Vol. 47: 107-116, 1988 MARINE ECOLOGY - PROGRESS SERIES Mar. Ecol. Prog. Ser. l Published August 2 Ecophysiology and stress response of marine and brackish water Gammarus species (Crustacea, Amphipoda) to changes in salinity and exposure to cadmium and diesel-oil Michael Tedengren, Marie Arner, Nils Kautsky Ask6 Laboratory, Institute of Marine Ecology and Department of Zoology, University of Stockholm. S-106 91 Stockholm, Sweden ABSTRACT: There is a current debate whether organisms living in estuaries should be more sensitive to pollution than their marine relatives, but few comparative studies have been made. Amphlpods of the genus Garnrnarus are generally euryhaline and 5 species are common to the braclash Baltic Sea (7 %O S) and the North Sea (30 %O S). Laboratory measurements on the metabolism of G. duebeni and G. oceanicus from the 2 areas showed that respiration and 0xygen:nitrogen ratios of G. duebeni were generally less affected by salinity changes and/or additions of diesel oil and cadmium than G. oceanicus. We interpret this as a higher tolerance of G. duebeni to pollutants and salinity changes. probably coupled to this species being physiologically broad-niched and having evolved in, and being adapted to, physically more variable environments. The Baltic populations of G. duebeni and G. oceanicus were generally more sensitive to salinity changes and treatments with diesel-011 than their North Sea conspecifics. The higher sensitivity to pollutants of the Baltic populations may be due to a number of factors such as changes in the characteristics of toxic substances (metals) with salinity, the higher relative ionic concentration of a given amount of poisonous substance in the low-saline Baltic Sea than in the North Sea, and direct interactions of toxicants with membrane permeability and osmo- regulatory mechanisms, which are already under strain at low salinities. INTRODUCTION The widely distributed amphipod genus Gammarus inhabits marine, braclush- and fresh-water habitats. To cope with a range of environmental conditions in their particular aquatic habitats, species diversity and niche variability are high (e.g. Kolding 1985), yielding a promising material for experimental investigations on adaptation and underlying physiological mechanisms. Considerable experimental and theoretical work has been done on the physiological characteristics of Gamrnarus spp. (Kinne 1952, 1959, 1964, 1971, Buln- heim 1972, 1979, Dorgelo 1973, Sutcliffe 1984) as well as on Life cycle characteristics, population dynamics and genetics of the genus (Fenchel & Kolding 1979, Kolding & Fenchel 1979, Bulnheim & Scholl 1981, Kold- ing 1981, 1986, Kolding & Fenchel 1981). In the Baltic Sea 5 species of Ganlmarus are found that also inhabit the North Sea. These are G. oceanicus Segerstrble, G. O Inter-Research/Printed in F. R. Germany locusta (L.), G. salinus Spooner, G. zaddachi Sexton and G. duebeni Liljeborg. Although many studies have been performed on the effects of a variety of toxicants on marine organisms, there are relatively few on estuarine animals (McLusky et al. 1986), and estimation of the effects of a certain pollutant on estuarine ecosystems have often been derived from experiments with marine organisms that have been subjected to combinations of lowered sahn- ity and toxicants, and generally not from comparative studies between estuarine organisms and their marine conspecifics in their respective environments. Thus, the general sensitivity of the brackish Baltic Sea and the marine North Sea has been much debated (Gray 1981, Landner et al. 1982), some regarding the Baltic Sea as more sensitive and some considering the North Sea as more vulnerable to pollution. Those regarding the Baltic as more sensitive base their opin- ion on the concept that organisms living near to the