Parental males of the plainfin midshipman are physiologically resilient to the challenges of the intertidal zone

The decision of where to rear young is influenced by both the needs of offspring and the costs parents incur in certain rearing environments. Plainfin midshipman fish ( Porichthys notatus ) provide extended paternal care in rocky intertidal zones, where they experience regular bouts of aquatic hypoxia and air exposure during low tide events. We investigated the physiological responses of plainfin midshipman males to three conditions for 6 h that simulate what these fish naturally experience during tidal cycles while nesting: normoxia, progressive hypoxia, or air exposure. Hypoxia and air- exposed fish exhibited shifts in energy metabolites, driven largely by elevated lactate and glucose content and reduced glycogen content in several tissues (muscle, heart, liver, and brain), but the magnitude of these changes was relatively modest. Hematocrit increased most in air-exposed fish relative to normoxia-exposed fish, contributing to an increase in whole blood hemoglobin concentration. Air exposure reduced swim bladder oxygen content, suggesting that internal O2 stores are drawn upon during air exposure. In a second experiment, we found that aquatic surface respiration and gill ventilation frequency increased in hypoxia-exposed fish relative to normoxia-exposed fish. Overall, our results suggest that plainfin midshipman overcome the challenges of the intertidal environment through a variety of physiological strategies and exhibit little physiological disturbance in response to the fluctuating and extreme conditions created by regular low tides.