Fission or fusion: shoaling adaptations in green chromides (Etroplus suratensis) across multiple manipulations

Collective behaviour contributes towards increased fitness. However, in fission-fusion societies, the decision to participate in a group is based on the cost-benefit ratio derived under a given condition. In today's world, this ratio dramatically changes due to increased challenges in degraded habitats, impacting behavioural decisions towards social grouping. In this paper, we used fish shoal as a unit of collective behaviour and investigated variations in shoaling adaptations across a range of manipulations, mimicking challenges faced by the species under natural conditions. We used green chromides (Etroplus suratensis), a cichlid fish species, and characterized their shoaling behaviour under laboratory conditions for two group sizes, 4 and 8. We then examined the effects of different manipulations, for example food-deprivation and reduced space (both mimicking resource constrained conditions), and the presence of hetero species, including tilapia (Oreochromis spp., an alien species widespread within the habitats of E. suratensis), on their shoaling behaviour. Our results showed that E. suratensis formed shoals in both group sizes, 4 and 8. In subsequent analyses, we used group size 8 as a control and demonstrated that space reduction and food deprivation significantly impacted shoaling, reducing shoal splits and occurrences of solitary fish, while increasing incidences of single, cohesive shoals comprising all individuals. E. suratensis also formed mixed-species shoals with alien Oreochromis spp., but exhibited a large percentage (in absolute terms) of shoal splits. Overall, E. suratensis exhibited plasticity in their shoaling behaviour and increased fusion of shoals under challenging conditions, which was in contrast to showing more fission in the presence of the alien species.