The blue mussel (Mytilus edulis) is a very common bivalve species throughout Icelandic marine waters. It is only found in very shallow water up to the seashore. It lives attached to hard surfaces by byssus fibers and is thus often left high and dry when the tide goes out. It feeds primarily on planktonic algae that it filters from the water.
Mussels are a popular food and people have long harvested them from the beach at low tide. However, there are certain algae that the mussel ingests from time to time that cause them to be toxic to people while the algae are blooming. The Marine Research Institute of Iceland monitors these algae in the water and issues a warning when it becomes too prevalent in the water column. These toxins are quickly washed out of the mussel after the algal bloom ceases and the mussels are edible once again. This temporary toxicity is probably the main reason that mussels have only recently been commercially exploited, though they have long been used as bait.
In recent years, some Icelanders have begun farming blue mussels on submerged ropes. The young settle onto the ropes, attach with their byssus fibers and grow for two years before being harvested.
The horse mussel (Modiolus modiolus) is very much like the blue mussel in appearance but it is much larger and has a thicker shell. They inhabit the same types of habitat and live in much the same way way all around Iceland. Horse mussels tend to stay away from shallow water and the seashore, unlike blue mussels. This species has never been commercially harvested on a large scale, but some divers have made a bit of profit by collecting them in baskets and selling them directly to restaurants.