THE PENETRATION OF CLADOCERANS INTO SALINE WATERS

Cladocerans are essentially freshwater organisms, many of which have been able to penetrate slightly saline waters (up to 5 parts per thousand salinity), both thalassic and athalassic, some of which occur at higher salinities, and a few of which, mostly non-chydorids, penetrate still higher salinities (15-30 parts per thousand and even higher) and may be confined to these salinities. Three previous studies from Saskatchewan, Iran, and Germany (the latter including thalassic waters) have been analyzed, and records for the athalassic saline waters of the World have been summarized; all results show a decline, at some point, in species number against increasing concentration of salinity. Examination of samples for 67 waterbodies in southern Australia and 167 in South Africa, covering the full salinity range over which cladocerans occur, reveals much the same relationships. Graphical analysis was carried out using salinity intervals defined along a logarithmically-scaled conductivity axis. The mean number of chydorid taxa per site was considerably greater in Australia than in South Africa, and this was true for both freshwater (< 5 mScm-1) and saline (> 5 mScm-1) sites. In both countries, the number of chydorid taxa per site showed little variation with conductivity over the freshwater range but declined rather abruptly at conductivities > 5 mScm-1. For South Africa, there was also some indication of reduced numbers of chydorid taxa in the most dilute (< 0.2 mScm-1) waters, Non-chydorid taxa, which were analysed only on a generic basis, averaged much more numerous in Australian saline sites than in either South Africa saline sites or Australian freshwater ones. Mean number of non-chydorid taxa per site was about the same for Australian and South African freshwater sites. Plots of total number of taxa observed per conductivity interval had maxima in the 0.2-0.7 mScm-1 conductivity range and decreased at higher and lower conductivities. This trend to some extent only reflected the effect of the variation in number of sites per interval; a negative impact on chydorids of very low conductivities (< 0.2 mScm-1) is nevertheless suggested.