• dissolved oxygen;
  • killifish;
  • LDH;
  • protein;
  • RNA:DNA;
  • specific growth rate

Specific growth rate (GS) and white skeletal muscle composition were measured in the mummichog Fundulus heteroclitus over a period of 28 days at four levels of dissolved oxygen (DO): severe hypoxia (c. 1·2 mg O2 l−1), moderate hypoxia (3·0 mg O2 l−1), normoxia (7·1 mg O2 l−1) and hyperoxia (10·6 mg O2 l−1). The GS was calculated over 0–8, 0–14, 0–28 and 14–28 days, and muscle protein, lactate dehydrogenase (LDH), DNA, RNA and water were measured at 0, 8, 14 and 28 days. Exposure of fish to severe hypoxia was associated with significantly reduced GS, lower muscle protein content and lower RNA:DNA compared with other DO treatments. When calculated over the first and second half of the 28 day exposure, however, GS of fish in severe hypoxia increased significantly during the second two-week interval, to the same rate as that of normoxic fish. Muscle LDH activity and water content were not significantly affected by DO level. Neither moderate hypoxia nor hyperoxia significantly affected GS or any biochemical variable. The results demonstrate that F. heteroclitus can tolerate wide variation in ambient oxygen concentration and, during prolonged exposure to severe hypoxia, shows significant compensation for the initial negative effects on growth. The capacity of F. heteroclitus to grow over a wide range of DO probably contributes to its ability to exploit habitats characterized by marked variation in oxygen availability.