The morphological and physiological adaptations of benthic fish species associated with the oxygen minimum zone within Monterey Bay, California

Abstract

Oxygen minimum zones (OMZs) are globally distributed low oxygen regions which drastically affect biomass and diversity of benthic communities. Recently documented expansion of low-oxygen waters along both continental shelf and slope habitats has the potential to radically alter species depth distributions, especially for those species which do not have the capability to survive in reduced dissolved-oxygen environments. In order to understand how fishes thrive in persistently low oxygen, 10 species of benthic fishes were sampled from Monterey Canyon, California U.S.A., which features a prominent OMZ. Fish adaptation to low oxygen was assessed through observations of oxygen demand (metabolic measurements, Vmax) and oxygen supply (gill surface area). Six species commonly found in the OMZ were compared to four others that were restricted to waters above or below the OMZ. Comparisons were tested within three orders to minimize phylogenetic effects. The six OMZ inhabiting species had different methods of coping with low oxygen. Proximate metabolism showed no distinct trend toward aerobic suppression or heightened reliance on anaerobic metabolism as might be expected in areas of low oxygen but there was a wide range of enzyme activities reflecting differences in activity amongst the fishes. Flatfishes had the lowest enzyme activities but demonstrated no differences in metabolism with habitat type, yet the OMZ-dwelling Microstomus pacificus has 1.8-3 times larger gill surfaces than similarly sized flatfishes from higher oxygen waters. In scorpaeniform fishes, high aerobic metabolism was accompanied by large gill surface areas in two routine swimming OMZ-dwelling species (Anoplopoma fimbria, Careproctus melanurus), while low aerobic metabolism and small gills were observed in the sedentary Sebastolobus species. In gadiform fishes, no differences were measured in enzyme activity levels, but larger gill surfaces were observed in OMZ-dwelling Nezumia liolepis. Additionally, how each group attains larger gill surface area is highly divergent, with differences in primary filaments (lengths, counts) and secondary lamellae (bilateral area, density), suggesting no clear convergent trend across taxa and that compensation can occur many ways in gill architecture. In light of recent OMZ expansion, several species studied may have to shallow their vertical distributions. On the other hand, some species are OMZ specialists and will likely experience range expansion in the future.