Cohesion, order and information flow in the collective motion of mixed-species shoals

Abstract

Despite the frequency with which mixed-species groups are observed in nature, studies of collective behaviour typically focus on single-species groups. Here, we quantify and compare the patterns of interactions between three fish species, threespine sticklebacks (Gasterosteus aculeatus), ninespine sticklebacks (Pungitius pungitius) and roach (Rutilus rutilus) in both single- and mixed-species shoals in the laboratory. Pilot data confirmed that the three species form both single- and mixed-species shoals in the wild. In our laboratory study, we found that single-species groups were more polarized than mixed-species groups, while single-species groups of threespine sticklebacks and roach were more cohesive than mixed shoals of these species. Furthermore, while there was no difference between the inter-individual distances between threespine and ninespine sticklebacks within mixed-species groups, there was some evidence of segregation by species in mixed groups of threespine sticklebacks and roach. There were differences between treatments in mean pairwise transfer entropy, and in particular we identify species-differences in information use within the mixed-species groups, and, similarly, differences in responses to conspecifics and heterospecifics in mixed-species groups. We speculate that differences in the patterns of interactions between species in mixed-species groups may determine patterns of fission and fusion in such groups.

Towards an ultimate explanation for mixed‐species shoaling

“…If a species preference for mixed‐species shoaling behaviour is learned rather than innate, it may be that selection acts upon between‐species information transfer and learning ability rather than mixed‐species shoaling behaviour itself. Rapidly progressing research into the cognitive ability of fishes and the ability of fish to communicate within‐ and between species is providing important insight in this field, with the evidence demonstrating that fish are capable of recognizing, communicating with, and learning from heterospecifics (Warburton & Lees, ; Ward, Holbrook, Krause, & Hart, ; Ward et al, ; Ward & Webster, ; Webster, Ward, & Hart, ). Irrespective of how selection may act upon mixed‐species shoaling behaviour, shoal choice experiments and investigations of resource use demonstrate that there are unique costs and benefits associated with the formation of mixed‐species shoals relative to single‐species shoals.…”

“…Underlying all the mechanisms, we propose as linked to mixed‐species shoaling is the assumption that shoal members are capable of efficiently transferring information among heterospecifics. Recent research has demonstrated that shoaling fish respond rapidly to the movements of heterospecific shoal‐mates (Ward et al, ), are capable of recognizing and responding to heterospecific alarm cues (Chivers et al, ), and prefer to shoal with heterospecifics from familiar habitats (Ward et al, ; Webster et al, ) suggesting benefits associated with habitat‐specific information transfer. However, further research across more mixed‐species shoaling systems and considering a variety of information (such as information about the location and quality of food) should be undertaken to understand the extent and limitations of between‐species information transfer as well as the role that social learning plays in mixed‐species shoaling behaviour.…”

“…Studies of single-species shoals have demonstrated that shoaling behaviour more generally is a heritable adaptation (Seghers, 1974;Wright et al, 2003). Further, research suggests that fishes are capable of recognizing and communicating with heterospecifics (Ward et al, 2018) and that there are unique benefits associated with the formation of mixed-species shoals (e.g. -Ramirez et al, 2017;Alevizon, 1976;Mathis & Chivers, 2003).…”

“…Underlying all the mechanisms, we propose as linked to mixedspecies shoaling is the assumption that shoal members are capable of efficiently transferring information among heterospecifics. Recent research has demonstrated that shoaling fish respond rapidly to the movements of heterospecific shoal-mates (Ward et al, 2018), are capable of recognizing and responding to heterospecific alarm cues (Chivers et al, 2002), and prefer to shoal with heterospecifics from familiar habitats Webster et al, 2008) suggesting benefits associated with habitat-specific information transfer.…”

Social Recognition and Social Attraction in Group-Living Fishes

“…Nonetheless, such groups are known to fragment along species lines when under threat of predation (Wolf, 1985). Moreover, mixed species groups can be less cohesive and less aligned than single species groups and social information may flow less readily between heterospecifics than between conspecifics in some mixed species shoals (Ward et al, 2018). It seems likely that an active preference for associating with conspecifics works alongside passive assortment in shoal formation, and that both are important in maintaining shoal coherence.…”