Multiple Sites of Gas Exchange

Abstract

SYNOPSIS. For many years the general body surface has been recognized as the primitive site of respiratory gas exchange in the animal kingdom. Even in simple animals such as the sea anemones, however, some specialization has occurred. In epifaunal species the tentacular crown appears to be the major site of gas exchange while in one infaunal species dermal papillae are more important. In both, the columnar body wall remains an additional site of O2 uptake. In the polychaete annelids an evolutionary trend can be seen from the predominance of the general body surface over simple, metameric gills to the predominance of highly specialized and branched anterior gills over the general body surface. n the oligochaetes reversion to the general body surface was followed in at least one species to specialization of the tail region to form a “lung.” Multiple sites clearly exist in the molluscs, but their relative importance is poorly known. Several sites exist in the echinoderms, the best known being the approximately equal allocation of O2 uptake between modified tube feet and respiratory trees in the holothurians. Multiple sites of gas exchange persist even in the crustaceans, in temporal as well as spatial dimensions. Water breathers revert to cutaneous O2 uptake following a molt, and air breathers have evolved an array of gas exchangers adapted to their new medium. This array is so diverse that it does not provide a single selection pressure for hemocyanin O2 binding properties which, when compared broadly, remain indistinguishable from those of water breathers found in similar thermal regimes.