An unusual double membrane structure, or 'annulus' located within leaf blade epidermal cells of the euryhaline seagrass Halophila ovalis may be an important site for ion transport and storage during osmotic adjustment. Cuttings of marine and estuarine H. ovalis were cultured for 4 weeks in enriched seawater at 35 parts per thousand and 25 parts per thousand, respectively (the salinities in which they were growing at the time of collection) and 15 parts per thousand below these habitat salinities. In a second experiment, plants growing at low salinity were returned to their habitat salinities. Paradermal sections of severed leaves revealed that the double membrane structure was in fact tonoplast encircled by protoplast and present only in the cells of marine plants. Cell responses differed between marine and estuarine plants. Marine H. ovalis plants were intolerant of prolonged exposure to low salinity (20 parts per thousand). Plant mortality reached 75% within 3 weeks, and plants continued to deteriorate following their return to marine salinity (35 parts per thousand). Estuarine H. ovalis were healthy at 20 parts per thousand and plants grew well at 10 parts per thousand, but appeared stressed after 4 weeks. Chloroplasts were swollen and few in number, while the number of cells with visible vacuoles increased under low salinity conditions. Leaf senescence was accelerated by approximately 1 week when compared to control plants grown at habitat salinities. Cell surface area, leaf and internode length and rhizome diameter decreased with prolonged exposure to low salinity. These structural changes were reversed when estuarine plants were returned to 25 parts per thousand salinity. Differences in the salinity tolerance of marine and estuarine H. ovalis may result from acclimation or the emergence of ecotypes, as occurs in algae with wide salinity distributions. (C) 1999 Elsevier Science B.V. All rights reserved.