ON THE DISTRIBUTION OF NEREIS DIVERSICOLOR IN RELATION TO SALINITY IN THE VICINITY OF TVÄRMINNE, FINLAND, AND THE ISEFJORD, DENMARK

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Abstract
1. A comparative field study of the distribution of Nereis diversicolor near Tvärminne, south Finland and in the Isefjord, Denmark shows that although the species penetrates into nearly fresh water in the Isefjord, its distribution near Tvärminne is limited at summer salinities of over 4°/00. 2. The apparent salinity optimum of N. diversicolor on the border of the polyhaline Isefjord is lower than on the "marine-dominated" beach at Millport, Scotland, but in both instances there may be a restriction of the potential range because of interspecific competition. 3. The paradoxical situation is seen near Tvärminne of N. diversicolor being among the first of the characteristic brackish-water fauna to drop out as salinity lessens, whereas in the Isefjord as well as in British estuarine situations it penetrates further toward fresh water than the rest of its associates. 4. It is suggested, on the basis of hydrographic conditions in the Tvärminne area, that salinities prevailing in summer cannot be the limiting factor for N. diversicolor in the Baltic Sea, but that critically low salinities occurring in spring while temperatures are still very low may adversely affect the osmoregulation and/or reproduction of the species. Furthermore, there may be a hydrographic (as well as a purely physiological) barrier to the spread of the species into oligohaline waters, in which connection the time of breeding and the lack of a long planktonic stage in the life history may be important.