Just as wildebeest are the main grazers of the Serengeti, so Calanus are the great grazers of the Atlantic and Arctic oceans, feeding on aquatic meadows of phytoplankton, microscopic plant-like algae that bloom in spring. Calanus filter the chlorophyll-rich phytoplankton out of seawater using rapid movements of their feathery mouthparts and their sense of touch.
The bodies of Calanus are transparent, which may explain why the famous eighteenth century Norwegian bishop and scientist Johan Ernst Gunnerus named them after the philosopher Kalanos (Calanus), who refused to wear clothes! For an apparently "simple" animal, Calanus has a complicated life cycle. The cycle begins in spring when adult females release batches of 50 or more eggs into the water. The eggs hatch a day or so later and, being cold-blooded, develop at a rate that is largely controlled by water temperature. Like all crustaceans, Calanus has a rigid external skeleton (exoskeleton) that it must shed in order to grow and develop. In total, there are 12 development stages to their life cycle. During the first 6 they are known as nauplii. These "baby" stages adopt a swimming-by-jumping approach to movement and look like tiny pulsating hands.

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