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The ecological survey of the three river estuaries showed that parasites' biomass exceeded that of the ecosystem's top predators, said University of California-Santa Barbara's Armand Kuris in the journal Nature.

The team surveyed hundreds of species living on river estuaries -- one in California and two in Mexico's Baja California -- including 138 species of infectious parasite, Kuris said. Parasites represented as much as 3 percent of the overall biomass of the ecosystem in some areas.

While dealing with only one type of ecosystem, the figure is higher than previous estimates of parasite biomass in other ecosystems, such as coral reefs, the authors said.

They found the most prevalent parasite in the river estuaries was a trematode species that infects and castrates snails. The survey shows that, in some estuaries, the snail biomass controlled by trematodes was greater than that of free-living snails.

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