Chronic hypoxia exposure

A subset of species (F. rathbuni, L. parva, L. goodei) were also exposed for 28 days to constant hypoxia or to nocturnal intermittent hypoxia (12 h hypoxia:12 h normoxia, matched to the photoperiod), as previously described (Borowiec et al., 2015, 2018). Constant hypoxia (P_O2 set point of 2 kPa O₂ with a 0.1 kPa hysteresis) was maintained by bubbling the water with nitrogen gas, mediated by a feedback loop using a fibre-optic oxygen sensor (Loligo Systems, Tjele, Denmark) to regulate the action of a solenoid valve controlling the flow of nitrogen. Intermittent hypoxia was maintained using the same feedback loop as for constant hypoxia, except that hypoxia (2 kPa) was only maintained at night (19:00 h to 07:00 h local time), and the water was bubbled with air to reoxygenate and maintain normoxia during the day. Respirometry and hypoxia tolerance measurements (see below) were conducted after the completion of chronic exposure. Average body mass for fish acclimated to intermittent hypoxia was as follows: F. rathbuni, 1.44±0.22 g and 1.08±0.24 g (t_LOE only); L. parva, 0.41±0.04 g and 0.55±0.08 g (t_LOE only); L. goodei, 0.32±0.02 g and 0.41±0.04 g (t_LOE only). Average body mass for fish acclimated to constant hypoxia was as follows: F. rathbuni, 1.00±0.20 g and 1.26±0.24 g (t_LOE only); L. parva, 0.33±0.05 g and 0.45±0.08 g (t_LOE only); L. goodei, 0.31±0.03 g and 0.34±0.04 g (t_LOE only). See Fig. 4 for the body mass of normoxia-acclimated fish.

Relationship between Ṁ_O2 and body mass in normoxia. (A) Absolute values (means±s.e.m.). (B) Phylogenetically independent contrasts. Least squares linear regressions (solid line) and 95% confidence bands (indicated by dotted lines) are shown, with equations and P-values reported in each panel. Species abbreviations and sample sizes for absolute values were as follows: Fc, F. confluentus (N=4); Fd, F. diaphanus (N=12); Fh, F. heteroclitus (N=7); Fr, F. rathbuni (N=15); Lg, L. goodei (N=15); Lp, L. parva (N=15). Other statistical information is reported in Table 1.