Abstract
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 (Vliveand Vdead; water velocity at which live and dead fish first move backwards against the current, respectively) measured on a smooth Perspex®surface. Gyrinocheilus aymonieri has the greatest station-holding ability (Vlive= 59.1 cm·s–1, Vlive– Vdead= 45.7 cm·s–1). Fish with high Vdeadand low Vlive– Vdeadvalues 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 Lacépède, 1803), whereas those with low Vdeadand high Vlive– Vdeadvalues place a greater emphasis on suction (e.g., G. aymonieri and the genus Pterygoplichthys Gill, 1858). Stream fishes maximize slipping speed through high densities (1.06–1.15 g·cm–3), high frictional coefficients (0.12–1.2 on a Perspex®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.
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