The life cycle of all species of lampreys contains a microphagous larval phase, which is spent in freshwater (Potter, 1980). After a number of years, the larvae (ammocoetes) undergo a radical metamorphosis into young adults that, in the case of anadromous species, migrate downstream to the sea where they feed on predominantly teleost fishes (Potter and Hilliard, 1987). Following the completion of their marine trophic phase, adult lampreys re-enter freshwater and migrate to upstream areas, where spawning and then death occur (Hardisty and Potter, 1971).
Ammocoetes of the sea lamprey Petromyzon marinus are unable to osmoregulate in environments in which the osmolality exceeds that of their own serum, i.e. 225 mosmol kg–1 (Beamish et al., 1978; Morris, 1980). Although 50% of larval P. marinus died within 24 h of being placed in water of 350 mosmol kg–1, i.e. about one-third of full-strength seawater, the recently metamorphosed young adults of anadromous species can readily be acclimated to full-strength seawater and are then able to maintain their serum osmolality at 260 mosmol kg–1 (Beamish et al., 1978). This ability to change from hyper-osmotic regulation in freshwater to hypo-osmotic regulation in seawater is so effective that more than 80% of young adult P. marinus can even survive direct transfer from fresh to full-strength seawater (Potter and Beamish, 1977). Similar results have been obtained with young adults of the anadromous lampreys Lampetra fluviatilis and Geotria australis (Potter and Huggins, 1973; Potter et al., 1980). The ability to osmoregulate in hypertonic environments is lost by adult lampreys soon after they have completed their marine trophic phase and embarked on their upstream spawning migration (Morris, 1956, 1958; Pickering and Morris, 1970).