Predator evasion as an explanation of diurnal vertical migration by zooplankton

Diurnal vertical migrations of planktonic crustaceans are widespread phenomena both in freshwater lakes and in the sea. The adaptive significance of this behaviour is unclear. This movement from the warm surface layers, which provide abundant food, to colder, deeper water with its poor food supply must place the migrating animals at a disadvantage compared with individuals remaining near the surface. The reduced availability of food and the energy consumed in migration result in reduced growth and reproductive capacity of migrating individuals. Moreover, the development time of the eggs carried in the brood pouch or in egg sacs is prolonged in cold water and the birth rate is reduced. We have studied vertical migration in a deep lake and report here that two very similar Daphnia species have different strategies. Daphnia hyalina shows a pronounced diurnal migration whereas Daphnia galeata remains near the surface. Although the non-migrating D. galeata has a much higher birth rate than D. hyalina, the latter is numerically dominant, as D. galeata suffers a high mortality near the surface. These results support the hypothesis that predator avoidance is one of the most important factors in vertical migration.