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The animal species that are at risk of extinction in the USA are molyte. among these there are also mussels. They are eight species of freshwater mussels. Reliant on healthy streams and rivers with clean, reliable water, freshwater mussels are some of the most endangered species in the US, home to more than half of the world's freshwater mussel species.
The mussels proposed for delisting due to extinction all lived in the US southeast, the US biodiversity spot for freshwater mussels. Freshwater molluscs are the most endangered group of organisms in the US, with 36 mussels and more than 70 freshwater snails already extinct.
They are the: flat pigtoe (Mississippi), southern acornshell (Alabama, Georgia, Tennessee), stirrupshell (Alabama), upland combshell, (Georgia, Alabama, Tennessee), green-blossom pearly (Tennessee, Virginia), turgid-blossom pearly mussel (Tennessee, Alabama, Arkansas), yellow-blossom pearly mussel (Tennessee, Alabama) and the tubercled-blossom pearly mussel (Alabama, Illinois, Indiana, Kentucky, Tennessee, West Virginia, southern Ontario, Canada).
The mussel is a lamellibranch mollusc, equipped with lamellar gills which absorb oxygen for respiration and which simultaneously retain the food for feeding, mainly made up of plankton and organic particles in suspension.
The valve, mainly composed of calcium carbonate, is externally black or black-purple, with thin circles of radial and concentric growth towards the pointed part; internally it has a mother-of-pearl color, but with a smooth surface.
The two shells are held together by a hinge with three or four teeth. The shape is roughly teardrop-shaped, with the valve margin rounded on one side and pointed and slightly curved on the other. Once opened, the mollusc shows the mantle which contains all the internal organs, including the reproductive ones.
The animal binds to the support through the byssus, made up of fibers composed of L-3,4-dihydroxyphenylalanine, a substance studied for its extraordinary tensile strength. For the proximal part of the fiber the resistance is 35 megapascals and for the distal part 75 megapascals.
The proximal part of the byssus can be stretched up to three times its length and therefore has a relatively high extensibility, which is surpassed only by spider silk, among natural fibers of animal origin. The distal part, more resistant to tearing, can be stretched up to twice its length.
Compared to hair, byssus threads have a smooth, scale-free fiber surface; the elliptical cross-section allows it to be distinguished from the silk fibers. The diameter of byssus fibers is between 10 and 45 micrometers, which means that they are among the finest natural animal fibers.
