Development and sex affect respiratory responses to temperature and dissolved oxygen in the air-breathing fishes Betta splendens and Trichopodus trichopterus.

Ventilation frequencies of the gills (f) and the air-breathing organ (f) were measured in juveniles and adults of the air-breathing betta (Betta splendens) and the blue gourami (Trichopodus trichopterus) in response to temperature and hypoxia. Ventilatory rates were evaluated after 1 h of exposure to 27 °C (control), 23 and 31 °C (PO = 21.0 kPa), after acute temperature changes (ATC) from 23 to 27, and 27 to 31 °C, and under progressive hypoxia (PH; PO =  ~ 21 to 2.5 kPa). Complex, multi-phased ventilatory alterations were evident across species and experimental groups revealing different stress responses and shock reactions (e.g., changes in temperature sensitivity (Q) of f between 1-h exposure and ACT in both species). Female and male gourami showed differences in Q over the temperature range 23-31 °C. No such Q differences occurred in betta. Juveniles of both species showed higher Q for f (~ 3.7) than f (~ 2.2). Adult fish exhibited variable Qs for f (~ 1.5 to ~ 4.3) and f (~ 0.8 to ~ 15.5) as a function of temperature, suggesting a switch from aquatic towards aerial ventilation in response to thermal stress. During PH, juveniles from both species showed higher f than adults at all oxygen levels. Females from both species showed higher f compared with males. Collectively, our results suggest that environmental cues modulate ventilatory responses in both species throughout ontogeny, but the actual responses reflect species-specific differences in natural habitat and ecology. Finally, we strongly suggest assessing physiological differences between male and female fish to avoid masking relevant findings and to facilitate results interpretation.