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Species Description

Triangle Floater

Alasmidonta undulata


Classification

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

Description

The Triangle Floater is a small-sized freshwater mussel that typically reaches a length of 2.7 inches (70 mm). Its shell is oval to nearly triangular in shape with distinctive, high umbos (rounded, raised structures on shell surface). The periostracum (outer shell surface) is yellow or golden brown and becomes dark greenish brown with age. The shell surface is also often marked with rays that may not be visible on older, darker specimens. The nacre (inner surface of the shell) is bluish-white.

Life Cycle

Some of the specific details about the complex life cycle of this rare mussel are not currently known, but the life history of Alasmidonta undulata is presumed to be similar to related species. Male Triangle Floater mussels release sperm into rivers and creeks with slow to moderate currents. 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. The parasitic glochidia must find and attach to the gills or fins of the appropriate host fish to complete development. Fish hosts have not been identified 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 sand, gravel or mud bottom of a river or creek.

Natural History

Triangle Floater l arvae (glochidia) are parasitic upon tissue of fish hosts while completing the metamorphosis into juvenile mussels. Despite being called the Triangle Floater, adult mussels are typically sessile and are found attached or buried in the sand, mud or gravel bottoms of creeks and rivers with slow to moderate currents. Within its broad range, the Triangle Floater has been found in a wide variety of habitats. It has been found in the sand and mud of slow-moving creeks, the gravel bottom of moderately flowing rivers and even in rapid flowing riffles. Adult mussels are filter feeders and usually feed upon plankton and detritus from their aquatic environment. Triangle Floater 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

This freshwater mussel is found in Atlantic Coast drainages, from the Ogeechee River system of Georgia to the St. Lawrence River system in Canada . The Triangle Floater is also found in the Appalachian-Chattahoochee-Flint River system of Georgia , Alabama and Florida .

Conservation Status

Within Georgia , the Triangle Floater is listed as Rare and Imperiled. Like many freshwater mussels, this species has likely been adversely affected by habitat degradation, pollution and excess sedimentation. Excess sedimentation can smother mussels, causing suffocation and inhibiting effective filter-feeding.

Similar Species

The Triangle Floater is fairly distinct within Georgia . Its small size, high umbos and oval or triangular shape distinguish it from most freshwater mussels within its range in Georgia .