JOYNER-MATOS, J.*; CHAPMAN, L. J.; JULIAN, D.; Univ. of Florida, Gainesville; McGill Univ., Montreal; Univ. of Florida, Gainesville: Elevated dissolved oxygen level influences fingernail clam (Sphaerium sp.) stress protein expression and population distribution in a Ugandan papyrus swamp

Abiotic factors can affect species distribution by stressing organisms to their physiological limits. In a population distributed along an abiotic gradient, organisms on the distribution edge may be more stressed than those in the center, where conditions may be benign. We tested this hypothesis using cellular-level biomarkers. The freshwater clam Sphaerium sp. inhabits papyrus (Cyperus papyrus) swamp/river systems in Uganda. In this system, ecotonal gradients from normoxia to hypoxia occur along streams feeding into the swamps. Across one such gradient, DO levels were lowest in the swamp (mean DO = 0.82 � 0.18 mg/L), 3.05 � 0.07 mg/L in the ecotone, and 5.46 �0.24 mg/L in the stream. Average clam density was high but patchy in the swamp (mean 74.9 �15.4 clams/0.5m², range 8-246 clams/0.5m²). Clam densities decreased along the ecotone, and clams were absent in stream sites. Across the gradient, clam abundance was negatively related to DO (r = -0.629, p < 0.0001). The internally-brooding clams likely spend their entire life cycle in extreme hypoxia. Elevated DO may influence their distribution since hyperoxia increases endogenous free radical production, causing oxidative damage. To test whether edge clams were stressed, we analyzed expression levels of 10 stress proteins (CuZnSOD, MnSOD, GPx, HO-1, ubiquitin, Hsp60, Hsp70, Grp75, sHSP, and OGG1-m) in clams from four sites. Seven of the proteins (but not GPx, Grp75, or HO-1) had the following pattern: clams from the dense interior of the swamp, ~200 m from stream input, had low expression levels, whereas clams at swamp sites 90m and 30m from the distribution edge had increased expression, and clams at the distribution edge had significantly decreased protein expression (p < 0.021). While this pattern could indicate that edge clams were not stressed, an alternative interpretation is that edge clams were stressed beyond a critical level at which elevated stress protein expression could not be maintained.