- Kingdom: Animalia
- Phylum: Mollusca
- Class: Bivalvia
- Order: Mytiloida
- Family: Mytilidae
- Scientific name: Modiolus modiolus
- Norwegian: O-skjell
The horse mussel grows bigger than the common mussel. It can reach a length of 20 cm, 5-15 cm is common. The juvenile shells are often blue, but they tend to turn reddish brown as they grow older.
Buried or partly buried in gravel. It is most frequent on subtidal depths from 10 to 25 meters, but there are registrations from 150 meters depth. They are often aggregated in large groups.
It is common in the North Sea, from northern parts of Norway to the Bay of Biscay.