DIFFERENCES IN THE DURATION OF EGG DIAPAUSE OF LABIDOCERA AESTIVA (COPEPODA: CALANOIDA) FROM THE WOODS HOLE, MASSACHUSETTS, REGION.

The duration of diapause of Labidocera aestiva eggs collected from the field and reared in the laboratory was determined at 5°C. A clear seasonal trend was observed. Diapause eggs produced in the early fall required a much longer exposure to cold to yield a 50% hatch (CT) (i.e., the duration of diapause was longer) than eggs produced later in the fall. Eggs produced by laboratory animals that were reared at 14°C, 8L-16D, required a shorter period of chilling to terminate diapause than the eggs of animals reared at 19° C, 12L-12D. Considerable variation in the CT value was also observed among laboratory cultures that were all reared under identical conditions, but which differed in terms of selection history. The results indicate that both the genotype of the egg and the conditions prevailing during oocyte formation influence the duration of diapause. Eggs that were stored at 5°C for periods longer than 300 days no longer hatched upon warming. It is suggested that the variation in the duration of diapause is an adaptation that promotes synchronization of hatching by ensuring that all individuals terminate diapause at approximately the same time, and survival during the winter by conferring cold-hardiness. Synchronizing the onset of post-diapause development is also discussed as an alternative mechanism for achieving synchronous hatching.