Dreissena

Dreissena is a genus of small freshwater mussels in the family Dreissenidae in the class Bivalvia. They are found attached to firm substrates by threads from underneath the shells and are the only freshwater bivalves to attach to hard substrates in high densities while having a planktonic larval stage. They are considered the most aggressive freshwater invader in the world because of their ability to invade environments in every one of their life cycle.

== Description == thumb|497x497px|Dreissena image of different stripe patterns Dreissena have prominent dark and light banding on the shell but can have many different variations. The outer covering is a well polished light tan color with dark bands that can either be smooth or zigzagging in shape. The shell shape is either trigonal or triangular with sharply pointed umbos. Under the umbos is the hinge plate that is broad and well developed. A ridge runs from umbos to the posterior point forming a shoulder. The byssal threads are secreted by the byssal gland posterior to the food.