Seasonal niche partitioning and coexistence of amphimictic and parthenogenetic lineages of Heterocypris barbara (Crustacea: Ostracoda)

Abstract: Sympatry of amphimictic and parthenogenetic lineages in species with mixed reproductive systems is rarely observed in nature. On Lampedusa Island (Pelagie Islands, Italy), amphimictic and parthenogenetic lineages of Heterocypris barbara (Gauthier and Brehm, 1928) co-occur in a temporary pond. Their sympatric persistence calls for an ecological differentiation. We investigated the role of seasonal variation of temperature and photoperiod conditions by two different approaches: microcosms set up by inundation of dry sediments from the temporary pond and life-table experiments. Microcosms recreate conditions similar to the field and in their sediments random samples of resting eggs of both amphimictic and parthenogenetic females are stored. Life-table experiments supplied individual-based estimates of survivorship, adult life span, fecundity, and sex ratio in the progeny. We carried out the experiments at 24 °C and a photoperiod of 12 h light (L) : 12 h dark (D) (simulating fall conditions) and at 16 °C and a photoperiod of 10 h L : 14 h D (simulating winter conditions). Males and amphimictic females were the most numerous forms at 24 °C and 12 h L : 12 h D; parthenogenetic females were dominant at 16 °C and 10 h L : 14 h D. Life-table experiments showed that amphimictic forms do not complete development at 16 °C and 10 h L : 14 h D. Our results suggest that sympatry of amphimictic and parthenogenetic females in the field depends on seasonal niche partitioning and the storage effect of resting eggs that allows survival through adverse-season conditions.