Induction of embryonic dormancy in the Calanoid copepod Acartia hudsonica: proximal cues and variation among individuals

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In this study of Acartia hudsonica, the change in egg types from subitaneous eggs to dormant eggs was investigated. Temperature and photoperiod were hypothesized to act as the proximal environmental cues inducing this population to produce dormant eggs, and the variation in the dormant egg responses of individual females was quantified. Some field-collected females produced some dormant eggs early in the spring (March and April), but most of the switch to dormant eggs occurred in late June. Short-term experiments showed that increased temperatures increased the percentage of dormant eggs produced, whereas increased length of day did not. Long-term experiments showed a different pattern from short-term experiments, indicating that duration of exposure to particular temperatures or photoperiods may influence dormant egg production. Under certain conditions, it appeared that nearly all copepods were capable of producing dormant eggs, although identical experimental conditions did not affect all individuals similarly. In several experiments that revealed a significant treatment effect, between 42% and 85% of the variance detected could be attributed to variation among individuals. Also, the fraction of individuals producing dormant eggs generally increased with increasing water temperature. In Narragansett Bay, A. hudsonica appears to produce dormant eggs primarily in response to increased water temperatures although an effect of photoperiod could not be ruled out. Furthermore, individual responses vary greatly. Appreciation of such variation in an important life history trait, such as embryonic dormancy, is important in understanding how this population may be adapted to its environment.