Marta C Soares | Search PubMed

PubMed2026

Body tactile stimulation intensifies appeasement behavior in a cichlid fish.

Social touch, provided by body tactile stimulation (TS), is known to trigger appeasement behavior and social stability in mammals. Recent studies have shown that TS also plays a role in regulating social interactions in fish by reducing aggression between individuals. While this effect may stem from decreased aggression in dominants, the potential contribution of increased appeasement behavior in subordinates has been overlooked. Here, we investigated whether TS facilitates the onset of appeasement behavior in the cichlid fish Geophagus iporangensis, a species that exhibits clear dominance-subordination relationships. Isolated individuals were assigned to one of two treatments, either with TS (provided by plastic sticks lined with silicone bristles) or without it for 21 days. Then, each fish was paired with a socially experienced dominant individual to enable rapid hierarchy resolution. Initial appeasement scores did not differ between treatments. However, individuals who experienced TS exhibited significantly higher appeasement scores during the final 5 min of the contest than the control group. Additionally, fish under TS showed an increase in the appeasement score after 10 min of agonistic interaction, while it did not change in the treatment without TS. Overall, this study provides the first evidence that TS can mitigate aggressive interactions by intensifying subordinates' appeasement behavior, thereby affecting social dynamics similarly to mammals.

aggression↗

PubMed2025

The hedonic impact of cleaner-client fish interactions is mediated by the opioid system.

According to the 'incentive salience hypothesis' reward processing involves two main components, including the motivation to obtain a reward (i.e. incentive salience or 'wanting') and the hedonic pleasure felt during its consumption (i.e. hedonic impact or 'liking'), which are dissociable. The processing of these hedonic mechanisms is suggested to be mediated by opioid neurotransmission; however, most evidence comes from humans and other mammals. Here we argue that, in mutualistic associations, client fish seek to interact with cleaner fish not only due to the immediate benefits of being cleaned but also because of the hedonic impact of tactile stimulation, modulated by the opioid system. We used a conditioned place preference (CPP) paradigm to test the hedonic dimension of motivation to be cleaned and found that de-parasitized client fish (the butterflyfish Chaetodon auriga) preferred the compartment paired with cleaner fish (Labroides dimidiatus). Treatment with the μ-opioid receptor agonist The µ-opioid receptor agonist 2-Ala-4-mephe-5-gly-enkephalin (DAMGO) dose-dependently increased preference, while treatment with the antagonist naloxone decreased it. To test for fish incentive salience, we used a detour task, with barriers being added after each trial to demand higher response strength to reach the target and found that client fish can circumvent barriers to reach a cleaner-adjacent compartment. Response strength (number of barriers added before 'giving up') was not affected by treatment with either naloxone or DAMGO. Our results show that cleaner-client interactions are hedonically positive for clients, an effect that is mediated by the opiodergic system. Visual contact with cleaners is sufficient to activate incentive salience programmes, but no evidence of participation of the opioidergic system in these programmes was found. Overall, we provide the first evidence confirming the participation of the opioidergic system in motivation to be cleaned in mutualistic associations.

Animals↗