|Dreissenidae, the closed mussels, are a family of fresh and brackish water mussels and includes the highly invasive zebra and quagga mussels. The shells may be triangular or oblong with a narrow pointed beak. One side of the shell is slightly bent inward (concave) while the other side is curved outwards (concave). A shelf-like plate (septum) is located internally below the beak. They attach to hard substrates using byssal threads. Some grow in vast numbers and are known to clog intake pipes, sink buoys (because of the weight) and foul the bottoms of boats and any structure sitting in the water. There is only 1 species found in Texas coastal water, the dark false mussel, and it has not caused any ecological problems, so far.