ADAPTATIONS TO ENVIRONMENTAL OXYGEN LEVELS IN INFAUNAL AND EPIFAUNAL SEA ANEMONES

1. Aerobic metabolism in Haloclava producta, a burrowing sea anemone, is largely or wholly dependent upon the oxygen supply in the water overlying its burrow. 2. This superficial water is brought through the burrow by an irrigation cycle of peristaltic waves. About 33% of the oxygen in the irrigation current is withdrawn by the animal. 3. Externally H. producta is exceedingly vermiform, studded with numerous thin walled hollow blisters and body surface area largely consists of the columnar body wall. M. senile, in comparison, is more robust, relatively smooth walled, and most of its total surface area is tentacular. Internally, the body wall of H. producta is an order of magnitude thinner than that of M. senile, and H. producta has fewer and smaller mesenterial septa and longitudinal retractors. 4. These morphological considerations suggest that vermiformicity and simplicity of internal organization are adaptive to low or unstable oxygen environments. The modifications associated with H. producta are not uniquely actinian, and are paralleled in completely unrelated phyla; thus they may represent a general mode or pattern of adaptation to burrow existence. 5. The morphological adaptations seem to be correlated with efficient use of the oxygen in the irrigation current, and with rapid restoration of internal pO₂'s following transient periods of anoxia. 6. H. producta is much more resistant to prolonged anoxia than M. senile, and it has a lower oxygen consumption rate at air saturation. 7. After exposure to anoxia, both species show a compensatory increase in the fraction of dissolved oxygen which they can remove from a closed system. 8. After exposure to anoxia, both species show an increased rate of oxygen consumption (both in relative and absolute terms) over wide ranges of oxygen concentration. 9. Different individuals of M. senile have very different absolute metabolic response curves to diminishing oxygen concentration, but all individuals examined showed the same relative decrease in oxygen consumption rate over a given change in oxygen concentration. This relative change is independent of acclimation temperature, experimental temperature, weight and previous exposure to anoxia. Different individuals of H. producta also have different absolute metabolic response curves to diminishing oxygen concentration, but a single relative response comparable to that in M. senile does not exist.