Fish predation on a Daphnia hybrid species complex: A factor explaining species coexistence?

Recent studies on the life histories of Daphnia hybrids and their parental species have revealed that hybrids can combine an intermediate size with a relatively high reproductive rate, which might explain their success in many European lakes. Based on this information, we formulated the temporal hybrid superiority hypothesis to explain the coexistence of parental species and their hybrids. Our hypothesis states that in temporally changing environments, such as lakes, hybrids can coexist with their parental species because hybrid individuals have a higher fitness compared to the parental individuals during specific periods of the year. This is in contrast to the hypothesis that hybrids have a lower fitness than the parentals, which should be compensated by recurrent hybrid production. To test this hypothesis we studied population dynamics and seasonal changes in body size of two Daphnia species (0. galeata and D. cucullata) and their interspecific hybrid D. cucullatu X D. galeata in Tjeukemeer, a shallow eutrophic lake in the Netherlands. In 1989, the year with the highest fish predation pressure, the larger D. gale&a was first replaced by the smaller hybrid that was soon succeeded by the even smaller D. cucullata. In 1990 the Daphnia community was dominated by the hybrid and in I991 by D. galeatu. Deviations of the instantaneous rate of increase, averaged over the three species, showed a positive relationship with fish predation risk for D. cucullata, a negative relationship for D. gale&a, and no relationship for the hybrid. This suggests a higher influence of fish predation on D. galeata. We conclude that the selective advantage of the hybrid over D. galeatu under fish predation, expected from life-history experiments, is in agreement with our field data. Interspecific hybridization in freshwater cladocerans has gained a lot of attention in the last decade (see Schwenk and Spaak 1995 for review). After the discovery of intermediate morphological forms (Einsle 1966; Lieder 1983; FlijRner and Kraus 1986), evidence for hybridization was obtained by means of allozyme electrophoresis (Hebert 1985; Wolf and Mort 1986; Spaak and Hoekstra 1993; Spaak 1996). Hybrids are known to be widespread in the genus Daphnia. Because most Daphnia species are cyclic parthenogens at least during the growing season when parthenogenesis dominates, hybrids can stay in the Daphnia community without being regularly formed by the parental species. Most theories on the maintenance of hybrid zones are based on hybrid inferiority. For example, the tension zone model (Barton and Hewitt 1985, 1989) predicts that hybrid zones are maintained by a dynamic equilibrium resulting I Present address: EAWAG/ETH, Department ,of Limnology, oberlandstrasse 133, 8600 Dtibendorf, Switzerland. * Present address: Province of Zeeland, Department of Environmental and Public Works, PO. Box 165, 4330 AD Middelburg, The Netherlands.