The migration of Pontoporeia affinis apparently is initiated by the disappearance of light, and the extent of the migration is modified by the thermal structure of the water column. Observations of the age structure of the migration population, structural modifications of pelagic individuals, and the percentage of the population that migrates give rise to the contention that the migration is not to feed, but possibly has adaptive value in maintaining genetic continuity in otherwise isolated benthic popuations.