The Dark False Mussel
A brackish water bivalve mollusk native to the Gulf of Mexico, the Dark False Mussel, Mytilopsis leucophaeata Conrad, 1831 has been introduced to the mid-Atlantic American states, Europe, and south to northeastern Brazil, possibly via ballast water in ships or among living Oysters. Dark False Mussels have non-descript, light-colored shells that may grow to about ¾ inch. They may be found locally on Sanibel in brackish water ponds and in the canals at the eastern end of the island. The Mussels, which belong to the same family (Dreissenidae), as the infamous Zebra Mussels, will congregate in large clumps around underwater vegetation and hard structures. They are not as pervasive and potentially damaging an introduced species as the Zebra Mussel, though.
Shell of the Dark False Mussel, Mytilopsis leucophaeata, and a typical aggregation of specimens growing around a submerged twig.