HEAVY-METAL COTOLERANCE IN A CHROMIUM TOLERANT STRAIN OF DAPHNIA-MAGNA

After 21 days of exposure to 5 ppb Cr the descendants of a single chromium tolerant individual of Daphnia magna, produced 67% more neonates than animals of the same age from the stock culture. At the end of the experiment 93% of the chromium tolerant descendants were still alive, but no individual from the stock culture survived. In 10 ppb Cr the differences in reproduction rate and mean life expectancy between the groups diminished. The intrinsic rate of population growth (r) for chromium tolerant individuals was significantly higher than for stock animals in both chromium concentrations. Primiparous chromium tolerant individuals were significantly larger than stock animals. Chromium tolerance was not enhanced over seven successive generations exposed to 5 ppb Cr or over six successive generations exposed to 10 ppb Cr. In water with copper (5 ppb and 10 ppb) or nickel (50 ppb and 100 ppb) chromium tolerant individuals showed a significantly higher tolerance for the demographic parameters examined, compared with animals from the stock culture. Contrary to this, chromium tolerant individuals were more sensitive to 100 and 150 ppb Zn than animals from the stock culture were.