Respiration and the function of brachiopod punctae

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Abstract
When tied shut, three genera of punctae 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 Spirifer-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. O2-deprived articulates gape widely and close slowly when disturbed, but there is no evidence of O2-debt. They regulate O2-consumption at the same rate from saturation to 0.5 ml .times. l-1, and thus appear adapted to low O2-levels. These results have implications for the energetic efficience of articulates and the evolutionary patterns of bivalve molluscs v. articulates.