Towards an ultimate explanation for mixed‐species shoaling

Abstract

The formation of social groups has important impacts on fitness for many animal species, with differences in group compositions resulting in a range of fitness outcomes for individuals. 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. Among fishes, mixed‐species shoaling is a common form of social grouping behaviour displayed across a range of marine and freshwater ecosystems. Research explaining mixed‐species shoaling shows some overlap with explanations for single‐species shoaling; however, it also demonstrates that distinct differences between species give rise to unique cost‐benefit trade‐offs which need to be incorporated into conceptual models of mixed‐species shoaling behaviour. Unique predation related trade‐offs may arise from inefficiency of the confusion effect, 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. We review the literature on fitness outcomes associated with mixed‐species shoaling and present a new theoretical framework to explain the cost‐benefit trade‐offs for individuals within mixed‐species shoals. The framework incorporates both trade‐offs arising from differences between species and those arising from group size, the former having been largely ignored due to a focus on single‐species shoaling. Our framework is designed to inform future research striving to explain mixed‐species shoaling behaviour.

Predation avoidance and foraging efficiency contribute to mixed‐species shoaling by tropical and temperate fishes

“…Because shoals in open habitats were characterized by large numbers of planktivorous species, it is likely that such habitats are associated with a greater abundance of planktonic food. In tropical coral reef systems, the movement of water adjacent to reefs has been shown to provide increased foraging efficiency for planktivores feeding further from the reef structure (Hamner et al, 1988 Irrespective of the underlying reason for differences, the results of this study and that by Smith et al (2018) in combination suggest that mechanisms associated with both shoal size and composition drive benefits for vagrant A. vaigiensis within mixed-species shoals (Paijmans et al, 2019).…”

“…Irrespective of the underlying reason for differences, the results of this study and that by Smith et al . () in combination suggest that mechanisms associated with both shoal size and composition drive benefits for vagrant A. vaigiensis within mixed‐species shoals (Paijmans et al ., ).…”

“…Indeed, mixed-species shoaling by fishes may afford key benefits to individuals which ultimately increase growth and survival (e.g., Camacho-Cervantes et al, 2014;Sackley & Kaufman, 1996;Smith et al, 2018). The net benefits gaind from mixed-species shoaling theoretically result from multiple fitness trade-offs predominantly associated with predation risk and foraging efficiency (Paijmans et al, 2019). Predation risk may be reduced for individuals in mixed-species shoals via multiple mechanisms, including predator encounter dilution, the relatively high vigilance of heterospecifics or a predator's preference for heterospecifics.…”

“…Foraging benefits may be afforded to individuals within mixedspecies shoals via multiple mechanisms, including collective-effort searching for food, the circumvention of territorial competitors or diet partitioning between species (Paijmans et al, 2019). Diet analysis from mixed-species schools of tuna revealed significant differences in both the diet and feeding times of yellowfin tuna Thunnus albacares (Bonnaterre 1788) and skipjack tuna Katsuwonus pelamis (L. 1758), suggesting that the partitioning of food between species may minimize within-shoal food competition (Alatorre-Ramirez et al, 2017).…”

“…Furthermore, A. vaigiensis that form mixed-species shoals with temperate fishes display increased growth rates relative to those within single-species shoals, suggesting that there are energetic benefits associated with mixed-species shoaling (Smith et al, 2018). These findings provide key insight, but further research is required to (a) understand the interactive effects of water temperature, predation threat, foraging efficiency and shoaling behaviour on the growth and survival of vagrant A. vaigiensis; and (b) determine whether mixedspecies shoaling by A. vaigiensis is due to low densities of conspecifics (constraint hypothesis) or whether there are in fact some specific benefits, apart from merely shoal size, driving A. vaigiensis to preferentially shoal with heterospecifics in temperate ecosystems (adaptive hypothesis) (Paijmans et al, 2019). Smith et al (2018) reported no significant difference between the size of mixed-and single-species shoals, or the distance from shelter for A. vaigiensis within each shoal type.…”