VARIATION IN THE DIAPAUSE RESPONSE OF LABIDOCERA AESTIVA (COPEPODA: CALANOIDA) FROM DIFFERENT LATITUDES AND ITS IMPORTANCE IN THE EVOLUTIONARY PROCESS

¹ Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543

Labidocera aestiva were collected from three distantly separated sites along the Atlantic coast of the United States. Their offspring were reared in the laboratory under several combinations of photoperiod and temperature to assess the diapause response and genetic similarity of the populations. The results indicate that L. aestiva is not genetically homogeneous throughout its range. Differences in the percentage of subitaneous and diapause eggs produced by laboratory-reared and field-collected populations reflected the unique combination of environmental conditions occurring in the geographic regions from which the animals originated. In the Virginian province where planktonic stages of the species are seasonally present, diapause egg production is triggered by short day length photoperiods and cool temperatures. On the other hand in the Carolinian province where the planktonic stages tend to occur year-round diapause eggs are rarely produced by either field-collected females or their laboratory reared offspring. This suggests that most of these animals lack the genetic capacity to produce diapause eggs. Diapause may lead to the reproductive isolation of populations if gene flow is reduced or prevented owing to their separation on a temporal scale.

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