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Diapause allows aquatic organisms to survive periods of drydown in intermittent pools. However, often not all of the individuals hatch in response to a filling event. This prolonged diapause (diapause lasting through one or more favorable periods) is the likely consequence of several factors. We address the possibility that prolonged diapause in fairy shrimp in Southern California vernal pools is a bet-hedging adaptation to unpredictability in the duration of filling events. Under the simplest bet-hedging model of selection on prolonged diapause, the fraction of eggs hatching in any filling event should approximate the fraction of filling events lasting long enough for successful reproduction.

We quantified filling durations in seven pools of coastal San Diego County, California for all winter and spring events between 1983 and 1996. The distributions of filling durations did not differ significantly among pools, so selection should be similar across pools. When the 189 filling durations were combined into a single estimated probability distribution, approximately 28% of the filling events lasted 17 days or longer, the developmental time to first reproduction (under field conditions) in the fairy shrimp Branchinecta sandiegonensis (Branchiopoda: Anostraca) inhabiting these pools. Previous laboratory hatching experiments determined a maximum hatching rate of 28% at 10 degreesC, with lower hatching percentages at higher and lower temperatures.