Nonapeptide levels in male cleaner fish brains during interactions with unfamiliar intra and interspecific partners

In nature, cleaner fish face complex situations arising from a highly demanding socio-environment that includes conspecific and mutualistic relationships. The nonapeptide arginine vasotocin (AVT) is known to play a relevant role in the expression of mutualistic behaviour in the Indo-Pacific bluestreak cleaner wrasse (Labroides dimidiatus). However, the role of the second nonapeptide isotocin (IT) in mediating social behaviour has yet to be established. Here we investigated whether there is a link between the distribution of active nonapeptides across brain regions and cleaners' social behaviour. Male cleaner fish were exposed to different social contexts: (i) an unfamiliar male conspecific (conspecific context), (ii) an unfamiliar interspecific partner (blond naso tang, Naso elegans; mutualistic context) and (iii) a ball (non-social context). Furthermore, to distinguish between physicochemical and visual stimulations, cleaners were also exposed to an unfamiliar conspecific or an unfamiliar interspecific partner placed in a separate, smaller aquaria while inside the experimental tank. We then measured biologically available AVT and IT levels in distinct brain macro-areas, forebrain, optic tectum, cerebellum and brain stem, using high-performance liquid chromatography with fluorescence detection. We did not find any association between the levels of AVT and social contexts. On the other hand, we found lower levels of IT in the forebrain of cleaners in contact with a conspecific compared to those introduced to a client and a client inside another aquarium. Additionally, cleaners displayed aggression but only towards other conspecifics. These findings indicate that (i) IT in cleaners' forebrain is linked with mutualistic engagement between cleaners and clients, (ii) stimulation of IT pathways in the forebrain is probably linked with the visual recognition of potential clients and (iii) physicochemical signals are essential to trigger aggressive displays in male conspecifics.