Gas exchange in a cerianthid

Analysis of the determinants of oxygen exchange in Ceriantheopsis americanus, a tubicolous anthozoan, indicates that in the natural habitat the site of oxygen entry is restricted to the tentacular crown. Predicted rates of oxygen diffusion solely across the marginal tentacles are sufficient to account for observed uptake rates in intact animals. The anatomical restriction of the site of exchange is, in part, a result of the absence of active irrigation of the tube. This pattern of gas exchange is more like that in epifaunal anemones than in a previously studied burrowing anemone. Gradual decline in ambient pO2 leads to a gradual decrease in oxygen consumption rate. Although the partial pressure differential across the exchange surface drops with declining ambient pO2, this decrease is not sufficient in itself to explain the relationship between oxygen consumption rate and ambient pO2. We suggest that differential oxygen sensitivities of the epidermis and gastrodermis and changes in surface area and diffusion distance during expsure to lowered pO2 may modify the oxygen consumption response.