WHY SWIM UPSIDE-DOWN - A COMPARATIVE-STUDY OF 2 MOCHOKID CATFISHES

Some catfishes in the genus Synodontis and its allied genera (family Mochokidae) swim upside down and exhibit reverse countershading. We demonstrate a potential respiratory function for this behavior through laboratory observations of upside down (Synodontis nigriventris) and right side up (Synodontis afrofischeri) species exposed to low PO2. Both species used aquatic surface respiration (ASR) at the air-water interface when PO2 was <15 mm Hg. With decreasing PO2, S. nigriventris increased the percentage of ASR time spent upside down, did not emerse its body during ASR, and had very modest surface activity levels. Synodontis afrofischeri used a vertical posture for ASR that caused emersion of its snout and required constant swimming to maintain position; it used active forward motion during ASR at very low PO2 and made repeated trips to the bottom. The vertical posture and increased swimming activity associated with ASR by S. afrofischeri is probably less efficient than the inverted ASR of S. nigriventris.