Stress response of a freshwater clam along an abiotic gradient: too much oxygen may limit distribution

1. Distribution and abundance patterns are influenced by cellular responses to abiotic stressors. 2. The freshwater clam Sphaerium sp. inhabits swamp/stream systems in Uganda, in which dissolved O 2 (DO), pH and water transparency form ecotonal gradients along tributary streams. Along a swamp/stream transect, clam abundance was negatively related to DO, pH and transparency; clams were most abundant in the hypoxic/acidic swamp, less abundant in the ecotone, and absent from stream sites (neutral pH and higher DO). 3. We tested the hypothesis that individuals living in low abundance at the distribution edge have increased stress by assessing cellular-level stress indicators in clams from four sites: dense swamp interior (reference site), swamp margin, ecotone and stream. Compared with swamp interior clams, stream clams had decreased RNA : DNA ratio (decreased protein synthesis and growth) and increased DNA and RNA oxidation (free radical damage). Of seven stress proteins related to general damage and free radical detoxification, expression of nearly all were higher in margin and ecotone clams than in swamp interior or stream clams. 4. This pattern, which includes the first measurement of DNA and RNA oxidation in a natural population, demonstrates that stress increases along the transect, inversely to clam abundance. Additionally, this pattern is consistent with free radical stress, which suggests that inhabiting swamp conditions minimizes oxidative damage in this species.