Oxygen uptake, diffusion limitation, and diffusing capacity of the bipectinate gills of the abalone, Haliotis iris (Mollusca: Prosobranchia)

Extant abalone retain an ancestral system of gas exchange consisting of paired bipectinate gills. This paper examines the hypothesis that fundamental inefficiencies of this arrangement led to the extensive radiation observed in prosobranch gas exchange organs. Oxygen uptake at 15 °C was examined in the right gill of resting adult blackfoot abalone, Haliotis iris Martyn 1784. Pre- and post-branchial haemolymph and water were sampled and oxygen content, partial pressure (Po2), pH, and haemocyanin content measured; in vivo haemolymph flow rate was determined by an acoustic pulsed-Doppler flowmeter. During a single pass across the gills, mean seawater Po2 fell from 138.7 Torr to 83.4 Torr, while haemolymph Po2 rose from 37.2 Torr to 77.0 Torr raising total O2 content from 0.226 to 0.346 mmol L− 1. Haemolymph flowed through the right gill at a mean rate of 9.6 mL min− 1 and carried 0.151 to 0.355 mmol L− 1 of haemocyanin (mean body mass 421 g). Only 34.7% of the oxygen carried in the arterial haemolymph was taken up by the tissues and less than half of this was contributed by haemocyanin. A diffusion limitation index (Ldiff) of 0.47–0.52, a well-matched ventilation–perfusion ratio (1.2–1.4) and a diffusing capacity (D) of 0.174 μmol O2 kg− 1 Torr− 1 indicate that the gills operate efficiently and are able to meet the oxygen requirements of the resting abalone.