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

Abstract

Ventilation frequencies of the gills (fG) and the air-breathing organ (fABO) 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 1h of exposure to 27°C (control), 23 and 31°C (PO2 = 21.0kPa), after acute temperature changes (ATC) from 23 to 27, and 27 to 31°C, and under progressive hypoxia (PH; PO2 = ~ 21 to 2.5kPa). 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 (Q10) of fG between 1-h exposure and ACT in both species). Female and male gourami showed differences in Q10 over the temperature range 23-31°C. No such Q10 differences occurred in betta. Juveniles of both species showed higher Q10 for fABO (~ 3.7) than fG (~ 2.2). Adult fish exhibited variable Q10s for fG (~ 1.5 to ~ 4.3) and fABO (~ 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 fG than adults at all oxygen levels. Females from both species showed higher fG 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.