The influence of changing temperature on the life history of Daphniopsis ephemeralis

We present evidence which suggests that for Daphniopsis ephemeralischanges in water temperature may directly cue changes in sexratio, ephippial and parthenogenetic egg production and indirectlydetermine female body size. Field data suggested that over anannual cycle, a population of D. ephemeralis comprised animalswith three distinct life history phases. The first was madeup of exephippial females which hatched in the fall. They produceda second group made up of large parthenogenetic females whichcould produce more than one overwintering generation. In thespring these parthenogenetic females gave rise to a third lifehistory phase which comprised males and small ephippial females.Field enclosure experiments suggested that the small size ofspring females was not related to predation. Laboratory experimentssuggested that ephippial egg production at small body sizeswas associated with ‘increasing’ temperature. Theseexperiments also suggested that the fall reproductive pattern(parthenogenetic eggs and large body sizes) was associated with‘decreasing’ temperatures. We conclude that directionalityof seasonal temperature change may be the prime factor responsiblefor D. ephemeralis life history changes observed in the field.