DIEL VERTICAL MIGRATION BY DAPHNIA-LONGISPINA IN A SPANISH LAKE - GENETIC SOURCES OF DISTRIBUTIONAL VARIATION

Individuals of many zooplankton species undergo diel vertical migratio n in which they move toward the surface after sunset and return to dee per waters at dawn. However, even when vertical migration can be clear ly demonstrated by mapping the distributions of individuals at noon an d midnight, at any given time the population is usually scattered over a broad range of depths. This paper focuses on a major potential sour ce of the variance associated with depth distributions. We have used e lectrophoretic analysis of isozymes to identify ''clones'' of Daphnia longispina in a Spanish lake. In September 1991, two clones constitute d 55% of the individuals collected at noon and midnight on each of two successive days. While both displayed significant diel vertical migra tion, one clone was consistently located deeper in the water column th an the other clone. Our results constitute the first demonstration fro m a held study of Daphnia that a single population is genetically subd ivided into groups having different patterns of diel vertical migratio n. Intrapopulational genetic variation for vertical migration patterns may explain a significant portion of the variability in the depth dis tributions of zooplankton populations.