What do fish make of mirror images?

Abstract

Fish act aggressively towards their mirror image suggesting that they consider it another individual, whereas in some mammals behavioural response to mirrors may be an evidence of selfrecognition. Since fish cannot self-recognize, we asked whether they could distinguish between fighting a mirror image and fighting a real fish. We compared molecular, physiological and behavioural responses in each condition and found large differences in brain gene expression levels. Although neither levels of aggressive behaviour nor circulating androgens differed between these conditions, males fighting a mirror image had higher immediate early gene (IEG) expression in brain areas homologous to the amygdala and hippocampus than controls. Since amygdalar responses are associated with fear and fear conditioning in other species, higher levels of brain activation when fighting a mirror suggest fish experience fear in response to fights with a mirror image. Clearly, the fish recognize something unusual about the mirror image and the differential brain response may reflect a cognitive distinction.

‘Winner effect’ without winning: Unresolved social conflicts increase the probability of winning a subsequent contest in a cichlid fish

“…That mirror-stimulated males mounted an androgen response in our study is consistent with findings in another cichlid, Astatotilapia burtoni [23]. Mirror-stimulated androgen levels in Pundamilia were similar to levels observed in tests with real intruders where focal animals fought a rival enclosed in a transparent tube [35].…”

Acute embryonic anoxia exposure favours the development of a dominant and aggressive phenotype in adult zebrafish

“…Because fish treat their mirror image as an intruding individual and respond with aggressive behaviours [23], our results suggest that the adults derived from embryonic anoxia are more aggressive than those derived from embryonic normoxia. Similarly, neonatal male rats exposed to anoxia are more aggressive as adults [24].…”

Short communication: Response of rainbow trout (Oncorhynchus mykiss) to mirror images

“…If a fish is not able to recognize itself in the mirror, the next question is whether it will consider the mirrored images to be fellow individuals. Small fighting fish may react to their mirror image by courting or attacking it (Ros et al, 2006;Van der Waal, 2016); however, Desjardins & Fernald (2010) image vs. fighting a real opponent were different such that they concluded that "clearly, the fish recognize something unusual about the mirror image and the differential brain response may reflect a cognitive distinction". In the study of Balzarini et al (2014), one of three sympatric cichlid species (Neolamprologus pulcher) showed a correlated response between a mirror test and a live opponent test, whereas in two others (Telmatochromis vittatus and Lepidiolamprologus elongatus) there was no such relationship.…”