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.