When tied shut, three genera of punctate terebratulids from the Northeast Pacific[Terebratulina unguicula(Terebratulidina),Terebratalia transversaandLaqueus californianus(Terebratellidina)] take up O2from the external water (at one‐third the rate when gaping). The impunctate rhynchonellidHemithiris psittaceadoes not, indicating that punctae admit O2when the valves are closed. The shells ofT. transversavary from smooth and globose toSpirifer‐like alate and costate morphs. The latter have more punctae and greater uptake of O2through the shell. The alate/costate morphs are most abundant where turbidity is likely to prompt closure. Punctael respiration is faster through the thin shells ofLaqueusthan the thick ones ofTerebratalia.O2deprived articulates gape widely and close slowly when disturbed, but there is no evidence of O2debt. They regulate O2consumption at the same rate from saturation to 0.5 ml·l1, and thus appear adapted to low O2levels. These results have implications for the energetic efficience of articulates and the evolutionary patterns of bivalve molluscs v. articul