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.

浅滩行为通常由鱼类表现出来，被认为可以减少捕食并提高觅食效率。浅滩依赖于个体之间的协调，浅滩内个体之间更高的凝聚力和一致性提供了这种行为的更大净收益。虽然单一物种经常以同种群体的形式聚集在一起，但经常观察到混合物种的浅滩，并已被确定为所涉及的多个物种个体适应性的重要决定因素。尽管它们很普遍，但混合物种浅滩的结构以及个体获得保护免受捕食者和提高觅食效率的机制不如单物种浅滩那么容易理解。事实上，混种浅滩可能不如单种浅滩那么协调，提出了一个有趣的问题，即为什么鱼类会形成混合物种的浅滩，而这种行为可能不如单一物种的浅滩有益。在这里，我们使用原位立体视频技术来比较浅滩内部和之间的凝聚力和对齐差异，对于包含热带流浪印度太平洋中士少校雀鲷的混合和单一物种浅滩，Abudefduf vaigiensis，在温带水域。正如预期的那样，混合物种浅滩比单一物种浅滩更不整齐。然而，在混合物种的浅滩中，同种比异种更具有凝聚力和一致性，这表明在更大的混合物种浅滩中形成了协调的单物种亚群。亚群的形成可能会降低与物种之间差异相关的成本，从而提高混合物种浅滩的收益。因此，混种浅滩中可能存在多层次的社会结构，可以促进流浪A. vaigiensis 的生长和生存在温带地区。更广泛地说，这项研究强调了在试图了解个人经历的成本 - 收益权衡时考虑混合物种浅滩的详细内部结构的重要性。