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A new species of naked amoeba, Platyamoeba pseudovannellida n.sp., is described on the basis of light microscopic and fine structural features. The amoeba was isolated from the Salton Sea, California, from water at a salinity of ca. 44%. Locomotive amoebae occasionally had a spatulate outline and floating cells had radiating pseudopodia, sometimes with pointed tips. Both these features are reminiscent of the genus Vannella. However, the surface coat (glycocalyx) as revealed by TEM indicates that this is a species of Platyamoeba. Although salinity was not used as a diagnostic feature, this species was found to have remarkable tolerance to fluctuating salinity levels, even when changes were rapid. Amoebae survived over the range 0 per thousand to 150 per thousand salt and grew within the range 0 per thousand to 138 per thousand salt. The generation time of cells averaged 29 h and was not markedly affected by salt concentration. This is longer than expected for an amoeba of this size and suggests a high energetic cost of coping with salinity changes. The morphology of cells changed with increasing salinity: at 0 per thousand cells were flattened and active and at the other extreme (138 per thousand) amoebae were wrinkled and domed and cell movement was very slow. At the ultrastructural level, the cytoplasm of cells grown at high salinity (98 per thousand was considerably denser than that of cells reared at 0 per thousand.

The salinity tolerance of naked amoebae collected from sites ranging from ca. 0& to 160& were compared in laboratory experiments. Amoebae were collected from hypersaline ponds around the perimeter of the Salton Sea, California, where salinities averaged 160&, and directly from the shoreline waters of the Sea where salinities were generally between 44 and 48&. Naked amoebae were also collected from the intertidal zone of a Florida beach, a habitat subject (on occasion) to salinity fluctuations within the range 6-85&. From these combined sites, 6 clones of amoebae were isolated for salinity tolerance experiments (2 marine beach isolates, 2 Salton Sea isolates, and 2 hypersaline pond isolates). A seventh clone, Acanthamoeba polyphaga, a common freshwater/soil amoeba, was obtained from a Culture Collection. Laboratory experiments compared the effects of gradually changing culture salinity versus no salinity acclimatization. Growth rate and culture yield were used as indices of effect. Generally, amoebae were tolerant over a wide range of salinity conditions (in terms of growth and yield) and were not markedly influenced by preconditioning to salinity changes throughout the experiments. Overall, the freshwater amoeba Acanthamoeba grew between 0 and 12&, the marine clones grew in the range of 2-120&, and the Salton Sea clones reproduced between 0 and 138 &. The hypersaline clones were the most resilient and grew between 0 and 270& salt. The survival and activity of large populations of naked amoebae in sites subject to salinity fluctuations suggest that they should be considered in future studies to better understand their, as yet, undefined ecological role.

Numbers of naked amoebae (Gymnamoebae) inhabiting the lower intertidal zone of two sandy beaches were estimated using a novel enrichment cultivation method. Samples were collected between June and September, 1999. Beach sand at Kames Bay, Isle of Cumbrae, Scotland contained on average 2604 amoebae cm−3 while at Dania Beach, Florida, USA, sand harboured 4236 amoebae cm−3. This is the first study to focus on the abundance of naked amoebae inhabiting a sandy beach. These numbers are higher than densities generally reported for shallow subtidal sands and show that amoebae must be considered in future studies on the dynamics of sandy beach communities.