Breathing with fins: Do the pectoral fins of larval fishes play a respiratory role?

Convective water flow across respiratory epithelia in water-breathing organisms maintains transcutaneous oxygen (O2) partial pressure (PO2) gradients that drive O2 uptake. Following hatch, larval fishes lack a developed gill and the skin is the dominant site of gas transfer, yet few studies have addressed the contribution of convective water flow to cutaneous O2 uptake in larvae. We hypothesized that the pectoral fins, which can generate water flow across the skin in larvae, promote transcutaneous O2 transfer and thus aid in O2 uptake. In zebrafish (Danio rerio), the frequency of pectoral fin movements increased in response to hypoxia at 4 days post-fertilization (dpf), but the response was blunted by 15 dpf, when the gills become the dominant site of O2 uptake and was absent by 21 dpf. In rainbow trout (Oncorhynchus mykiss), PO2 measured at the skin surface of ventilating larvae was lower when the pectoral fins had been surgically removed, directly demonstrating that fins contribute to convective flow that dissipates cutaneous PO2 boundary layers. Lack of pectoral fins compromised whole animal O2 consumption in trout during hypoxia, but this effect was absent in zebrafish. Overall, our findings support a respiratory role of the pectoral fins in rainbow trout, but their involvement in zebrafish remains equivocal.