The Costs and Benefits of Mixed-species Shoaling for a Vagrant Tropical Damselfish

Social grouping behaviour is displayed by animals across terrestrial and aquatic ecosystems globally. The evolution of such sociality is attributed to multiple benefits enhancing individual growth, survival and reproduction. Recent interest in mixed-species grouping, which extends from a large body of literature invested in understanding single-species grouping, highlights novel complexities of group formation which relate to phenotypic, behavioural and physiological differences that naturally exist between species. Considering that mixed-species grouping is common among social animals and has been shown to enhance growth and survival, further research striving to explain the phenomenon will be important for comprehensively understanding social grouping behaviour and the role it plays in structuring animal communities. The explanations put forward to date for mixed-species grouping focus heavily on explanations for single-species grouping, but also demonstrate distinct differences between species, leading to unique cost-benefit trade-offs which need to be incorporated into conceptual models of mixed-species grouping behaviour. Unique predation related trade-offs may arise from inefficiency of the confusion effect within prey groups, variation in vigilance between species and unequal species-preferences shown by predators, whilst unique foraging related trade-offs may arise from diet partitioning, variations in foraging behaviour, and differences in competitive abilities between species. Considering these unique trade-offs, understanding mixed-species grouping behaviour requires critical consideration beyond the framework developed for single-species groups. Social grouping behaviour displayed by fishes, termed shoaling, provides an ideal system for investigating mixed-species grouping because fish exhibit a wide range of shoal forms and functions. Mixed-species shoaling has been described across multiple systems and importantly, species have been found to form both mixed- and single-species shoals of varying size and composition. Such systems provide ideal models because they allow for the drivers and outcomes of mixed-species grouping to be identified and quantified by comparing cost-benefit trade-offs for individuals in mixed relative to single-species shoals. For this thesis, I used shoaling by the tropical vagrant (expatriated to temperate waters from the tropics) damselfish Abudefduf vaigiensis (family Pomacentridae) in association with temperate heterospecifics as a model system to investigate cost-benefit trade-offs for individuals within mixed-species shoals. I undertook a series of laboratory- and field-based investigations to tease apart the interacting mechanisms operating within mixed-species shoals. In situ surveys revealed significant differences in shoal composition and size between exposed and sheltered habitats, suggesting an interplay between species composition, shoal size, predation threat and foraging opportunity. Further, comparison of mixed- and single-species shoal densities suggested that mixed-species shoaling is adaptive rather than driven by constraints in conspecific density. Following in situ surveys, laboratory-based choice experiments revealed that A. vaigiensis showed equal preference for mixed- and single-species shoal compositions, regardless of predation threat. These results suggest that differences between species are not costly in terms of predator protection, contradicting the widely accepted theory that oddity within animal groups enhances predation threat. Finally, I undertook in situ and laboratory-based investigations of shoal structure and information transfer which revealed that species segregate within mixed-species shoals and display distinct shoal structures. This outcome explains how A. vaigiensis and temperate heterospecifics within mixed species shoals gain benefits from differences between species and large shoal size, without incurring costs associated with oddity. Identifying how individuals benefit from the formation of mixed-species groups is fundamental to our understanding of why animals from mixed-species groups and how such interspecific behaviour may influence individual fitness and broader ecosystem function. In this thesis, I have identified and described complex predation and foraging cost-benefit trade-offs arising from differences between species, in addition to shoal size. This demonstrates that single- and mixed-species groups, although similar in some ways, need to be considered using distinct theoretical frameworks. Future research needs to critically consider differences between species and internal group structure to fully appreciate the role of mixed-species social grouping behaviour in animal communities