Gas exchange and active ventilation in a starfish, Pteraster tesselatus

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1. The oxygen uptake of the cushion star Pteraster tesselatus was independent of ambient tension down to about 70 mm Hg and showed an average value of 10.5 ml/kg/hr in normoxic water in the range 10–12° C. The average uptake decreased approximately 2.65 times when the temperature was lowered from 10–12 to 5° C. 2. Oxygen extraction from the respiratory water current averaged 18 % at normoxic conditions. Effectiveness of oxygen removal was slightly higher than 50%; this is a very high value when compared with those of the crab Cancer magister and the cephalopod Octopus dofleini. 3. The exceptional ability of Pteraster to regulate its O2 uptake is based on the active irrigation of the dermal branchiae in the nidamental cavity. About 90 % of the total O2 uptake is derived from the active ventilation, the remaining 10 % being ascribed to passive diffusion through the body surface and tube feet. Ventilation values, measured with a termal velocity probe directly over the osculum, averaged 70 ml/min. 4. The transfer factor for Pteraster at 10–12° C was 0.0048 ml O2/kg/min/mm Hg, as compared with 0.0062 for Octopus dofleini and 0.0043 for Cancer magister. 5. Normal CO2 tensions in the coelomic fluid ranged between 1.00–1.40 mm Hg; coelomic fluid pH values ranged between 7.33–7.46 when the animals were in well aerated water at 11–12° C. 6. An effective barrier exists between the large fluid compartment of the stomach and the coelomic fluid, as indicated by measurements in both compartments. 7. The ability of Pteraster to regulate O2 uptake and the values for coelomic fluid tensions under different conditions, indicate that the principal determinant of the O2 uptake rate is the O2 tension prevailing at the tissues level.