However, according to a study reported in the September 2019 San Francisco Estuary and Watershed Science, Hyalella collected in the wild from Cache Slough are resistant to pyrethroids. Genetic analysis revealed that, collectively, Hyalella in the slough have four mutations for pyrethroid resistance. They also have a fifth mutation for resistance to organophosphates and likely carbamates, two additional major classes of pesticides that are applied extensively on agricultural and urban lands. “Hyalella are special,” explains co-author Helen Poynton, a molecular ecotoxicologist at the University of Massachusetts in Boston. “They reproduce quickly and have a short life span, which makes them highly adaptable.” Pesticide interests have seized on findings of pyrethroid resistance in Hyalella elsewhere in the wild to argue that water quality regulations for these pollutants should be relaxed. But the authors found that pyrethroid concentrations in Cache Slough were high enough to harm fish, for example by reducing the numbers and viability of offspring as well as by disrupting neurodevelopment and behaviors like predator avoidance. In addition, Hyalella are a mainstay of fish diets, and other researchers have shown that when fish gobble these crustaceans in contaminated waters, the pesticides transfer and build up in their own flesh. Ironically, their ability to survive pesticides may ultimately threaten Hyalella themselves. “They have one superpower: pesticide resistance,” Poynton says. “But the tradeoff is that they are weaker for other kinds of stress and they reproduce less.” Plummeting Hyalella populations would spell further trouble for Delta smelt and other imperiled species that depend on these crustaceans for food.