Lateralization of trunk muscle volume, and lateralization of swimming turns of fish responding to external stimuli.

A significant bias towards right-hand startle C-bends was found in vibration-stimulated zebra- and goldfish, but not in guppies and four Cichlid species. The goldfish right bias was significantly larger if they turned their head towards than away from the vibratory stimulus. In an undisturbed situation the fast swimming-turns of isolated goldfish and grouped zebrafish were significantly right-biased, especially so when attacked by group mates. In contrast, the slow turns were significantly left-biased, except for female zebrafish showing significantly right-biased slow turns during periods of non-attack by group mates. The contrast in left-right-bias between fast and slow turns may perhaps be explained by a white-muscle-mass bias to the right versus a red-muscle bias to the left, especially so in the anal region of the trunk of the zebrafish, because fast swimming is mainly powered by white muscle in contrast to red-muscle-powered slow swimming. Fish taxa that occur in open-water habitats and which are more often exposed to predatory fish might have evolved stronger muscular and behavioral lateralizations than more substrate-bound fish taxa.