Once jellies get to a new habitat, "they're very good at sliding in and taking advantage of excess food," says Monty Graham, an assistant professor of marine sciences at the University of South Alabama whose team sighted the massive spotted jellyfish bloom in 2000. Excess food, according to Graham and other researchers, is often a result of overharvesting of the fish that feed on the same resources. Eutrophication, a process in which nutrient-rich runoff from agriculture, sewage-treatment plants and other sources boosts algae growth, also contributes. The algal blooms, as they're called, reduce the amount of available oxygen in the water, thus killing off competitors like fish. Jellyfish, on the other hand, can thrive in this oxygen-deprived environment. The dwindling numbers of predators, especially the critically endangered leatherback turtle (Dermochelys coriacea), is another factor.