We review current knowledge and understanding of the biology and ecology of the calanoid copepod Calanus helgolandicus
in European waters, as well as provide a collaborative synthesis of data from 18 laboratories and 26 sampling stations
in areas distributed from the northern North Sea to the Aegean and Levantine Seas. This network of zooplankton
time-series stations has enabled us to collect and synthesise seasonal and multi-annual data on abundance, body size,
fecundity, hatching success and vertical distribution of C. helgolandicus. An aim was to enable comparison with its congener
Calanus finmarchicus, which has been studied intensively as a key component of European and north east Atlantic marine
ecosystems. C. finmarchicus is known to over-winter at depth, whereas the life-cycle of C. helgolandicus is less well
understood. Overwintering populations of C. helgolandicus have been observed off the Atlantic coast between 400 and
800 m, while in the Mediterranean there is evidence of significant deep-water populations at depths as great as 4200 m.
The biogeographical distribution of C. helgolandicus in European coastal waters covers a wide range of habitats, from open
ocean to coastal environments, and its contribution to mesozooplankton biomass ranges from 6% to 93%. Highest abundances
were recorded in the Adriatic and off the west coast of Spain. C. helgolandicus is generally found in 9–20 C water,
with maximum abundances from 13–17 C. In contrast, C. finmarchicus is found in cooler water between 0 and 15 C, with
peak abundances from 0 to 9 C. As water has warmed in the North Atlantic over recent decades, the range of C. helgolandicus
and its abundance on the fringes of its expanding range have increased. This review will facilitate development of
population models of C. helgolandicus. This will not only help answer remaining questions but will improve our ability to
forecast future changes, in response to a warming climate, in the abundance and distribution of this important species.
