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In Southampton Water the copepod Acartia bifilosa presents a diapause reproductive strategy, where there is a switch from subitaneous to diapause egg production around May, prior to the species' disappearance from the water column between June and October. The effect of temperature and photoperiod on the production of diapause eggs by A. bifilosa was studied in an attempt to determine the primary cues for its summer diapause. A parallel study on the effect of temperature on metabolic efficiency of A. bifilosa and the non-diapause species A. discaudata, defined by the species' 'scope for growth' (SfG), was examined as a potential, ultimate reason behind the diapause stage. Photoperiod was identified as the primary proximate cue that induced diapause in A. bifilosa, and this response was temperature-mediated. Diapause was triggered by a 13:11 h light:dark photoperiod (day length), corresponding to a late-April photoperiodic regime, and resting eggs were produced even at temperatures as low as 5°C. A very low number of diapause eggs were, however, also produced after 6 d at a 12:12 h light:dark photoperiod at elevated temperatures between 14 and 20°C, but the mean percentage produced was significantly less (p The ultimate cause of the over-summering strategy of A. bifilosa is currently unknown, but the SfG assay indicated that at 10°C, SfG was twice that at 20 or 5°C, and so it may diapause to avoid the higher temperatures in summer. This pattern contrasted with the SfG of A. discaudata, which suggested a simple, positive relationship with temperature. In the field, competition is greatly reduced in the winter months, so A. bifilosa has a better chance of survival, even with its lower SfG.

Photoperiod and temperature regulation of diapause egg production in Acartia bifilosa from Southampton Water