The respiratory circulation was investigated in air-breathing and waterbreathing Holthuisana transversa von Martens by analysis of the distribution of radioactive microspheres injected into the haemocoel at seven locations. The gills and putative lungs (branchiostegites and membraneous thoracic walls) both trap approximately 90% of the microspheres entrained in their afferent circulations. The main blood supply to the branchiostegites is from the venous sinuses and constitutes a substantial fraction of the total venous return, which is consistent with earlier inferences, based on morphological information, of their possible involvement in gas exchange. In airbreathing crabs, a mechanism exists which directs a greater proportion of the total venous return via the lungs. From the sinus at the base of walking leg 2, the ratio lung: gill flow was estimated as 86.9: 13.1 ± 5.7% in hydrated crabs that had been air-breathing for more than 1 day, and 19.5:80.5 ± 7.12% in water-breathers. A factor in this circulatory switch may be an increase in branchial resistance in air caused by surface tension of water adherent to the gill lamellae. The direct arterial circulation to the gills represents about 3% of cardiac output and is therefore an insignificant component of the total respiratory circulation. Patterns of microsphere distribution among different gills and different regions of the lung provide information on flow patterns within the thoracic sinus. Neither the thoracic sinus as a whole nor the inf rabranchial sinuses can be considered as reservoirs of truly mixed venous blood in H. transversa.

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