Experiments with mice, rats, and dogs have shown that a degree of 25% deuteration causes (sometimes irreversible) sterility, because neither gametes nor zygotes can develop. High concentrations of heavy water (90%) rapidly kill fish, tadpoles, flatworms, and "Drosophila". The only known exception is the anhydrobiotic nematode "Panagrolaimus superbus", which is able to survive and reproduce in 99.9% D2O. Mammals (for example, rats) given heavy water to drink die after a week, at a time when their body water approaches about 50% deuteration. The mode of death appears to be the same as that in cytotoxic poisoning (such as chemotherapy) or in acute radiation syndrome (though deuterium is not radioactive), and is due to deuterium's action in generally inhibiting cell division. It is more toxic to malignant cells than normal cells but the concentrations needed are too high for regular use. As may occur in chemotherapy, deuterium-poisoned mammals die of a failure of bone marrow (producing bleeding and infections) and of intestinal-barrier functions (producing diarrhea and loss of fluids).