Oxygen Uptake in Sea Anemones: Effects of Expansion, Contraction, and Exposure to Air and the Limitations of Diffusion

Weight-specific rates of oxygen consumption ({aastex} {amsbsy} {amsfonts} {amssymb} {bm} {mathrsfs} {pifont} {stmaryrd} {textcomp} {portland,xspace} {amsmath,amsxtra} {wasysym} {empty} {10}{9}{7}{6} {document}V̇_O₂{document}) in Metridium senile and Anthopleura elegantissima are lower in air than in water; the effect is pronounced in larger specimens, suggesting that retraction of the tentacles and cessation of direct irrigation of the coelenteron during emersion limit oxygen delivery to internal tissues in these individuals. Because of an inverse relationship between body weight and percent of total body surface provided by tentacles in small to intermediate-sized M. senile, the smallest specimens show a disproportionate reduction of {aastex} {amsbsy} {amsfonts} {amssymb} {bm} {mathrsfs} {pifont} {stmaryrd} {textcomp} {portland,xspace} {amsmath,amsxtra} {wasysym} {empty} {10}{9}{7}{6} {document}V̇_O₂{document} upon contraction. In A. elegantissima, the weight-related reduction of {aastex} {amsbsy} {amsfonts} {amssymb} {bm} {mathrsfs} {pifont} {stmaryrd} {textcomp} {portland,xspace} {amsmath,amsxtra} {wasysym} {empty} {10}{9}{7}{6} {document}V̇_O₂{document} in water upon contraction is less than the reduction of {aastex} {amsbsy} {amsfonts} {amssymb} {bm} {mathrsfs} {pifont} {stmaryrd} {textcomp} {portland,xspace} {amsmath,amsxtra} {wasysym} {empty} {10}{9}{7}{6} {document}V̇_O₂{document} in air; this result implies that tissue collapse and the resultant increases in diffusion distances are involved in the greater reduction of {aastex} {amsbsy} {amsfonts} {amssymb} {bm} {mathrsfs} {pifont} {stmaryrd} {textcomp} {portland,xspace} {amsmath,amsxtra} {wasysym} {empty} {10}{9}{7}{6} {document}V̇_O₂{document} in air. The body wall of M. senile is less permeable to O2 than that of A. elegantissima, possibly because a greater proportion of the latter is due to mesoglea; this difference, together with the greater absolute thickness of the body wall in large specimens of M. senile, accounts in part for the greater weight-related depression of {aastex} {amsbsy} {amsfonts} {amssymb} {bm} {mathrsfs} {pifont} {stmaryrd} {textcomp} {portland,xspace} {amsmath,amsxtra} {wasysym} {empty} {10}{9}{7}{6} {document}V̇_O₂{document} in air in M. senile than in A. elegantissima. After 4 h of exposure to air, the subtidal species M. senile shows an increase in {aastex} {amsbsy} {amsfonts} {amssymb} {bm} {mathrsfs} {pifont} {stmaryrd} {textcomp} {portland,xspace} {amsmath,amsxtra} {wasysym} {empty} {10}{9}{7}{6} {document}V̇_O₂{document} in water relative to {aastex} {amsbsy} {amsfonts} {amssymb} {bm} {mathrsfs} {pifont} {stmaryrd} {textcomp} {portland,xspace} {amsmath,amsxtra} {wasysym} {empty} {10}{9}{7}{6} {document}V̇_O₂{document} in water prior to aerial exposure; the absence of this phenomenon in the intertidal species A. elegantissima may be an adaptation for energy conservation in an intertidal habitat.