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AdultAcartiacongeners,A.bifilosa,A.clausi,A.discaudataandA.tonsa, have distinct seasonal and spatial distribution patterns in Southampton Water (UK), reflecting patterns of temperature and salinity, respectively. The effect of these factors on other life stages, hatch success and naupliar survival was investigated by exposing the congeners to a range of salinity (15.5-33.3) and of temperature (5-20°C).A.clausiis known to prefer more saline waters, and showed highest hatch success at 33.3 salinity.A.tonsais most tolerant to dilution, and at 15.5 salinity it had the highest hatch success of all the congeners. Hatch success in bothA.bifilosaandA.discaudatawas similar over the range of salinities investigated, confirming that they are intermediate species in terms of spatial distribution. The nauplii of all species survived well at the higher salinities and best at 33.3, which allows for differential transport of the poorly swimming nauplii to the mouth of the estuary until size and swimming ability increase, after which they can then return to regions of preferred salinity. The summer species,A.clausiandA.tonsashowed higher hatch success at 20°C, whereasA.discaudata, which is present in the water column all year round, showed no significant temperature-related differences in hatch success.A.bifilosa, which diapauses over summer, showed significantly higher hatch success at 10°C than at 20°C. The physiological relationship between temperature and development time was clear; naupliar survival of all species was highest at 20°C and all congeners reached the first copepodite stage (CI) significantly faster at 20°C. However, no consistent pattern was seen for salinity. It would appear that the adult Acartidae in Southampton Water remain in regions of their preferred salinity and lay eggs there which hatch well. However, because the nauplii are not good swimmers, they are swept towards the mouth of the estuary and into areas of higher salinity, where they remain and develop into more advanced stages before moving back up the estuary to take up their adult distribution pattern. [ABSTRACT FROM AUTHOR]