The present study investigates the influence of salinity and temperature on the life history of two estuarine bacterivorous nematode species, Pellioditis marina and Diplolaimelloides meyli, isolated from the mesohaline zone of the Westerschelde Estuary, SW Netherlands. Gravid females and adult males were inoculated in petri dishes containing agar layers of nine (for P. marina) or five (for D. meyli) different salinities, from almost freshwater to higher than marine, and incubated at a temperature of 20°C, to study the impact of salinity; agar layers with a salinity of 20‰, incubated under each of six different temperatures from 5 to 30°C, served to study the effect of temperature. Daily and total fecundity, development time and sex ratio were quantified, and preadult mortality was estimated. The results are compared to those of a partner study on the influence of salinity and temperature on respiration, assimilation and scope for production in the same nematode species. Salinity had relatively minor effects on fecundity, development times and sex ratio in both species, but strongly impacted juvenile viability at the extremes of the salinity range: at salinities close to 0 and 40‰, preadult mortality was more than 80% in P. marina; it was 100% at 5‰ in D. meyli. Both species had an (near) optimal fitness at salinities of 10 to 30‰. Temperature had a pronounced influence on both nematodes over the entire range studied. Diplolaimelloides meyli still reproduced and matured at temperatures exceeding 30°C, while P. marina had an upper temperature limit for reproduction of 25°C. Development times of D. meyli were more temperature-dependent than those of P. marina: the mean development time from adult to adult for the latter nematode ranged from 2 days at 25°C to 7 days at 9°C. The development time of D. meyli increased from 7 days at 25–30°C to 63 days at 10°C, temperature below which no reproduction occurred. Female-biased sex ratios were found in D. meyli at low temperatures and in P. marina under optimal salinity conditions. The life history results largely agree with the predicted scope for production, but discrepancies were found near the extremes of the abiotic range of both species. It is emphasized that the ranges observed are characteristic of populations, not of species; they may to an extent have been influenced by culture conditions. A comparison of the present results with literature data on other P. marina populations demonstrates that some populations of this species may still reproduce successfully under conditions which are lethal to other populations, raising the question as to whether cryptic species rather than populations of a single species are involved. |
