Temperature acclimation and other influences on embryonic duration in the copepod Pseudocalanus sp.

Abstract

Embryos of Pseudocalanus sp. captured near Halifax, Nova Scotia, Canada, in spring take longer to develop than do those from the warmer waters of summer and autumn. This is related to the larger size of females and their eggs in the colder seasons. An experiment revealed size-assortative mating and the influence of male size as well on embryonic duration. The seasonal change is contrary to the slight reduction of embryonic duration after short-term cold acclimation of females in the laboratory. There is no evidence that embryos in the cold season develop more rapidly than expected from the overall relationship between embryonic duration and female size. However, they do, like embryos after short-term cold acclimation, develop relatively more rapidly at low temperatures; i.e., there is clockwise “rotation” of the rate-temperature response, equivalent to the temperature adaptation that has been demonstrated among species of copepods. This seasonal “rotation” may also be related to seasonal variations in size, since larger females from a given season produce embryos that show such “rotation”. Thus, there is no evidence for seasonal compensation as a consequence of temperature acclimation underlying changes that are attributable to differences in size. Evidently there is also a strong genetical component in duration of development of embryos produced by individual females.