Adaptation for accuracy or for precision? Diel emergence timing of the intertidal insect Pontomyia oceana (Chironomidae)

Abstract

Synchronization of reproduction is obviously under strong selection, but it is not always clear whether the selection is for accuracy of timing, that is, minimal deviation from the environmental optimum, or alternatively, for precision of timing, that is, least extent of spread from the population mean. The former is usually adaptive for exploiting certain environmental conditions, whereas the fitness gain of the latter is usually associated with co-occurrence with conspecifics. We studied the intertidal midge Pontomyia oceana, which has a life cycle of about 30 or 45 days culminating in a highly synchronous swarming of the adult stage that lasts only about 1–2 h. We found that the external proximate factors, controlling their diel swarming, comprise two cues, that is, sunrise and sunset. These two cues, however, are not in existence to improve the accuracy of timing, as their diel swarming times differ widely, with respect to light condition or to tidal condition, in different evenings. Rather, the function is to improve the precision of timing. During the metabolic processes in preparation for emergence, waiting for the cues reduced variation in the population. The extent of dispersion in emergence (swarming) time in an evening is much smaller, when two cues are used compared with the inferred one-cue situations. The short adult life of the marine midges must have rendered mate-finding a stringent selective force despite their apparent high densities. The same principle, that is, using multiple cues to improve precision of timing, may be applicable to many long-lived, free-spawning species, for example, corals, where gametes can fertilize effectively only within a short time after release.