Factors regulating the reproductive phenology of Onychodiaptomus birgei (Copepoda: Calanoida)1

Abstract

Reproduction in a population of Onychodiaptomus birgei inhabiting a shallow montane pond consisted of alternating pulses of subitaneous and diapausing eggs. Unlike the single seasonal transition from subitaneous to diapausing eggs typical of many populations of diaptomids, female O. birgei tend to switch from the production of subitaneous eggs as they age regardless of time of year. The reproductive phenology of individual females was investigated in the laboratory to determine the proximate factors inducing diapausing egg production. Dissolved oxygen concentrations below the minimum observed in the field (<3 mg liter−1) had no effect on diapause induction. Photoperiod (L:D 8:16, 12:12, 16:8 h) had a detectable effect only at very high copepod densities (>1,000 individuals liter−1). High copepod densities increased and low food levels decreased the incidence of diapausing egg production. Increases in the proportion of subitaneous clutches in the field were preceded by peaks in fifth copepodid abundance. Field studies with in situ enclosures corroborated the finding that young females initially produce subitaneous eggs and then switch to diapausing egg production.