The Adaptation of Gunda Ulvae to Salinity

Abstract

1. The environment of the triclad Gunda ulvae has been studied. This organism lives on the sea shore in the estuaries of very small streams. 2. The components of the external medium are (a) stream water, which is rich in Ca and CO₃, and (b) Atlantic sea water. These are mixed in different proportions in different parts of the estuary. 3. An analysis of the stream water is given. 4. The habitat of the organism is described. This extends roughly from high-water neap tides to low-water neap tides. A faunistic survey is given. 5. The conditions which control the limits of the habitat of Gunda are discussed. Between the upper limit of occurrence of Gunda and the place of occurrence of fresh-water forms there is a region devoid of fauna. This region corresponds roughly with the span between high-water neap tides and high-water spring tides. 6. Salinity determinations have been made on samples taken from the actual places where Gunda occurred. It is shown that Gunda has to withstand changes from completely fresh to undiluted sea water. It may normally be exposed to either extreme for several hours. 7. Salinity determinations made continually throughout the range of Gunda show that its environment may vary from one in which it is subjected to the action of sea water for only about I hour at high tide to one in which the sea water is only diluted to about 10 per cent. of its normal strength for a few hours during low tide.