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Species Description

Downy Rainbow

Villosa villosa


Classification

Phylum: Mollusca
Subphylum:
Class: Bivalvia
Order: Unionoida
Family: Unionidae

Description

The Downy Rainbow is a small freshwater mussel that usually measures less than 2.5 inches (63.5 mm) in length. It has a fairly thin shell that is moderately inflated or swollen and oval-shaped. The umbos (rounded, raised structures on shell surface) are moderately swollen and often decorated with several fine ridges. The periostracum (outer shell surface) is generally rough and greenish yellow, dark green or brownish black. The shell surface is also decorated with faint blue, green or yellow rays that are usually not visible to the naked eye. The nacre (inner surface of shell) is bluish-white.

Life Cycle

Some of the details about the complex life cycle of this mussel are not currently known, but the life history of Villosa villosa is presumed to be similar to related species. Male Downy Rainbow mussels release sperm into the slow to moderate current of creeks and rivers. Sperm enters females through siphon-like regions and fertilization of eggs occurs within female shells. These fertilized eggs develop into special larva called glochidia. Glochidia continue to develop and are released into the water column when fully matured. Parasitic glochidia must find and attach to the gills or fins of the appropriate host fish to complete development. The Largemouth Bass (Micropterus salmoides) and Bluegill Sunfish (Lepomis macrochirus) have been identified as suitable hosts for this species. The glochidia parasitize a fish host for a variable length of time, likely depending upon water temperature, fish species and other factors. Larvae transform into juvenile mussels on the fish and then release from the host to find a suitable substrate, often the mud, sand or silt bottom of a creek, reservoir or river.

Natural History

Downy Rainbow l arvae (glochidia) are parasitic upon tissue of fish hosts while completing the metamorphosis into juvenile mussels. Adult mussels are typically sessile and are found attached or buried in the mud, sand or silt bottoms of creeks and rivers. This species appears to prefer a habitat of slow to moderate flowing water and has been found in a number of reservoirs. Adult mussels are filter feeders and usually feed upon plankton and detritus from their aquatic environment. Downy Rainbow mussels bring water from their habitat into their shells through specialized regions that are similar to the true siphons of clams. The water is then filtered over its gills and food particles are trapped and eventually digested.

Range

south to central Florida . It was also found within the Apalachicola-Chattahoochee-Flint (ACF) River system of Georgia and Florida . It is presently found in Chattahoochee River tributaries and in the Flint River and its tributaries within western portions of Georgia . Further studies and surveys should be conducted to determine its present range in Florida , from the St. Marys River to central Florida .

Conservation Status

This species is listed as Uncommon or Rare in Georgia . However, populations appear to be relatively stable and more tolerant of the construction of impoundments (dams and reservoirs) than most freshwater mussels. The Downy Rainbow has likely been affected by excess sedimentation, pollution and habitat degradation.

Similar Species

The Downy Rainbow is most likely to be confused with the Southern Rainbow (Villosa vibex). However, the Downy Rainbow has a thicker shell, distinctive roughened surface and is typically more elongate in shape.