Hypoxia effects on gill surface area and blood oxygen-carrying capacity of the Atlantic stingray, Dasyatis sabina.

Atlantic stingrays, Dasyatis sabina, are common residents of shallow-water seagrass habitats that experience natural cycles of severe hypoxia during summer months. We hypothesized that stingrays exposed to hypoxic episodes would improve their hypoxia tolerance by increasing branchial surface area and altering blood oxygen-carrying capacity. To this end, we compared critical oxygen minimum, gill morphology, and hemoglobin/hematocrit levels in a control group of Atlantic stingrays held at continuous oxygen saturations of 80-90% (≥5.5 mg/l), to treatment groups exposed to a 7-h hypoxic interval at 55% (~4.0 mg/l), or 30% oxygen saturation (~2.0 mg/l). Stingrays in hypoxic treatment groups significantly improved their hypoxia tolerance. Critical oxygen minimum values fell from 0.7 ± 0.11 mg/l in control fish to 0.4 ± 0.05 and 0.4 ± 0.06 mg/l in the 55 and 30% saturation treatment groups, respectively. Mass-specific gill surface area between control fish and the 30% saturation treatment group increased by 1.7-fold, from 85 to 142 mm(2)/g. Although stingrays did not show an increase in hematocrit or hemoglobin levels, production of more efficient hemoglobin isoforms could not be ruled out. An increase in hypoxia tolerance allows Atlantic stingrays to forage for longer times and across a wide range of hypoxic habitats that are less accessible to predators and competitors.