Supplementary material from "Mechanisms of Social Behavior in the Anti-Social Blind Cavefish (Astyanax mexicanus)"

The evolution of social behavior in Astyanax mexicanus, which exists as a sighted, surface-dwelling morph and a blind, cave-dwelling morph, provides a model for understanding how environmental pressures shape social behaviors. We compared the shoaling behavior of blind and surface A. mexicanus to that of zebrafish (Danio rerio), and examined the effects of nutritional state and the neuropeptides isotocin (IT) and arginine vasotocin (AVT) on their social behavior. Blind cavefish not only fail to form shoals, but actively avoid conspecifics, with hunger further diminishing their social cohesion. Administration of low doses of AVT and an IT antagonist partially restored social behavior in blind cavefish, reducing distances between individuals, whereas surface fish exhibited minimal or opposite responses to these hormonal manipulations. Our findings suggest that the loss of shoaling behavior in blind cavefish is not a consequence of visual impairment alone, as they remain capable of detecting and responding to others. Instead, this behavior likely reflects an adaptive response to their resource-poor, predator-free cave environment, where shoaling may be disadvantageous. The differing responses to nonapeptides between the morphs indicate that blind cavefish may have lost the motivation to shoal rather than the ability, highlighting how ecological pressures can shape social behavior.