Effects of temperature on acid-base regulation, gill ventilation and air-breathing in the clown knifefish, Chitala ornata

Chitala ornata is a facultative air-breathing fish, which at low temperatures shows an arterial PCO2 (PaCO2) level only slightly elevated above that of water-breathers. By holding fish with in-dwelling catheters in temperatures from 25-36°C and measuring blood gasses, we show that this animal follows the ubiquitous poikilotherm pattern of reducing pHa with increasing temperature. Surprisingly, the temperature increase caused an elevation of PaCO2 from 5 to 12 mmHg while the plasma bicarbonate concentration remained constant at around 8 mmol−1. Temperature increase also gave rise to a larger fractional increase in air-breathing than gill ventilation frequency. These findings suggest that air-breathing, and hence the partitioning of gas exchange, is to some extent regulated by acid-base status in air-breathing fish and that these bimodal breathers will be increasingly likely to adopt respiratory pH control as temperature rises, providing an interesting avenue for future research.