Ecological and evolutionary aspects of integumentary respiration: Body size, diffusion, and the invertebrata

Abstract

SYNOPSIS. Most animal phyla lack specialized respiratory surfaces and all phyla contain groups that, for some part of their life history, depend entirely upon integumental diffusion of respiratory gases. Animals that are diffusion-limited, yet function aerobically are generally small with large surface areas and there has been convergence for this among all phyla including the coelomate invertebrates. Acoelomates lack specialized respiratory structures but have highly modified integuments, functional specializations, and features ranging from symbioses to air gulping that compensate for diffusion limitation. The diversity of structures functioning for integumentary respiration is much greater among invertebrates than vertebrates. Among the higher invertebrates with respiratory surfaces, accessory integumentary O2 uptake is usually 20 to 50% of total respiration. The high diffusion constant of O2 in air minimizes boundary effects on gas transfer and permits larger body size, although this is limited by dry conditions. Terrestrial annelids and flatworms, both confined to moist habitats, are larger than aquatic forms which often have accessory gills. Size differences between terrestrial forms in these two phyla reflect the presence of a circulation in the annelids. Ontogenetic transitions from skin breathing to other respiratory structures occur among marine invertebrates and vertebrates. Vertebrates apparently exercise greater integratory control over integumental respiration through adjustment of ventilation and perfusion; however, it is not known if these processes occur in some invertebrates. © 1988 by the American Society of Zoologists.