Respiratory characteristics of the blood pigments of three worms from an intertidal mudflat

Abstract

Three worms living in an intertidal mudflat near Auckland have respiratory pigments with different oxygen-binding properties. The burrowing sipunculid Xenosiphon mundanus has hemerythrin contained within coelomic cells. Oxygen is co-operatively bound to the hemerythrin (Hill's coefficient, n, = 1.8), resulting in a sigmoidal oxygen-binding curve of high oxygen affinity (halfsaturation tension, P₅₀, = 7.0 mm Hg at pH 7.5 and 20°C). Oxygen release is regulated by pH, and the Bohr effect is quantified by Ф = Δ log P₅₀ / ΔpH = - 0.30. The errant polychaete Glycera sp. has hemoglobin-charged coelomocytes with low oxygen affinity (P₅₀ = 13.5 mm Hg at pH 7.3 and 20°C)? and the oxygen-binding curve is essentially hyperbolic and insensitive to pH. The burrowing polychaete Abarenicola affinis has a vascular (circulating) erythrocruorin which binds and releases much oxygen for small changes in PO₂ (Hill's n = 3.8) and is insensitive to pH (Ф = -0.09). The physiological properties of these pigments cannot be related to the availability of oxygen in the near-environment, or to the habits of the animals, but appear to be dictated by the level of body organisation, particularly with regard to the gas exchange surfaces.