Upside-down catfish

Abstract: Synodontis nigriventris is a surface-feeding facultative air-breather that swims inverted with its zoological ventral side towards the water surface. Their near-surface drag is about double the deeply submerged drag (due to wave drag) and roughly twice the sum of frictional and pressure drags. For streamlined technical bodies, values of wave drag augmentation near the surface may be five times the deeply submerged values. However, the depth dependence of drag is similar for fish and streamlined technical bodies, with augmentation vanishing at about 3 body diameters below the surface. Drag ;inverted' is approximately 15% less than that ;dorsal side up' near the surface. Consistent with this, at any given velocity, tailbeat frequency is lower and stride length higher for inverted swimming in surface proximity (P 0.05). At the critical Froude number of 0.45, speeds in surface proximity correspond to prolonged swimming that ends in fatigue. To exceed these speeds, the fish must swim deeply submerged and this behaviour is observed. Inverted swimming facilitates efficient air breathing. Drag dorsal side up during aquatic surface respiration is 1.5 times the value for the inverted posture. Fast-starts are rectilinear, directly away from the stimulus. Average and maximum velocity and acceleration decrease in surface proximity (P<0.05) and are higher inverted (maximum acceleration: 20-30 m s(-2); P<0.05) and comparable to locomotor generalists (e.g. trout). Mechanical energy losses due to wave generation are about 20% for inverted and 40% for dorsal side up, and lower than for trout fast-starting in shallow water (70% losses); bottom effects and large amplitude C-starts (c.f. relatively low amplitude rectilinear motions in S. nigriventris) enhance resistance in trout. S. nigriventris probably evolved from a diurnal or crepuscular 'Chiloglanis-like' benthic ancestor. Nocturnality and reverse countershading likely co-evolved with the inverted habit. Presumably, the increased energy cost of surface swimming is offset by exploiting the air-water interface for food and/or air breathing.

Abstract: SUMMARY Synodontis nigriventris is a surface-feeding facultative air-breather that swims inverted with its zoological ventral side towards the water surface. Their near-surface drag is about double the deeply submerged drag (due to wave drag) and roughly twice the sum of frictional and pressure drags. For streamlined technical bodies, values of wave drag augmentation near the surface may be five times the deeply submerged values. However, the depth dependence of drag is similar for fish and streamlined technical bodies, with augmentation vanishing at about 3 body diameters below the surface. Drag `inverted' is approximately 15% less than that `dorsal side up' near the surface. Consistent with this, at any given velocity, tailbeat frequency is lower and stride length higher for inverted swimming in surface proximity (P<0.05). Deeply submerged, there are no significant differences in drag and kinematics between postures (P>0.05). At the critical Froude number of 0.45, speeds in surface proximity correspond to prolonged swimming that ends in fatigue. To exceed these speeds, the fish must swim deeply submerged and this behaviour is observed. Inverted swimming facilitates efficient air breathing. Drag dorsal side up during aquatic surface respiration is 1.5 times the value for the inverted posture. Fast-starts are rectilinear, directly away from the stimulus. Average and maximum velocity and acceleration decrease in surface proximity (P<0.05) and are higher inverted (maximum acceleration: 20–30 m s–2; P<0.05) and comparable to locomotor generalists (e.g. trout). Mechanical energy losses due to wave generation are about 20% for inverted and 40% for dorsal side up, and lower than for trout fast-starting in shallow water (70% losses); bottom effects and large amplitude C-starts (c.f. relatively low amplitude rectilinear motions in S. nigriventris) enhance resistance in trout. S. nigriventris probably evolved from a diurnal or crepuscular `Chiloglanis-like' benthic ancestor. Nocturnality and reverse countershading likely co-evolved with the inverted habit. Presumably, the increased energy cost of surface swimming is offset by exploiting the air–water interface for food and/or air breathing.

Abstract: The catfish Synodontis nigriventris shows a unique habit taking a stable upsidedown posture in free water regardless of an above, one-sided illumination. This upsidedown posture can be observed when the catfish is apart from objects because the catfish usually orients its ventral side towards the water bottom or objects due to a so-called ventral substrate response. Thus, it is not easy to study the mechanism of the upside-down posture. To resolve this problem, the frequency of the upside-down posture was measured by using various sizes of vessel in which the catfish was kept. Video analysis showed that the frequency of the upside-down posture depended on the space size around the catfish. The smaller the size became, the higher the frequency of the upside-down posture became. Furthermore, the frequency of the upside-down posture depended on the shape of the vessel bottom. Curved-bottom vessels induced the upside-down posture more frequently than flat bottom. These findings suggest that a small, curved-bottom vessel is ideal for researching the upside-down postural control mechanism.