Single-species subgroups form within mixed-species shoals of tropical and temperate fishes

Shoaling behaviour is commonly displayed by fishes and is thought to reduce predation and increase foraging efficiency. Shoaling relies on coordination between individuals, with higher cohesion and alignment among individuals within a shoal providing greater net benefits of this behaviour. Whilst single species often shoal together in conspecific groups, mixed-species shoaling is frequently observed and has been identified as an important determinant of individual fitness for the multiple species involved. Despite their prevalence, the structure of mixed-species shoals and the mechanisms by which individuals gain protection from predators and enhance their foraging efficiency are not as well understood as for single-species shoals. In fact, mixed-species shoals may be less coordinated than single-species shoals, raising the intriguing question of why fishes form mixed-species shoals when this behaviour could be less beneficial than single-species shoaling. Here we used in situ stereo-video techniques to compare within and between shoal differences in cohesion and alignment, for mixed- and single-species shoals containing the tropical vagrant Indo pacific sergeant major damselfish, Abudefduf vaigiensis, in temperate waters. As expected, mixed-species shoals were less aligned than single-species shoals. However, within mixed-species shoals conspecifics were more cohesive and aligned than were heterospecifics, suggesting coordinated single-species subgroups formed within larger mixed-species shoals. The formation of subgroups may mitigate costs associated with differences between species, therefore enhancing benefits of mixed-species shoaling. As such, multiple levels of social structure may exist within mixed-species shoals that could facilitate growth and survival for vagrant A. vaigiensis in temperate regions. More broadly, this research highlights the importance of considering detailed internal structures of mixed-species shoals when trying to understand cost–benefit trade-offs experienced by individuals