Adaptive mechanism of the bimodal emergence dates in the intertidal midge Pontomyia oceana

Abstract

The intertidal midge Pontomyia oceana (Diptera: Chironomidae) emerges semilunarly, about 15 d apart, in southern Taiwan. Fertilized eggs laid on the same night develop into adults that show emergence peaks about 30 and 45 d later. We tested 2 hypotheses: (1) 'two adaptive peaks'; adult midges emerging in each peak are adapted to certain environments and (2) 'bet-hedging'; late emergence is not adaptive in itself but acts as an insurance against risk of total loss of offspring from a single emergence. In our experiment, the percentage of the cohort that emerged in the first peak had a strong environmental component, but individuals emerging in 2 peaks did not differ in plastic traits such as fecundity, male head length and male thorax length. The 'two adaptive peaks' hypo- thesis is not supported. Offspring of most individual mothers showed bimodal emergence under homogeneous laboratory conditions. Highly unpredictable wind speed may cause variable success rates of mating on the water surface. By allocating some offspring to a second available window, the midges ensure higher fitness in the long-term in an unpredictable environment. The offspring of some females, nevertheless, only emerged in the first peak. We propose an intermittent bet-hedging strategy in which midges need not produce offspring in both peaks in every generation. As the benefits of bet hedging are only realized over multiple generations, offspring can be distributed in multiple peaks even if offspring do not emerge in 2 peaks in every generation.