| Dreissena Temporal range:
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| A shell (without byssus) of Dreissena polymorpha | |
Scientific classification 
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| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Mollusca |
| Class: | Bivalvia |
| Order: | Myida |
| Family: | Dreissenidae |
| Genus: | Dreissena Beneden, 1835 |
| Species | |
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See text | |
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.