COMPARISON OF THE FAST-START PERFORMANCES OF CLOSELY-RELATED, MORPHOLOGICALLY DISTINCT THREESPINE STICKLEBACKS (GASTEROSTEUS SPP)

Fast-start escape performances for two species of threespine stickleba ck, Gasterosteus spp., were investigated using high-speed cinematograp hy (400 Hz). The two fishes (not yet formally described, referred to h ere as benthic and limnetic) inhabit different niches within Paxton La ke, British Columbia, Canada, and are recent, morphologically distinct species. All escape responses observed for both species were double-b end C-type fast-starts. There were no significant differences between the species for any linear or angular parameter (pooled averages, both species: duration 0.048s, distance 0.033m, maximum velocity 1.10 m s( -1), maximum acceleration 137 m s(-2), maximum horizontal angular velo city 473.6 rad s(-1) and maximum overall angular velocity 511.1 rad s( -1)). Benthics and limnetics have the greatest added mass (M(a)) at 0. 3 and 0.6 body lengths, respectively. The maximum M(a) does not includ e the fins for benthics. but for limnetics the dorsal and anal fins co ntribute greatly to the maximum M(a). The deep, posteriorly placed fin s of limnetics enable them to have a fast-start performance equivalent to that of the deeper-bodied benthics. Both the limnetic and benthic fishes have significantly higher escape fast-start velocities than the ir ancestral form, the anadromous threespine stickleback Gasterosteus aculeatus, suggesting that the high performance of the Paxton Lake sti cklebacks is an evolutionarily derived trait. In this biomechanical st udy of functional morphology, we demonstrate that similar high fast-st art performance can be achieved by different suites of morphological c haracteristics and suggest that predation might be the selective force for the high escape performance in these two fishes.