Familiarity among kin has minor effects beyond grouping for escape response in a shoaling fish

Prey have evolved cost-effective antipredator manoeuvres to evade predators, and living in a group is a common strategy employed to improve survival. Associating with familiar conspecifics, particularly kin, may further enhance these strategies by improving the flow of reliable information concerning the type and magnitude of nearby threats. This study investigated the influence of familiarity among shoalmates on the activity and escape responses of a shoaling damselfish. We examined the routine swimming and fast-start response of juvenile spiny Chromis (Acanthochromis polyacanthus, Bleeker 1855) when tested alone, with familiar kin, or with unfamiliar conspecifics. Solitary fish were less active, moved more slowly than fish in groups and were more likely to exhibit a substantially delayed (>0.05 s) fast-start response to a startle stimulus. Fish with familiar kin exhibited the highest escape speeds and travelled the greatest distances during the initial response, and there was a trend for familiar fish to perform better than non-familiar fish. Interestingly, the time taken to initiate a response (response latency: ~0.01 s) for those undertaking a non-delayed fast start was unaffected by treatment. Shoaling with familiar kin enhanced some key aspects of the fast-start escape response, which may contribute to predator confusion effects and increase the likelihood of survival for individuals within familiar groups.