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 (O-2) partial pressure (Po-2) gradients that drive O-2 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 O-2 uptake in larvae. We hypothesized that the pectoral fins, which can generate water flow across the skin in larvae. promote transcutaneous O-2 transfer and thus aid in O-2 uptake. In zebrafish (Danio rerio), the frequency of pectoral fin movements increased in response to hypoxia at 4 days postfertilization (dpf), but the response was blunted by 15 dpf, when the gills become the dominant site of v uptake, and was absent by 21 dpf. In rainbow trout (Oncorhynchus mykiss). Po-2 measured at the skin surface of ventilating larvae was lower when the pectoral tins had been surgically removed, directly demonstrating that fins contribute to convective flow that dissipates cutaneous Po-2 boundary layers. Lack of pectoral fins compromised whole animal O-2 consumption in trout during hypoxia, but this effect was absent in zebrafish. Overall, our findings support a respiratory role of the pectoral tins in rainbow trout, but their involvement in zebrafish remains equivocal.