Temperature and salinity constraints on the life cycle of two brackish-water nematode species

The present study investigates the influence of salinity and temperature on the life history of two estuarine bacterivorous nematode species, Pelliodi tis marina and Diplolaimelloides meyli, isolated from the mesohaline zone o f the Westerschelde Estuary, SW Netherlands. Gravid females and adult males were inoculated in petri dishes containing agar layers of nine (for P. mar ina) or five (for D. meyli) different salinities, from almost freshwater to higher than marine, and incubated at a temperature of 20 degrees C, to stu dy the impact of salinity; agar layers with a salinity of 20 parts per thou sand, incubated under each of six different temperatures from 5 to 30 degre es 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 inf luence of salinity and temperature on respiration, assimilation and scope f or production in the same nematode species. Salinity had relatively minor e ffects on fecundity, development times and sex ratio in both species, but s trongly impacted juvenile viability at the extremes of the salinity range: at salinities close to 0 and 40 parts per thousand, preadult mortality was more than 80% in P, marina; it was 100% at 5 parts per thousand in D, meyli . Both species had an (near) optimal fitness at salinities of 10 to 30 part s per thousand. Temperature had a pronounced influence on both nematodes ov er the entire range studied. Diplolaimelloides meyli still reproduced acid matured at temperatures exceeding 30 degrees C, while P. marina had an uppe r temperature limit for reproduction of 25 degrees 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 degrees C to 7 days at 9 degrees C. The development time of D. m eyli increased from 7 days at 25-30 degrees C to 63 days at 10 degrees C, t emperature below which no reproduction occurred. Female-biased sex ratios w ere found in D. meyli at low temperatures and in P. marina under optimal sa linity conditions. The Life history results largely agree with the predicte d scope for production, but discrepancies were found near the extremes of t he abiotic range of both species. It is emphasized that the ranges observed are characteristic of populations, not of species; they may to an extent h ave been influenced by culture conditions. A comparison of the present resu lts with literature data on other P. marina populations demonstrates that s ome populations of this species may still reproduce successfully under cond itions which are lethal to other populations, raising the question as to wh ether cryptic species rather than populations of a single species are invol ved. (C) 2000 Elsevier Science B.V. All rights reserved.