Respiration and the function of brachiopod punctae

When tied shut, three genera of punctate terebratulids from the Northeast Pacific Terebratulina unguicula (Terebratulidina), Terebratalia transversa and Laqueus californianus (Terebratellidina)] take up O₂ from the external water (at one-third the rate when gaping). The impunctate rhynchonellid Hemithiris psittacea does not, indicating that punctae admit O₂ when the valves are closed. The shells of T. transversa vary from smooth and globose to Spirife r-like alate and costate morphs. The latter have more punctae and greater uptake of O₂ through the shell. The alate/costate morphs are most abundant where turbidity is likely to prompt closure. Punctael respiration is faster through the thin shells of Laqueus than the thick ones of Terebratalia. 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. articulates.