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

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