Depth-related trends in metabolism of benthic and benthopelagic deep-sea fishes

The metabolism of several animal groups declines with depth even after adjustments for size and temperature. The ``visual-interactions hypothesis'' explains this trend as the result of declining light levels that reduce the distances over which predators and prey interact. This reduction relaxes the selective pressure for locomotory capacity, and reductions in metabolic rate follow. The decline in visual interactions and metabolism is most pronounced among pelagic species, as this environment affords no refuge from predators. The visual-interactions hypothesis thus predicts less depth-related variation among benthic species. However, it has been tested primarily with pelagic animal data. Summarizing many scattered studies and data sets to further test the hypothesis, here we analyze the data for benthic and benthopelagic fishes. Oxygen consumption rates declined significantly with depth in benthopelagic and, to a lesser extent, benthic species. Trends in muscle metabolic enzyme activities generally corroborated these patterns. The anaerobic capacity of the white muscle indicated the greatest decline in pelagic and the smallest reduction in benthic species, as expected. Similar trends were not found in aerobic capacity, but this result may reflect a paucity of enzyme data for benthic species. Most of the studied fishes live off of California, where the presence of an oxygen-minimum zone may influence some of the patterns observed. This preliminary analysis of data clearly illustrates that temperature and body mass cannot explain the variability evident in metabolism. Rather, some covariate of habitat depth acts to influence metabolism in benthic and benthopelagic fishes. The general trends are explained by the visual-interactions hypothesis, but considerably more data are required to account for the variation in metabolism and lifestyle that is apparent. In particular, regional comparisons are needed to separate the influences of environmental factors, such as oxygen, which covary with depth.