Energy metabolism and respiratory gas exchange

Abstract

‘Respiration’ in the energy budget classically includes both aerobic and anaerobic processes, but in the strict sense only the former is respiration, whereas the latter is fermentation. During environmental hypoxia and activity, anemones employ fermentative pathways of energy metabolism similar to those found in other euryoxic invertebrates. Both aerobic and anaerobic energy metabolism draw heavily on amino acid catabolism, probably owing to the protein-rich diet of sea anemones. Lacking O₂-carrying respiratory pigments and capable of only limited convection of the ambient seawater, sea anemones seem particularly at the mercy of the Fick equations for diffusive O₂ exchange. Their compensation for decreased O₂ availability involves morphological, cytological, and compositional features that change parameters in Fick’s first equation to enhance delivery of O₂ to respiring tissues. Morphological adaptations include the predominantly two-dimensional, laminar body plan, which minimizes diffusion distances and results in O₂ consumption being more nearly directly proportional to body mass than in most Metazoa. In consequence of their variable-volume hydrostatic skeleton and distensible body wall, sea anemones also have a system of gas exchange utilizing variable surface-area and diffusion-distance. These parameters are altered behaviourally in response to changing O₂ availability and demand. The slow nature of their muscular contractions is reflected in a more modest scope for activity in sea anemones than in other metazoans.