Calanus helgolandicus

The female has an average size of about 2.9 millimetres (0.11 in) and the male has an average size of about 2.7 millimetres (0.11 in).

Calanus helgolandicus is found in the Atlantic, from the North Sea south to the western coast of Africa. It can also be found in the Mediterranean and Black Sea. Because of increasing temperatures due to climate change, its range is expanding northward. It is usually found in waters between 9 and 20 °C (48 and 68 °F) in temperature, with maximum abundance being between 13 and 17 °C (55 and 63 °F).

Calanus helgolandicus overwinters as a stage V copepodite. However, only those with enough energy stores can overwinter; the rest stay at the surface. In fact, it has been observed that overwintering copepods are heavier than those that stay at the surface. During this overwintering process, the copepodites moult into adult females. After moulting, the females breed the next spring. In the absence of food during this time, they can lay their first few eggs from accumulated energy stores. Although this is true, it is mainly an income breeder, reproducing using recently gained energy. It has a maximum of about three generations per year. In the central North Sea, it is most abundant during spring and autumn. It is likely that warmer temperatures can facilitate egg production for more of the year.