It is predicted that two species of crayfish (Astacus leptodactylus and Pacifastacus leniusculus) introduced into Britain and now widely spread in the wild may colonize the estuarine environment as they have done within their home ranges. The only crayfish native to British waters, Austropotamobius pallipes, is not known from the estuarine environment. Adults of all three species were found to have a similar osmotic response when transferred to a range of seawater concentrations (salinities of 7, 14, 21 and 28 where 100% seawater equals 35) for 48-h periods. The osmolality of the haemolymph increased significantly as the salinity increased. All three species hyper-regulated in freshwater and low salinities (? and 14), and hyporegulated in high salinities (21 and 28). The transition from hyper- to hyporegulation was found to be: 497 mOsm kg(-1) - A. pallipes, 470 mOsm kg(-1) - A. leptodactylus and 523 mOsm kg(-1) - P. leniusculus. Both A. leptodactylus and P. leniusculus hyper-regulated over a 6-week period in freshwater and salinities of 7 and 14. In reverse transfer experiments, no mortalities occurred in either species when they were transferred directly back to freshwater after having been exposed to salinities of 7, 14: and 21 for a 9-week period. Survival of 0+ juvenile A. leptodactylus and P. leniusculus was good (50% or over) in salinities up to 28 over a 9-week period. Pacifastacus leniusculus was better able to survive full-strength seawater than A. leptodactylus although mortality was high (75%) after 2 weeks. Adults did not survive as well as 0+ juveniles and 100% mortality was recorded for both species in salinities of 28 and 35 within 3 weeks. Berried female A, leptodactylus and P. leniusculus survived exposure to salinities of 7, 14 and 21 whilst incubating their eggs, but only in a salinity of 7 (20% seawater) did the eggs hatch into stage 1 juveniles which subsequently moulted into stage 2 juveniles. Such stage 2 juveniles survived for at least 2 weeks in a salinity of 7. The three species tested were found to have the physiological capacity to survive in seawater up to a salinity of 28, at least in the short term, despite having being acclimated to freshwater for many generations. Tolerance experiments with A. leptodactylus and P. leniusculus indicated that 0+ juveniles and adults are well adapted for surviving salinities of at least 21 in the long term and of being transferred directly back into freshwater. However, their ability to colonize the estuarine environment may be restricted to areas of low salinity (i.e. 7) due to the adverse effects of seawater on egg development and hatching. (C) 1997 Academic Press Limited.
