Photoperiod and temperature as inducers of gamogenesis in a dicyclic population of Scapholeberis armata Herrick (Crustacea : Cladocera : Daphniidae)

The induction of gamogenesis in a dicyclic population of Scapholeberis armata from a small pond at 40 degree N in Pennsylvania, USA, was studied. Animals were collected at the time of gamogenesis. Test animals were observed bidaily or daily for up to 37 days. Occurrence of molts and of parthenogenetic broods or ephippia was recorded. At short photoperiod, ephippial females developed, usually after one or more parthenogenetic broods, among both spring and fall animals. The response was stronger (higher % sexual females) among fall clones, and was enhanced in both groups by 15 degree C water. Differences in the responsiveness of spring and fall clones to inducing conditions suggest some differences in genotypes of spring and fall subpopulations in this pond. It is proposed that variable responses among genotypes to factors inducing gamogenesis may be the basis for the differences and clines in the timing of gamogenesis found among ecologically and latitudinally separated populations.