A comparison of startle response in two morphs of the brook stickleback (Culaea inconstans): Further evidence for a trade-off between defensive morphology and swimming ability

It is generally thought that predation pressure drives the evolution of long spines and robust pelvic girdles in sticklebacks (Pisces: Gasterosteidae). However, the lack of these traits in some environments with intense predation pressure suggests that there may be a limit to which apparently defensive structures benefit sticklebacks. This paper focuses on a trade-off between defensive morphology and escape response performance in a genetically based polymorphic population of brook stickleback (Culaea inconstans), in which fish have fully developed pelvic girdles and pelvic, spines ('complete' morph) or completely lack these defensive structures ('absent' morph). Startle response experiments revealed that the absent morph did better in all aspects of startle performance (displacement, maximum velocity, acceleration) than the complete morph. The absent morph also bent deeper than the complete morph during the initial phase of the startle response. The pelvic girdle polymorphism in this population may be maintained because individuals with a complete pelvic girdle and spines may be able to escape more easily from predators once they are captured, whereas individuals lacking these structures may better avoid capture when they are attacked.