Sea shells or bivalves are among the most common animals in the ocean. However, many of them cannot be seen so easily, as they bury themselves in soft sediments. Many are also quite small. A few large species do also burrow into the sediment and only extend siphons up to the surface to suck in seawater to get oxygen and filter food.
The best known and probably the most abundant in Northern Icelandic waters is the ocean quahog (Arctica islandica). The ocean quahog has the longest lifespan of all the known animals in the world. The oldest one recorded was more than 500 years old, found north of Iceland.
A few large species are also found on the bottom surface, the most abundant are blue mussels (Mytilus edulis) and Iceland scallop (Chlamys islandica). Blue mussels are farmed in Iceland, the larvae attach to ropes hung in the water and are allowed to grow there for up to two years before being harvested. Scallops are unusual among the bivalves as they can swim short distances. This ability is mostly used to escape from predatory sea stars.
Bivalves that have been harvested commercially in Northern Iceland are these three mentioned above. There are other large species, as well. The more common large bivalves are the Greenland cockle (Serripes groenlandicum), the hairy cockle (Ciliatocardium ciliatum), the soft-shell clam (Mya arenaria), and the truncate softshell (Mya truncata).
There are also many species of very small bivalves where the seafloor is soft off the coast of Iceland. In Eyjafjörður the most common of these are: smooth nut clam (Nucula tenuis), northern yoldia (Yoldia hyperborea), flexuose cleftclam (Thyasira flexuosa), and chalky macoma (Macoma calcarea). All of these species live buried in the sediment.
Some bivalves have adapted to a specific type of substrate. The small saddle oyster (Heteranomia squamula) lives attached to uneven rocks, shells, and other hard materials. The oval piddock (Zirfaea crispata) bores into soft stones and leaves only a siphon exposed to draw in water. The big-ear shipworm (Psiloteredo megotara) does not look like a sea shell at all but as a worm. It bores into waterlogged wood, such as pier stantions and driftwood.