Aspects of mating, reproduction, and co‐occurrence in three freshwater calanoid copepods

SUMMARY. 1. We examined the seasonal distributions and mating behaviour of Epischura lacustris, Diaptomus minutus and Diaptomus oregonensis to evaluate several proposed mechanisms for reproductive isolation. There was no divergence in onset of maturity or timing of breeding periods between sympatric diaptomid populations; size displacement between co‐occurring diaptomid species did not result in concomitant reduction in mating frequency between heterospecific pairs in the laboratory. 2. Divergence in mating behaviour was supported by observations that, in conspecific mating experiments, males of both diaptomid species discriminated between females bearing ripened ovaries (gravid) and those that did not (non‐gravid), and that, in heterospecific trials, males either mated indiscriminantly with all females, or engaged in mating very infrequently. 3. In mating experiments with Epischura. males did not discriminate between gravid and non‐gravid females, but mated exclusively with virgins of a specific relative size. Divergence in timing of breeding seasons between Epischura and the two diaptomid species suggests that even if mating between genera is possible, it does not occur often in nature. 4. We discuss the adaptive significance of different reproductive strategies of these three copepods and speculate on mechanisms that allow for coexistence of the closely related taxa in nature.