Nesting on high: reproductive and physiological consequences of breeding across an intertidal gradient

Nest site selection is a critical parental decision with profound fitness consequences, yet the physiological consequences of these decisions are rarely examined. Certain fishes and other aquatic organisms construct nests and provide parental care in the intertidal zone—an environment characterized by fluctuating water levels, which can exert intermittent and sometimes extreme abiotic stress on the animals that live there including dramatic changes in temperature and dissolved oxygen level. In this study, we used the plainfin midshipman fish, Porichthys notatus, to test whether (1) nest site preferences and reproductive success vary across an intertidal elevation gradient, and (2) fish that nest at higher elevations pay greater physiological costs due to prolonged exposure to more extreme abiotic conditions. We found that fish preferred nests lower in the intertidal zone, with larger males outcompeting smaller males for these sites. Broods at high elevations suffered greater offspring mortality than broods at lower elevations. The average microhabitat temperature of nests was also warmer and more variable at higher elevations compared to lower elevations. While isolated from the ocean during low tides, care-giving parents increased their use of anaerobic metabolism, and potentially draw upon oxygen reserves in the swim bladder. Our results suggest that the choice of nesting location can have profound effects on a parent’s physiology and may generate significant variation in reproductive success among individuals.