The distribution and salinity tolerance ofCiona intestinalis(L.) F.typicawith special reference to the waters around Southern Scandinavia

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Abstract
The ascidian, Ciona intestinalis, has a world-wide distribution and occurs in many different salinities. The distribution in the waters around southern Scandinavia in relation to the rather complicated and dynamic salinity conditions prevailing there is described. The outermost distribution limit lies at the entrance sills of the Baltic Sea proper in an average salinity of about 11‰. In a series of experiments it is shown that populations living under different salinity conditions have different tolerance ranges for the development offertilized eggs and larvae to metamorphosed individuals. The limit for complete development lies at 11 ‰. S which is in agreement with the above-mentioned figure for the geographical distribution limit for steady settlement of the species as such. The results of the experiments also show that the tolerance ranges for larvae, and still more for the adults, are wider than for the zygotes, and the importance of this fact for the dispersal and survival of the populations is discussed. In some experiments the influence of temperature on the salinity tolerance was studied. Extreme temperatures and salinities cooperate in getting poorer or no development at all. The tolerance ranges, especially for the zygotes, do not always exactly cover the most extreme salinity variations in the natural habitats of different populations which may often cause failure of spawning, especially when also the temperatures are sub-optimal. The different tolerance ranges from high salinity populations to low salinity populations are found to constitute a character gradient or dine, which, geographically seen, in reality is a 3-dimensional cline-network. It is supposed that this is brought about by phenotypic, genotypic and chronological factors.

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