© 1994 by The Society for Integrative and Comparative Biology
Morphological, Behavioral, and Physiological Characterization of Bimodal Breathing Crustaceans1
Department of Zoology and Wildlife Science, Auburn University Auburn, Alabama 36849-5414
SYNOPSIS Bimodal breathing crustaceans, while representing a stage in the transition from the aquatic to terrestrial habitat, also constitute a distinct group that can be characterized by morphological, behavioral, and physiological traits. Morphologically, this group displays reduced gill surface area and enlarged branchial chambers. The lining of the branchial chambers, the branchiostegites, also has increased surface area and is highly vascularized. The branchiostegites can be smooth, cutaneous epithelia, or they can have more complex evaginations or invaginations to further increase surface area. In addition, the thickness of the branchiostegal epithelium is greatly reduced, compared to that in the gills, thus minimizing the diffusion distance between air and hemolymph. These animals maintain a store of water in the branchial chamber that covers the gills and allows for simultaneous gas exchange with two media (air and water). There is also a partitioning of gas exchange, with oxygen uptake occurring preferentially from air across the branchiostegites, and carbon dioxide excretion occurring across the gills into the branchial water. One critical factor that appears to separate bimodal breathing crustaceans from fully terrestrial, exclusively air-breathers is the ability of the latter group to excrete CO2directly into air across the gills and branchiostegites. It is suggested that the incorporation of the enzyme carbonic anhydrase into the membrane fraction of the branchiostegites may have been one of the key molecular events which allowed for pulmonary CO2 excretion into air.