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

Life history variation and genotype composition were studied in two sp ecies of the Daphnia longispina group: the relatively large D. galeata (1.4 mm) and the smaller D. cucullata (0.9 mm). Several multi-locus g enotypes of these species were compared with genotypes of their inters pecific hybrid, D. cucullata X D. galeata. Both species and the hybrid co-occur in the shallow eutrophic Tjeukemeer. For two successive year s, laboratory life table experiments were conducted at three temperatu res (12.5 degrees, 17.5 degrees, and 22.5 degrees C) and two food leve ls (carbon concentrations 0.62 and 1.64 mu g/mL) to study the mechanis ms that permit coexistence of taxa in this Daphnia hybrid species comp lex. 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 i nstars, intrinsic rate of increase) among the Daphnia taxa. For size-r elated traits (e.g., size at maturity and offspring size) only one mai n effect, taxon, was found. Hybrids were intermediate to the parental species with respect to size at maturity. offspring size, and number o f newborns. For developmental traits (number of juvenile instars and a ge at maturity) the hybrids tended to have the lowest values for all c onditions. The intrinsic rate of increase (r) of the hybrid did not di ffer significantly from that of D. galeata, but was significantly high er than that of D. cucullata. We argue that essentially r will reach h igher values for the hybrids than for D. galeata, because of the stron g influence of size-selective fish predation. Our results are discusse d in reference to different theories about the maintenance of hybrid s pecies complexes in nature.