changes in the physical environment occur over very short distances in the ocean’s vertical plane. Temperature, pressure, light levels, and sometimes oxygen concentrations vary drastically within a kilometer’s journey of the surface. To flourish, open‐ocean fauna must accommodate the challenges posed by the environment within their biological characteristics.
Because small swimming species that are the focus of this book are ectotherms, temperature is the environmental variable with the most potential to influence survival, zoogeographic distributions, and biological rate processes such as metabolism and growth. Upper and lower lethal limits of open‐ocean species, like those of all ectotherms, are dictated by their physiological and biochemical characteristics. Limited live animal experimentation on species from a variety of open‐ocean systems as well as more extensive work with enzymes and membranes strongly suggest that open‐ocean pelagic species show the basic responses to temperature described here in detail. That is, we see no exotic or unusual adaptations to temperature. Rate measurements show Q10s in the range of 2–3 and temperature adaptation is observed when ecological analogues in polar and temperate systems are compared.
The increased pressure associated with mesopelagic depths has the potential to influence biochemical and physiological processes ranging from the ion transport necessary for nerve and muscle function to enzyme function in the anaerobic and aerobic pathways of intermediary metabolism. Animals that live at modest pressures (<100 atm) are either insensitive to it, as are the vertically migrating euphausiids, or show a slight acceleratory response as in the deeper‐living mesopelagic migrators. In contrast, surface‐dwelling species exposed to pressures outside those of their normal environment show excitement at low (50 atm) pressures, moribundity at higher pressures (150 atm), and death due to tetany at high pressures (200 atm). Adaptations to pressure include increases in the fluidity of biological membranes as well as slight changes in the structure of enzymes to confer pressure insensitivity.
Zones of minimum oxygen are present at intermediate depths throughout the world ocean, but in a few locations oxygen reaches levels low enough to limit animal life. Three such locations are coastal California, the eastern tropical Pacific, and the Arabian Sea. When there is oxygen present in sufficient quantities to enable extraction, such as off California, pelagic species have evolved mechanisms to live aerobically despite the vanishingly low oxygen. Such adaptations include a high gill surface area to allow for efficient extraction of oxygen, a well developed circulatory system, and an efficient means of ventilating the gills. Animals that migrate into regions of zero oxygen, such as in the Arabian Sea, use a strategy of minimizing accumulation of toxic end products by changing the end point of their anaerobic metabolism from lactate to ethanol.
Depth itself exerts a profound influence on the metabolic characteristics of pelagic species. In swimming species that are either visual predators or are preyed upon by visual predators, i.e. the crustaceans, squids, and fishes, metabolism declines profoundly with increasing depth of occurrence. A fish living at the surface has a metabolism about 50 times that of a species living at 1000 m. Weaker swimmers where vision plays less of a role, such as jellyfishes and chaetognaths, do not show an equivalent decline in metabolic rate with depth of occurrence. Benthic and benthopelagic fishes show a similar decline with depth of occurrence, but benthic crustaceans do not.
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