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During July of 1983, 1986, we measured rates of oxygen consumption of 234 individuals of 17 species of midwater crustaceans (orders Decapoda, Mysidacea, and Euphausiacea) off the Hawaiian islands at depths from the surface to greater than 1200 m. The routine metabolic rates declined with increasing depths of the species' occurrence to an extent greater than could be accounted for by depth-related changes in body size or water temperature. Most species appeared able to regulate their oxygen consumption down to the lowest oxygen partial pressures found in their depth range (20 mm Hg O2), but did not regulate to such low oxygen partial pressures as did similar midwater crustaceans off California, where oxygen levels reach as low as 6 mm Hg. Metabolic rates of the shallower-living, but not the deepest-living Hawaiian crustaceans were significantly higher than those of Californian crustaceans. This is interpreted as indicating that the metabolic rates of midwater crustaceans are not adapted specifically to differing levels of primary production and that the decline with depth of metabolic rates in these species is not the result of food limitation at depth. The data are, however, consistent with the hypothesis that lower metabolic rates at depth are due to the relaxation of selection pressures relating to visual predation near the surface.

Abstract. The variation with depth in water, lipid, protein, carbon and nitrogen contents (% wet weight) of 42 species of midwater fishes, collected in November 1976 off the west coast of Oahu in the Hawaiian Archipelago, was measured. The Hawaiian fishes show significant relationships between these components and depth of occurrence. The slopes of these relationships are not significantly different from those reported for midwater fishes from off California, USA. However, the fishes from Hawaii have significantly lower lipid levels and higher protein levels than do the species from off California. The deep-living Hawaiian species (500 m and deeper) have significantly lower lipid (% wet weight), but there is no significant difference in protein (% wet weight). The difference in lipid contents at all depths appears to be an evolved characteristic, with the greater lipid levels off California being selected for by greater spatial and temporal variation in the food supply for these fishes off the California coast than off Hawaii. The higher protein contents in the shallow-living Hawaiian fishes appear to reflect greater muscle power selected for in these fishes by the greater water clarity, and therefore greater "reactive distances", in the surface layers off Hawaii. These conclusions support the general hypothesis that the lower protein contents of bathypelagic fishes are not directly selected by food limitation at depth, but rather result from the relaxation of selection for rapid-swimming abilities at greater depths due to the great reduction at greater depths in the distance over which visual predator-prey interactions can take place. The lower lipid levels in the deeper-living species are apparently made possible by the reduced metabolic rates of these species which reduces their need for energy stores.

The energetic costs of swimming were deter mined for the bathypelagic mysid Gnathophausia ingens. Individuals over a large size range spontaneously swam at speeds from 5 to 6.5 cm/s. To maintain this speed, smaller animals swam at much higher relative swimming speeds than did larger animals. Routine rates of oxygen consumption were thus considerably higher in the smaller instars. The relationship between standard rates of oxygen consumption and animal size was slightly less than the standard log-log allometric slope of 0.75. Within the speed range of 0-8 cm/s, oxygen consump tion appeared to increase as a linear function of speed. Cost of transport was very high at low speeds. At 5.5 cm/ s, cost of transport was lower than that measured for other crustaceans, but higher than that of fish. Swim ming efficiency increased with speed. While the lower cost of transport and higher swimming efficiency may contribute to G. ingens ' reduced rates of oxygen con sumption as compared to those of shallower-living crus taceans, the major factor appears to be G. ingens' lower level of swimming activity.