Heterogeneous perfusion of the paired gills of the abalone Haliotis iris Martyn 1784: an unusual mechanism for respiratory control.

The abalone Haliotis iris retains the ancestral gastropod arrangement of a pair of bipectinate gills (ctenidia). The gills share a single branchial chamber, are supplied from a common haemolymph sinus and effectively support the whole of oxygen uptake by the animal. Using chronic indwelling cannulae and pulsed Doppler probes, post-branchial haemolymph oxygen partial pressures (PaO2) and haemolymph flow rates were measured in the left and right efferent ctenidial veins. During periods of internal hypoxia following emersion and handling, total branchial haemolymph flow (24.4+/-3.6 ml kg-1 min-1) was partitioned nearly equally between the left and right gills (13.3+/-2.6 and 10.8+/-1.4 ml kg-1 min-1, respectively) and their PaO2 values were similar (81.9+/-6.1 and 87.3+/-4.7 mmHg, respectively). In animals settled for >24 h, branchial haemolymph flow decreased to 9.1+/-2.1 ml kg-1 min-1, primarily resulting from a virtual shutdown of the left gill flow to only 4.6% of total flow (left, 0.41+/-0.34 ml kg-1 min-1; right, 8.6+/-2.0 ml kg-1 min-1). At rest, right gill PaO2 (85.5+/-6.8 mmHg) was essentially unchanged while PaO2 of the slowly perfused left gill rose to 105.3+/-10.2 mmHg, close to the PO2 of the exhalant seawater (104.5+/-3.1 mmHg). The aerobic metabolic scope of H. iris therefore appears to be met primarily by circulatory adjustments at the left gill, which at rest is highly perfusion limited (left Ldiff, 0.14+/-0.07; right Ldiff, 0.44+/-0.08).