ABSTRACT
Heteropsyllus nunni is a marine copepod that builds a cyst and dwells within it during a period of extended diapause. The field abundance of this copepod has been monitored for 10 years, but nothing is known about the cues that induce and terminate encystment. In the laboratory, different photoperiods and temperatures were tested for their effects on encystment and excystment. The photoperiod and temperature cues tested neither induced nor inhibited encystment in H. nunni. Encystment occurred in all treatments, regardless of temperature or photoperiod, suggesting that internal genetic cues, tied to a specific ontogenetic stage, must be the central causal factor. Copepods in the hot treatments encysted and excysted more rapidly than in the cold. Many copepods in the cold treatment encysted (though later than copepods in the hotter treatments), and most were still within the cyst at the end of the 23-week experiment. There were significantly more males within the full cysts than females. A concurrent field study confirmed the known seasonal patterns in the number of encystments relative to the number of free-living forms; i.e., encystment took place in the summer.
