Odd one in: Oddity within mixed-species shoals does not affect shoal preference by vagrant tropical damselfish in the presence or absence of a predator

Grouping behaviour displayed by animals is usually attributed to predation and foraging-related benefits. The mechanisms of predator protection and foraging efficiency are diverse and often produce conflicting drivers of grouping behaviour. One key conflict is that between group size and phenotypic oddity. Theoretically, individuals should choose the largest available group due to multiple mechanisms associated with “safety in numbers”. However, individuals should also choose the most phenotypically similar group members due to the “predator confusion effect”. This conflict is particularly important within the context of mixed-species groups because, while their formation may facilitate large group size, phenotypic differences between species may be costly due to oddity. To investigate the interacting effects of shoal size, composition and predator presence on grouping decisions, choice experiments were conducted using displaced tropical damselfish, Abudefduf vaigiensis, settling in temperate south-eastern Australia (termed vagrants) as a model species. Contrary to predictions, A. vaigiensis displayed no preference for single- over mixed-species shoals, with or without a predator present. These results suggest that shoal composition may not be an important driver of shoal choice in this system. Further, A. vaigiensis showed no preference for larger mixed-species shoals over smaller single-species shoals. These outcomes are discussed within the context of climate change-driven redistribution of A. vaigiensis in temperate south-eastern Australia.