Group size influences feeding behavior and the expression of appetite, stress, and neurotransmitter-related transcript expression in tiger barb (Puntigrus tetrazona)

Tiger barbs (Puntigrus tetrazona) are typical shoaling fish that can become stressed and aggressive when kept in small groups. To examine whether group size affects feeding, fish were housed in groups of 1, 2, 4, or 8 individuals per tank and monitored over a 10-day period. Overall feeding behavior, latency to feed, and the amount of food consumed were assessed daily. After 10 days under these conditions, fish were sampled for intestine and brain for transcript expression analyses. Genes examined included appetite regulators such as neuropeptide Y-npy, orexin and melanin-concentrating hormone-mch in the brain and cholecystokinin-cck and ghrelin in the intestine. We also analyzed brain expression of hormones related to social behavior and stress (oxytocin-oxt, corticotrophin-releasing factor-crf and thyrotropin-releasing hormone-trh) and genes related to the synthesis of serotonin (tryptophan hydroxylase-tph) and dopamine (tyrosine hydroxylase-th) and epinephrine (dopamine-β-hydroxylase- dbh). Isolated fish consumed significantly less food and exhibited longer latency to feed compared to all other groups. Fish housed in pairs also showed reduced intake and longer feeding latency relative to those in groups of 4. In the intestine, ghrelin expression was elevated in isolated fish relative to those in larger groups, while no differences were observed in ccka or cckb expression. In the brain, expression of orexigenic neuropeptides (npy, mch, and orexin) was higher in isolated and pair-housed fish compared to those in groups of 4 or 8. Expression of stress- and neurotransmitter-related genes (crf, trh, th, and tph1) was also significantly elevated in isolated fish. No significant group-size effects were found for oxt, tph2, dbh, or cart expression. Overall, our results suggest that small group sizes, particularly isolation, impair feeding behavior and upregulate orexigenic and stress-related gene expression, highlighting the importance of social environment in regulating physiological and behavioral responses in shoaling fish.