Comparison of population structure, vertical distribution and growth of sympatric, carnivorous, mesopelagic copepods, Paraeuchaeta glacialis and Heterorhabdus norvegicus, in the western Arctic Ocean

In the Arctic Ocean, the life cycles of interzonal omnivorous copepods have been studied, whereas little information is available on the life cycles of mesopelagic carnivorous species. Here, the life cycles of two mesopelagic carnivorous copepods (Paraeuchaeta glacialis and Heterorhabdus norvegicus) are described from vertically stratified samples collected at an annual ice-station (SHEBA) in the western Arctic Ocean during 1996-1997. Reproduction estimated to occur between January and March for both species. Vertical distributions and population growth varied between the species. Early copepodite stages and adult males of P. glacialis were distributed in deeper layers, whereas late copepodite stages and adult females were observed in shallower layers. The skewed sex ratio towards females was observed for adults, which may be related to the cease feeding and short longevity for adult males. In contrast, all the H. norvegicus life stages were distributed in the mesopelagic layer. Clear peaks of the young stages and their sequence suggest that H. norvegicus may complete its life cycle within 1 year. A specialized feeding mode (incorporating a venomous injection spine and large beak) could be a key trait facilitating the achievement of rapid growth in H. norvegicus in the food-limited mesopelagic layer.