Adults of the Wilbur Springs shore bug,Saldula usingeriPolhemus, live longer (8.5 cf. 2.0 d) than the closely relatedSaldula pexaDrake when cultured in the high salinity water ofS. usingeri's habitat (Wilbur Hot Springs, Colusa County, Calif.). However, survivorship of the two species is similar in the low salinity water ofS. pexa's habitat (Grizzly Springs, Lake County, Calif., 15 km from Wilbur Hot Springs). Tests with glycine-radiolabeled water indicate thatS. usingeriimbibes fluids from its prey, larvae of the brine flyParacoenia calidaMathis, rather than imbibing water from its habitat; in contrast,S. pexaimbibes water from its own habitat, and Wilbur Hot Springs water as well. By imbibing fluids from its prey,S. usingeriexpends less energy for osmoregulation and its intake of toxic ions from Wilbur Hot Springs water is reduced. By imbibing Wilbur Hot Springs water, the potentially coexistingS. pexalacks these advantages. Although abiotic differences may explainS. pexa's absence fromS. usingeri's habitats, susceptibility ofS. usingerito spider predation may explain the absence ofS. usingerifromS. pexa's habitats. Predatory spiders are virtually absent fromS. usingeri's habitats but are very abundant inS. pexa's habitats. In laboratory tests, the wolf spiderPardosa ramulosaMcCook (Araneae: Lycosidae) killed 8-fold as many of the sluggishS. usingeriadults as the skittishS. pexaadults. Sluggishness may be an advantage whenS. usingerioccurs along margins of lethal thermal effluents (as at Wilbur Hot Springs) but it may be a disadvantage in nonthermal habitats with mobile predators.