Gill surface area and metabolic enzyme activities of demersal fishes associated with the oxygen minimum zone off California

Abstract

Metabolic enzyme activities and gill surface areas were measured across 10 species of demersal fishes from Monterey Canyon, California, which features a prominent oxygen minimum zone (OMZ). Comparisons were made between species living both within and outside of the OMZ. Enzyme activities showed no significant trend toward aerobic suppression or heightened reliance on anaerobic metabolism in response to the OMZ. While flatfish species living both within and outside of the OMZ had similarly low enzyme activities, the OMZ‐dwelling Microstomus pacificus had 1.8–3 times larger gill surface area than comparably sized flatfishes from higher‐oxygen waters, suggesting a morphological adaptation to low oxygen. In scorpaeniform fishes, high aerobic metabolism was accompanied by large gill surface areas in two routine‐swimming OMZ‐dwelling species (Anoplopoma fimbria and Careproctus melanurus). Low aerobic activities and small gills were found in two Sebastolobus species, suggesting a low oxygen demand resulting from a more sedentary behavior compared to other Scorpaeniformes. In gadiform fishes, no differences were measured in enzyme activity levels, but larger gill surface areas were measured in OMZ‐dwelling Nezumia liolepis. These results indicate adaptation to low oxygen in a variety of ways that balance oxygen demand with supply, with no indication that these species rely on enhanced anaerobic metabolism. With both flatfishes and rattails, adaptation to OMZs is demonstrated through increased gill surface area.