Form and function of barbels in Osteoglossum bicirrhosum (Pisces Osteoglossidae) during aquatic surface respiration

Abstract

The barbels on the lower jaw of Osteoglossum bicirrhosum have a different shape from those of other teleosts. A thin longitudinal fold along the shaft is characteristic. The barbel consists of dermal tissue surrounded by a thin epidermal layer. O. bicirrhosum lacks accessory respiratory organs for breathing air. At oxygen concentrations at or below 2 ppm, young individuals begin "aquatic surface respiration" (ASR). While engaged in this behaviour, the fishes swim constantly at a regular speed with the barbels in a fixed position, the lower jaws motionless, and the opercula in continuous motion. This enables the fishes to survive for hours at oxygen concentrations of less than 0.5 ppm. Observations of the currents around the barbels reveal their hydrodynamic function in light of the current theories. The barbels improve the flow of surface water into the mouth