Multispecies collective waving behaviour in fish

Abstract

Collective behaviour is widely accepted to provide a variety of antipredator benefits. Acting collectively requires not only strong coordination among group members, but also the integration of among-individual phenotypic variation. Therefore, groups composed of more than one species offer a unique opportunity to look into the evolution of both mechanistic and functional aspects of collective behaviour. Here, we present data on mixed-species fish shoals that perform collective dives. These repeated dives produce water waves capable of delaying and/or reducing the success of piscivorous bird attacks. The large majority of the fish in these shoals consist of the sulphur molly, Poecilia sulphuraria, but we regularly also found a second species, the widemouth gambusia, Gambusia eurystoma, making these shoals mixed-species aggregations. In a set of laboratory experiments, we found that gambusia were much less inclined to dive after an attack as compared with mollies, which almost always dive, though mollies dived less deep when paired with gambusia that did not dive. By contrast, the behaviour of gambusia was not influenced by the presence of diving mollies. The dampening effect of less responsive gambusia on molly diving behaviour can have strong evolutionary consequences on the overall collective waving behaviour as we expect shoals with a high proportion of unresponsive gambusia to be less effective at producing repeated waves. This article is part of a discussion meeting issue 'Collective behaviour through time'.

Keywords: Gambusia eurystoma; Poecilia sulphuraria; collective behaviour; collective waves; mixed-species; predator–prey.

Figure 1.

Widemouth gambusia (Gambusia eurystoma, left) and sulphur molly (Poecilia sulphuraria, right) in their natural habitats. (Online version in colour.)

Figure 2.

(a) Species composition of shoals. Each column represents a single picture (N = 154). (b) Mixing of sulphur mollies and widemouth gambusia in relation to the observed number of fish per picture. Shown are observed proportions of mollies (blue) as well as the intervals between the 0.025 and the 0.975 quantiles (orange) expected for random mixing. (c) Body size distributions of Poecilia sulphuraria and Gambusia eurystoma caught at the same river locations. (d) Polarization, (e) nearest neighbour distance (NND) and (f) neighbour count within 2 BL in relation to the proportion of mollies in the respective photo. (Online version in colour.)

Figure 3.

(a) Oxygen-dependent ASR tendencies of Poecilia sulphuraria and Gambusia eurystoma. Time spent at the water surface performing ASR increased with decreasing oxygen concentrations for mollies while gambusia spent most of their time at the surface throughout the tested oxygen range. Each data point equals the cumulative time spent at the surface for one group of four adult fish (n = 19 groups per species). The half-maximal concentration at which fish spend 50% of their time at the surface (ASR₅₀) was estimated at 1.3 mg l⁻¹ DO for mollies. (b) Maximum sustainable swimming speed (U_crit) for mollies and gambusia in relation to body size. (Online version in colour.)

Figure 4.

(a) Proportion of fish diving after a simulated bird attack in con- and heterospecific shoals of Poecilia sulphuraria and Gambusia eurystoma. (b) Dive depth of both species. (c) Dive depth of only mollies in the mixed-species treatment when at least one gambusia was diving compared with none. In (b) and (c), only those individuals that performed a dive were included; bars denote means and error whiskers standard deviations. Asterisks show significance of p < 0.01 from post hoc comparisons; n.s., not significant.