LIFE-HISTORY VARIATION AND THE COEXISTENCE OF A DAPHNIA HYBRID WITH ITS PARENTAL SPECIES

Life history variation and genotype composition were studied in two species of the Daphnia longispina group: the relatively large D. galeata (1.4 mm) and the smaller D. cucullata (0.9 mm). Several multi-locus genotypes of these species were compared with genotypes of their interspecific hybrid, D. cucullata X D. galeata. Both species and the hybrid co-occur in the shallow eutrophic Tjeukemeer. For two successive years, laboratory life table experiments were conducted at three temperatures (12.5 degrees, 17.5 degrees, and 22.5 degrees C) and two food levels (carbon concentrations 0.62 and 1.64 mu g/mL) to study the mechanisms that permit coexistence of taxa in this Daphnia hybrid species complex. Three-way analyses of variance showed significant main effects (e.g., temperature, food level, taxon) for most reproductive traits (e.g., number of newborns, age at first reproduction, number of juvenile instars, intrinsic rate of increase) among the Daphnia taxa. For size-related traits (e.g., size at maturity and offspring size) only one main effect, taxon, was found. Hybrids were intermediate to the parental species with respect to size at maturity. offspring size, and number of newborns. For developmental traits (number of juvenile instars and age at maturity) the hybrids tended to have the lowest values for all conditions. The intrinsic rate of increase (r) of the hybrid did not differ significantly from that of D. galeata, but was significantly higher than that of D. cucullata. We argue that essentially r will reach higher values for the hybrids than for D. galeata, because of the strong influence of size-selective fish predation. Our results are discussed in reference to different theories about the maintenance of hybrid species complexes in nature.