In temperate seas, multi-voltine copepods show a pronounced seasonal variability in body size, which affects both their reproductive capacity and their accessibility to size -selective predators. Here, we studied seasonal changes of female prosome length from six common copepods, Acartia clausi, Centropages hamatus, Centropages typicus, Paracalanus parvus, Pseudocalanus elongatus, and Temora longicornis between 2000 and 2005 at the time series station Helgoland Roads, southern North Sea. We observed no significant effect of food (measured as phytoplankton carbon content) with size of adult females. Moreover, in none of the species investigated was prosome length significantly correlated with temperature when considering the whole year. Instead, all species had a period of temperature-related size, but for the size distribution during the rest of the year we distinguished two groups of species. Group 1 (Acartia clausi, Centropages hamatus, and Pseudocalanus elongatus ) had a resting phase with females of the same size persisting for > half a year, while in group 2 the time after the temperature-related phase was characterized by irregular size distributions. A female resting phase of Acartia clausi, Centropages hamatus, and Pseudocalanus elongatus has been hitherto unknown. Size distribution control after the temperature-related phase in group 2 is as yet not understood, but the awakening/hatching of resting eggs and/or copepodids may be found to be involved.

Samples were collected at Helgoland Roads using a Calcofi net (500 µm mesh, 100 cm diameter). When possible, 60 females of the six species Acartia clausi, Centropages hamatus, C. typicus, Paracalanus parvus, Pseudocalanus sp., and Temora longicornis were randomly sorted and their prosome length was measured with an interactive video image digitizing system to the nearest 20 µm (NIH Image 1.55) on a Wild M3/JVC at 40x magnification usually within 6 months after collection. According to Durbin and Durbin (1978) preservation in formalin caused a significant loss in dry weight, carbon and nitrogen in adult females, mostly during the first 24 h; however, it did not significantly affect the C:N ratio or the cephalothorax length of the animals.

Supplement to: Hirche, Hans-Juergen; Boersma, Maarten; Wiltshire, Karen Helen (2019): Partial decoupling from the temperature size rule by North Sea copepods. Marine Biology, 166(5)