Does hydrostatic pressure influence lumpfish (Cyclopterus lumpus) heart rate and its response to environmental challenges?

Studies on the effects of environmental changes with increasing depth (e.g. temperature and oxygen level) on fish physiology rarely consider how hydrostatic pressure might influence the observed responses. In this study, lumpfish (Cyclopterus lumpus, 200–400 g), which can exhibit vertical migrations of over 100 m daily and can be found at depths of 500 m or more, were implanted with Star-Oddi micro-HRT loggers. Then, their heart rate (f(H)) was measured in a pressure chamber when exposed to the following: (i) increasing pressure (up to 80 bar; 800 m in depth) at 10°C or (ii) increasing temperature (12–20°C), decreasing temperature (12 to 4°C) or decreasing oxygen levels (101–55% air saturation at 12°C) in the absence or presence of 80 bar of pressure. Additionally, we determined their f(H) response to chasing and to increasing temperature (to 22°C) at atmospheric pressure. Pressure-induced increases in f(H) (e.g. from 48 to 61 bpm at 12°C) were associated with hyperactivity. The magnitude of the rise in f(H) with temperature was greater in pressure-exposed vs. control fish (i.e. by ~30 bpm vs. 45 bpm between 5°C and 20°C). However, the relative increase (i.e. slope of the relationship) was not different between groups. In contrast, 80 bar of pressure eliminated the small (5 bpm) increase in f(H) when control fish were exposed to hypoxia. Exhaustive exercise and increasing temperature to 22°C resulted in a maximum f(H) of 77 and 81 bpm, respectively. Our research shows that pressure influences the f(H) response to environmental challenges and provides the first evidence that lumpfish have a limited capacity to increase f(H).