Effects of temperature fluctuations and photoperiod on hatching in the parasitic copepod Salmincola edwardsii

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Abstract
In laboratory experiments, we studied the effects of the day–night cycle (photoperiod and temperature fluctuations) on the initiation of hatching and hatching rate in egg sacs of the copepod Salmincola edwardsii, ectoparasitic on brook trout (Salvelinus fontinalis). Pairs of egg sacs were removed from adult female copepods; for each pair, one sac was placed under experimental conditions and the other one was kept under constant conditions, providing an ideal control. Photoperiod had no significant effect on the initiation of hatching or hatching rate. The observed effects of temperature fluctuations were associated with water temperature itself, and appeared independent of whether it was fluctuating or constant. We conclude that hatching in S. edwardsii is not rhythmical but spontaneous, showing no relationship with daily changes in host vulnerability.