It's all in the gills: evaluation of O2 uptake in Pacific hagfish refutes a major respiratory role for the skin.

Hagfish skin has been reported as an important site for ammonia excretion and as the major site of systemic oxygen acquisition. However, whether cutaneous O uptake is the dominant route of uptake remains under debate; all evidence supporting this hypothesis has been derived using indirect measurements. Here, we used partitioned chambers and direct measurements of oxygen consumption and ammonia excretion to quantify cutaneous and branchial exchanges in Pacific hagfish (Eptatretus stoutii) at rest and following exhaustive exercise. Hagfish primarily relied on the gills for both O uptake (81.0%) and ammonia excretion (70.7%). Following exercise, both O uptake and ammonia excretion increased, but only across the gill; cutaneous exchange was not increased. When branchial O availability was reduced by exposure to anteriorly localized hypoxia (∼4.6 kPa O), cutaneous O consumption was only slightly elevated on an absolute basis. These results refute a major role for cutaneous O acquisition in the Pacific hagfish.