RESPIRATORY ADAPTATIONS TO THE OXYGEN MINIMUM LAYER IN THE BATHYPELAGIC MYSIDGNATHOPHAUSIA INGENS

Abstract

1. The oxygen consumption rate in G. ingens is not surface proportional, but is not significantly different from weight proportionality or intermediate proportionality. 2. The critical partial pressure of oxygen in G. ingens is directly proportional to the regulated oxygen consumption rate. This relationship suggests that a respiratory rate of about 0.8 ml O2/kg wet wt/min (corresponding to a slow rate of swimming probably just sufficient to allow an individual to maintain its depth in the water column) is the maximum which G. ingens can sustain over long periods in situ in the minimum layer. 3. Respiratory water enters the gills along the ventral midline, travels through them first laterally and then dorsally, finally exiting past the two scaphognathites located anterior to the gills. No respiratory water enters at the posterior dorsal margin of the carapace. The large gill surface of G. ingens is almost certainly an important factor in its unusual aerobic abilities. 4. The ability of G. ingens to maintain a high ventilation volume and a high per cent utilization independent of each other is certainly important in explaining its aerobic abilities. The regulatory abilities of G. ingens are probably limited by its maximum ventilatory abilities. 5. It is suggested that the extreme stability of the deep sea made possible the evolution of the specialized aerobic adaptations found in G. ingens. This situation contrasts sharply with that of inhabitants of less stable low oxygen environments as well as "commuters" in stable low oxygen environments which can temporarily use anaerobic metabolism without an energy penalty and which would therefore gain negligible selective advantage from specialized aerobic adaptations.