Biomechanics of rheotaxis in six teleost genera

Biomechanical, behavioural, and morphological adaptations affecting lift and drag on tropical stream fish (five loricariids and one gyrinocheilid) are analyzed. The drag on fish attached to a surface is directly measured for the first time, allowing the lift coefficient to be inferred. Mean drag coefficients (0.2-0.9, based on projected frontal area) are 2 to 30 times the equivalent frictional coefficients for laminar flow for smooth blisters attached to a surface. Fineness ratios (total length/maximal height, l/h = 6.7-9.0), flattening (maximal body width/maximal height, b/h = 0.9-2.0), and lengthening (distance from end of rostrum to maximal height / total length, x/l = 0.17-0.22) are similar to optimal values for technical bodies of low drag (smooth blisters attached to surfaces: 10, 2, and 0.3 for l/h, b/h, and x/l, respectively). The station-holding ability of Gyrinocheilus aymonieri (Tirant, 1883) (closed oral sucker) is compared with that of the loricariids (open oral suckers) using live and dead slipping velocities (V-live and V-dead; water velocity at which live and dead fish first move backwards against the current, respectively) measured on a smooth Perspex (R) surface. Gyrinocheilus aymonieri has the greatest station-holding ability (V-live = 59.1 cm center dot s(-1), V-live - V-dead = 45.7 cm center dot s(-1)). Fish with high V-dead and low V-live - V-dead values rely more on frictional devices (e.g., spines and odontodes) for station-holding than on oral suction (e.g., fish of the genera Otocinclus Cope, 1871 and Hypostomus Lacepede, 1803), whereas those with low V-dead and high V-live - V-dead values place a greater emphasis on suction (e.g., G. aymonieri and the genus Pterygoplichthys Gill, 1858). Stream fishes maximize slipping speed through high densities (1.06-1.15ag center dot cm(-3)), high frictional coefficients (0.12-1.2 on a Perspex (R) surface), and high rheotactic suction pressures (26-173 Pa). In addition, a negative lift coefficient of -0.5 is calculated for the genus Chaetostoma von Tschudi, 1846.