Phylum mollusca is one of the most species rich animal phyla in the ocean. The three main groups of molluscs are sea shells (bivalvia), snails (gastropoda), and cephalopods (cephalopoda, i.e. squids and octopuses). Smaller groups include chitons (polyplacophora) and tusk shells (scaphopoda).

All molluscs have a soft body but often also have a hard shell for protection. Otherwise molluscs are very diverse. Bivalves are primarily sessile filter or detritus feeders while gastropods are slow-moving algae eaters or predators on sessile animals. Cephalopods are mostly active swimming predators on fishes and other cephalopods, among a wide array of other marine animals.

Most molluscs are too small or too rare to be economically worth collecting. Those that have been harvested from Icelandic waters are blue mussels (Mytilus edulis), ocean quahog (Arctica islandica), Iceland scallop (Chlamys islandica) and the common whelk (Buccinum undatum). Some other large edible species found in Eyjafjörður are the horse mussel (Modiolus modiolus) and the soft shell clam (Mya arenaria).

Octopuses have not been recorded in coastal waters of the north shore that we know of but small species can be found in the deep waters offshore just outside the fjord. Squids have been found in the fjords but are rare. Occasionally, the European flying squid (Todaroides sagittatus) migrates into the fjords and is fished. The mysterious giant squid (Architeuthis dux) has also been found there. In fact one of the first recorded instances in the world did happen in Northern Iceland. In 1790 a local farmer found a dying individual washed ashore in Arnarnes close to the chimneys. The farmer was also somewhat of a naturalist and wrote down a very accurate description of the beast.