Sheepnose, also known as bullhead or clear profit, is one of three Plethobasus genus species within the United States, all of which are currently listed as federally endangered. The species is characterized by its yellowish-brown color and single row of wide bumps, or tubercles, that run the length of its shell. Sheepnose are now considered extirpated from roughly 75 percent of its historical range. Primary risks that affect the species include contaminants, hydrological regime, landscape alterations, lack of connectivity and.
Evidence suggests sheepnose has an approximate life span extending up to at least 30 years.
Sheepnose reproduction requires that male and female individuals be located in close proximity within a mussel bed and rely on flowing water to facilitate fertilization. Sheepnose females will brood their eggs for a matter of weeks, with eggs becoming fully developed between mid-May to early August. Following, females release their glochidia, or larvae, into the water column in late summer months of July and August. When released, the glochidia are contained within structures called conglutinates that can resemble fish prey items, such as blood worms, or clear-colored mucus masses, each holding several hundred glochidia. A fish may either eat or swim through these structures, allowing the glochidia to adhere to the fish's gills or fins.
This host fish is a critical component of sheepnose reproduction and is required for glochidia to further develop into juvenile mussels. However, the host fish can't be just any species. Naturally, sheepnose has been observed to use mimic shiner (Notropis volucellus) and sauger (Sander canadensis) as host fish; however, it can use many more species of fish to complete this critical life stage within a laboratory setting. Once the sheepnose glochidia have developed into juvenile mussels, they drop off the host fish. If they land in an area of suitable habitat, they will continue to grow into an adult mussel.
The sheepnose life cycle is comprised of four stages. First, spawning occurs in the early summer during May through June. Then, fertilized eggs develop within the gills of a female mussel throughout the summer months from May through August. Once mature, these glochidia, or larvae, are then released into the water column where they attach to, or parasitize, a host fish. This typically occurs in the mid- to late-summer, from July through August, and the glochidia will remain attached to their host until they have developed into a juvenile mussel, at which time it will drop off. If the juvenile mussel lands in an area of suitable habitat, it will continue to grow into an adult mussel. It is thought that sheepnose mature into adults at roughly 5 years of age.
Sheepnose is a thick-shelled, medium-sized freshwater mussel species that reach nearly 5.5 inches in length. Many low, wide bumps run in a single file line down the outer shell surface, from the beak, which is defined as the swelling above the point where the two shell halves join, to the opposite shell edge. The rest of the shell surface is smooth, without bumps. It looks slightly pressed-in from the beak to the shell edge, which looks as similar to the pressed-in mark that the length of your finger would make on wet clay, and is parallel to the row of bumps. Young mussels may have two raised ridges, one on either side of the pressed-in mark. There are no differences between male and female shells of this species.
The external shell surface of sheepnose is generally smooth, shiny, rayless and light yellow, to a dull yellowish brown in color.
Sheepnose is generally found in medium to large stream systems, typically within shallow shoal habitats with moderate to swift currents over mixtures of coarse sand, gravel and clay. Individuals may occur in aquatic areas ranging from riffles of a few inches in depth to runs that exceed six meters in larger rivers.
Throughout their various life stages, sheepnose rely on varying food sources and feeding methods. Adult freshwater mussels, including sheepnose, feed by filtering suspended particles from the water column or sediments. These particles include phytoplankton, zooplankton, rotifers, protozoans, detritus and dissolved organic matter. Juvenile mussels collect food items from the surrounding sediments and water column. Glochidia attach to, and obtain, nutrients from the cells of host fish tissue.
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