A fingernail clam (Sphaerium sp.) shows higher reproductive success in hypoxic waters

Low dissolved O2, or hypoxia, is becoming increasingly prevalent in aquatic habitats and is considered to be stressful for aerobic organisms. However, hypoxia also can be beneficial by decreasing cellular stress, particularly that related to free radicals. Therefore, an animal’s ideal habitat may have the minimum O2 necessary to sustain aerobic metabolism, with excess O2 increasing the need to scavenge free radicals and repair free radical damage. Here we show that a natural population of small (<9 mm shell length) freshwater clams (genus Sphaerium Scopoli, 1777) lives along a dissolved O2 gradient from extreme hypoxia to moderate hypoxia. We tested the hypothesis that clams living in extreme hypoxia would have higher reproductive success than clams that live in moderate hypoxia. Clam abundance was highest in water with very low dissolved O2, conditions previously demonstrated to decrease cellular stress. The internally brooding clams reproduced year-round and had higher reproductive output in extreme hypoxia than in moderate hypoxia. The findings demonstrate that the apparent cellular-level benefits of hypoxia may translate into increased fitness, especially for