Evidence is presented to suggest that the salinity tolerance of the adult North Sea Asterias rubens is around 23‰, whereas that of the Baltic Sea animals is 8‰. This, together with other evidence outlined, is thought to be sufficient reason to justify the use of the term ‘physiological race’ where these two populations are concerned.

Adult A. rubens from the North Sea is incapable of any degree of weight regulation in diluted media within the ecological temperature range, size or state of the breeding cycle.

Similarly, it is incapable of any osmotic regulation and it is suggested that the two main coelomic cavities constitute an osmotic unity.