Copepods are the most abundant of the zooplankton. They are tiny crustaceans the largest of which are about the size of a grain of rice.
The most common and most abundant of the copepods is the red feed (Calanus finmarchicus). This species is the mainstay of the diet for capelin and herring, and therefore it plays an extremely important role in the Icelandic marine food web.
During the winter juvenile copepods mostly hibernate in deeper layers of the ocean. In the spring they migrate into surface waters, quickly reach maturity, and mate. Young hatch roughly a month later and remain in the surface water to feed and grow. These copepods will reach maturity and produce a generation of offspring by the fall of the same year. This second generation will migrate to a deeper depth to hibernate over the winter and repeat the cycle the following year.
The growth and development of red feed is dependent on water temperature and in colder water, there is usually only a single generation each year, rather than the two described above. Although the waters in northern Iceland are cold, there are sometimes two generations.
There are many species of copepod and distinguishing between them can be very difficult. In general, smaller species are found in fjords and close to shore while the largest species like the red feed cousins Calanus hyperborealis and Calanus glacialis are found in the cold offshore waters north of Iceland. Fourteen species of copepod have been identified from Eyjafjörður and of those, the small species Temora longicornis, Oithona setigera, and Acartia longiremis are the most common. In addition, a small population of red feed is present in the fjord.
Although copepods are a large proportion of the pelagic zooplankton, there are also many benthic species, that is, species that live on the seafloor and even some that burrow in the sediment. These copepods are very small and although they appear to be very common and abundant, very little is known about the species composition or life history of them.
Many copepods are parasitic. Among these species, the larval stages usually look similar to those of other crustaceans but the adult stages often look completely different. For example, sea lice (copepods of the genus Caligus) are flat and adapted to grip onto the skin of fish. This group can cause great harm to stocks of fish, especially those farmed in marine weirs. An even more unusual looking parasitic copepod is the cod worm (Lernaeocera branchialis). These look like some kind of hard blod-red worm that lives fastened to the gills of cod, from where they send tendrils into the fish’s heart to suck blood from it.