Hypoxia-induced physiological changes in two mangrove swamp fishes: sheepshead minnow, Cyprinodon variegatus Lacepede and sailfin molly, Poecilia latipinna (Lesueur)

1. Laboratory measurements (30° C and 30‰ salinity) were made of plasma osmolality, plasma chloride ion concentration, hematocrit, oxygen consumption and survival of sheepsheadminnow, Cyprinodon variegates Lacepede and sailfinmolly, Poecilialatipinna (Lesueur) under normoxic (150mm Hg) and hypoxic (40 mm Hg) conditions. 2. Significant increases in hematocrit and reductions in oxygen consumption were documented for both species. Plasma osmolality increased in sheepsheadminnows while in hypoxie conditions but plasma chloride did not change from values in 150mm Hg in either species. There was no mortality in either species during the 24 hr hypoxia survival tests. 3. Results suggest a strong tolerance of hypoxia in both species and use of aquatic surface respiration (ASR) by P. latipinna. 4. Low-level mortality occurs in both species but severe mortality occurs only in C. variegatus and may be due to synergistic environmental effects typical of mangrove swamp habitats.