Life in the Open Ocean. Joseph J. Torres

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Life in the Open Ocean - Joseph J. Torres


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must accommodate the colder temperatures, higher pressures, lower light levels, and, sometimes, lower oxygen levels of the mid‐depths within their suite of adapted characters. A fascinating consequence of the changing environment with depth is the metabolic response of many deep‐living species to the change: metabolic rate declines precipitously with species’ depth of occurrence. It far exceeds that which would be predicted by the changes in the physical environment alone.

Schematic illustration of relationship between routine respiration (solid line) and maximum respiration (dashed line) for groups of fishes with different minimum depths of occurrence.

      Source: Adapted from Torres et al. (1979), figure 1 (p. 190). Reproduced with the permission of Elsevier.

      Nearly forty years later, with investigations spanning the Atlantic, Pacific, Gulf of California, Gulf of Mexico, and Southern Ocean, and using a wide variety of different taxa, the trend has been found to be universal among many taxa. We now know a lot more about the decline in metabolism with depth, and a well‐accepted theory of why it occurs has been established.

      The fact that both pelagic crustaceans and fishes exhibited profound depth‐related declines in metabolism confirmed that the trend was real and not confined to one taxonomic group. The results in turn opened up a Pandora’s Box of questions. Why the decline occurs and how it is biologically achieved spring to mind as appropriate queries. In addition, one might wonder how widespread among oceanic taxa the decline is and whether it only occurs among pelagic species or whether it is also observed in bottom‐dwelling (benthic) species and species that swim just above the bottom, the benthopelagic species. Enough work has been done to answer many of those questions. It is an instructive journey through the literature to see the questions posed and answered and the explanations for the phenomenon evolve.

Schematic illustration of the relationship between water content and minimum depth of occurrence in a group of midwater fishes.

      Source: Adapted from Childress and Nygaard (1973), figure 1 (p. 1098). Reproduced with the permission of Elsevier.

Schematic illustration of metabolic rates of diverse marine species as a function of minimum habitat depth.
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