals bury themselves in the substrate of the river bottoms with just the edge of their partially opened shells exposed. They are usually found in mussel beds that contain at least fice other species, as noted in the 2004 recovery plan. River currents flow over the mussels as they siphon water for microorganisms such as algae and bacteria, which they use as food. Higgins eye are prey for wildlife like muskrats, otters and raccoons. They filter water which improves water quality and mussel beds create microhabitats on river bottoms that provide food and cover for other aquatic life.
A natural body of running water.
Higgins eye shells are usually yellow, greenish, reddish or brown with green rays. The inside of the shell is white with portions that are iridescent and areas that may be tinged with cream or salmon.
According to descriptions by F.C. Baker in 1928, the shell of the Higgins eye is oval or elliptical, somewhat inflated, and solid. The shell is made of up of two hinged, inflated halves with a rounded side and a pointed, in males, or squared sides in females. The shell can be up to 15 centimeters (6 inches) long according to the Minnesota Department of Natural Resources rare species guide.
There are currently no studies focusing specifically on the Higgins eye pearlymussel. However, published studies on the feeding mechanisms of unionid mussels by R.A. Tankersley and R.V. Dimock in 1992 and 1993 describe unionids as filter-feeders that remove small, suspended particles from the water column using the large lamellibranch gills as feeding organs. Feeding rate in bivalves is also known to be greatly influenced by temperature, food concentration, food particle size and body size, as documented by C.B. Jørgensen in 1975 and later in 1978 by T.E. Winter.
Male Higgins eye release sperm into river currents, often in packets, known as volvocoid bodies, and females downstream siphon the sperm to fertilize their eggs, as documented by S.L.H Fuller in 1974. After fertilization, females store developing larvae, or glochidia, in their gills until expelling them back into the current. D. Waller with the U.S. Geological Survey notes that the glochidia attach to a fish host, where they remain for approximately three weeks, at water temperatures of 20 to 22 Celius, as they transform into juveniles. The sauger, walleye, yellow perch, largemouth and smallmouth bass, as well as freshwater drum are considered suitable hosts for Higgins eye glochidia.
The juveniles then drop off their fish host and develop a byssal thread, which may assist in dispersal, and upon settling on suitable habitat on the river bottom. They use the byssal thread as a means of attachment, to prevent being swept away in water currents. Once settled, the juveniles mature into adult mussels and possibly live up to 50 years. Higgins eye is a long-term brooder, bradytictic, meaning that they spawn in the summer and larvae are retained in the marsupia, which is a pouch that protects eggs, through the winter until they are released the following spring or summer.
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