Calanus Copepod

A swimming oil tank

Calanus finmarchicus is a copepod, a small crustacean in the zooplankton, the animals that drift with ocean currents. A year-round study in a Svalbard fjord counted one as adult when its prosome, the front part of the body, measured 2.4 to 2.95 millimetres.

Its energy store is a large oil sac made of wax esters, built up while it is young and food is plentiful. In one analysis, lipids (fats and oils) made up 22 to 25 percent of the dry weight of its later juvenile stages and adults. It eats algae, but in the open Gulf of St. Lawrence, microplankton that feed on other organisms, including ciliates and dinoflagellates presumed to feed that way, made up a disproportionate share of its ration.

Sleeping through winter

Towards the end of summer, it descends to deeper water and spends the winter dormant, a state called diapause. In that Svalbard fjord, C. finmarchicus overwintered mainly at stage CV, the last juvenile stage, with 65 percent of them there. In the northernmost part of its range, its life cycle takes one year.

For copepods, lipid mass is directly related to how deep they can descend during diapause. Smaller reserves mean overwintering at shallower depths, where they are more prone to predation.

The lipid pump In the North Atlantic, this one species sequesters an amount of carbon equivalent to the sinking flux of detrital material, which is debris drifting down from above.

Food for fish and whales

Its lipid stores make it nutritionally valuable to fish and whales. It is widely distributed in the northern hemisphere and is the dominant mesozooplankton species in the North Sea and Norwegian Sea.

Atlantic currents regularly carry it in high numbers into the Arctic. It is expanding northwards, and it may potentially replace its larger relative Calanus glacialis if the climate continues to warm.

Plastic in the pantry

In a lab test, juvenile copepods exposed to nylon granules built up less lipid, and those exposed to nylon microplastics moulted, or shed their outer skeleton, earlier than unexposed ones. In a separate feeding experiment, all stages of C. finmarchicus ingested polystyrene fragments, and late juveniles (stage CV) and adults preferred aged microbeads over pristine ones.