Two new species and new host and distribution records of Gnathia Leach, 1814 (Crustacea, Isopoda, Gnathiidae) from Western Australia and the Great Barrier Reef, Australia.

Gnathiaantennacrassasp. nov. from seagrass beds off Rottnest Island, Western Australia is the first record of any gnathiid from the entirety of Western Australia; the male can be distinguished from congeners by the stout peduncular articles of the antenna. Gnathiataurussp. nov. is described from two adult specimens reared from praniza larvae found infecting elasmobranch fishes at Heron Island, southern Great Barrier Reef; the males can be distinguished from all congeners by the dorsally strongly elongate mandibles and smoothly rounded mediofrontal process on the anterior part of cephalosome. Gnathiaaff.maculosa Ota & Hirose, 2009 is recorded from Australia, together with further records of G.trimaculata Coetzee, Smit, Grutter & Davies, 2009 and G.grandilaris Coetzee, Smit, Grutter & Davies, 2008, all from elasmobranch fishes.

Cleaner fish are potential super-spreaders.

Cleaning symbiosis is critical for maintaining healthy biological communities in tropical marine ecosystems. However, potential negative impacts of mutualism, such as the transmission of pathogens and parasites during cleaning interactions, have rarely been evaluated. Here, we investigated whether the dedicated bluestreak cleaner wrasse, Labroides dimidiatus, is susceptible to and can transmit generalist ectoparasites between client fish. In laboratory experiments, L. dimidiatus were exposed to infective stages of three generalist ectoparasite species with contrasting life histories. Labroides dimidiatus were susceptible to infection by the gnathiid isopod Gnathia aureamaculosa, but were significantly less susceptible to the ciliate protozoan Cryptocaryon irritans and the monogenean flatworm Neobenedenia girellae, compared with control host species (Coris batuensis or Lates calcarifer). The potential for parasite transmission from a client fish to the cleaner fish was simulated using experimentally transplanted mobile adult (i.e. egg-producing) monogenean flatworms on L. dimidiatus. Parasites remained attached to cleaners for an average of 2 days, during which parasite egg production continued, but was reduced compared with that on control fish. Over this timespan, a wild cleaner may engage in several thousand cleaning interactions, providing numerous opportunities for mobile parasites to exploit cleaners as vectors. Our study provides the first experimental evidence that L. dimidiatus exhibits resistance to infective stages of some parasites yet has the potential to temporarily transport adult parasites. We propose that some parasites that evade being eaten by cleaner fish could exploit cleaning interactions as a mechanism for transmission and spread.

Access to Cleaning Services Alters Fish Physiology Under Parasite Infection and Ocean Acidification.

Cleaning symbioses are key mutualistic interactions where cleaners remove ectoparasites and tissues from client fishes. Such interactions elicit beneficial effects on clients' ecophysiology, with cascading effects on fish diversity and abundance. Ocean acidification (OA), resulting from increasing CO2 concentrations, can affect the behavior of cleaner fishes making them less motivated to inspect their clients. This is especially important as gnathiid fish ectoparasites are tolerant to ocean acidification. Here, we investigated how access to cleaning services, performed by the cleaner wrasse Labroides dimidiatus, affect individual client's (damselfish, Pomacentrus amboinensis) aerobic metabolism in response to both experimental parasite infection and OA. Access to cleaning services was modulated using a long-term removal experiment where cleaner wrasses were consistently removed from patch reefs around Lizard Island (Australia) for 17 years or left undisturbed. Only damselfish with access to cleaning stations had a negative metabolic response to parasite infection (maximum metabolic rate-MO2Max; and both factorial and absolute aerobic scope). Moreover, after an acclimation period of 10 days to high CO2 (∼1,000 µatm CO2), the fish showed a decrease in factorial aerobic scope, being the lowest in fish without the access to cleaners. We propose that stronger positive selection for parasite tolerance might be present in reef fishes without the access to cleaners, but this might come at a cost, as readiness to deal with parasites can impact their response to other stressors, such as OA.

The impact of long-term reduced access to cleaner fish on health indicators of resident client fish.

In many mutualisms, benefits in the form of food are exchanged for services such as transport or protection. In the marine cleaning mutualism, a variety of 'client' reef fishes offer 'cleaner' fish Labroides dimidiatus access to food in the form of their ectoparasites, where parasite removal supposedly protects the clients. Yet, the health benefits individual clients obtain in the long term from repeated ectoparasite removal remain relatively unknown. Here, we tested whether long-term reduced access to cleaning services alters indicators of health status such as body condition, immunity and the steroids cortisol and testosterone in four client damselfish species Pomacentrus amboinensis, Amblyglyphidodon curacao, Acanthochromis polyacanthus and Dischistodus perspicillatus To do so, we took advantage of a long-term experimental project in which several small reefs around Lizard Island (Great Barrier Reef, Australia) have been maintained cleaner-free since the year 2000, while control reefs had their cleaner presence continuously monitored. We found that the four damselfish species from reef sites without cleaners for 13 years had lower body condition than fish from reefs with cleaners. However, immunity measurements and cortisol and testosterone levels did not differ between experimental groups. Our findings suggest that clients use the energetic benefits derived from long-term access to cleaning services to selectively increase body condition, rather than altering hormonal or immune system functions.

Parasite infection directly impacts escape response and stress levels in fish.

Parasites can account for a substantial proportion of the biomass in marine communities. As such, parasites play a significant ecological role in ecosystem functioning via host interactions. Unlike macropredators, such as large piscivores, micropredators, such as parasites, rarely cause direct mortality. Rather, micropredators impose an energetic tax, thus significantly affecting host physiology and behaviour via sublethal effects. Recent research suggests that infection by gnathiid isopods (Crustacea) causes significant physiological stress and increased mortality rates. However, it is unclear whether infection causes changes in the behaviours that underpin escape responses or changes in routine activity levels. Moreover, it is poorly understood whether the cost of gnathiid infection manifests as an increase in cortisol. To investigate this, we examined the effect of experimental gnathiid infection on the swimming and escape performance of a newly settled coral reef fish and whether infection led to increased cortisol levels. We found that micropredation by a single gnathiid caused fast-start escape performance and swimming behaviour to significantly decrease and cortisol levels to double. Fast-start escape performance is an important predictor of recruit survival in the wild. As such, altered fitness-related traits and short-term stress, perhaps especially during early life stages, may result in large scale changes in the number of fish that successfully recruit to adult populations.

Practical methods for culturing parasitic gnathiid isopods.

The reliance of parasites on their hosts makes host-parasite interactions ideal models for exploring ecological and evolutionary processes. By providing a consistent supply of parasites, in vivo monocultures offer the opportunity to conduct experiments on a scale that is generally not otherwise possible. Gnathiid isopods are common ectoparasites of marine fishes, and are becoming an increasing focus of research attention due to their experimental amenability and ecological importance as ubiquitous, harmful, blood-feeding "mosquito-like" organisms. They feed on hosts once during each of their three juvenile stages, and after each feeding event they return to the benthos to digest and moult to the next stage. Adults do not feed and remain in the benthos, where they reproduce and give birth. Here, we provide methods of culturing gnathiids, and highlight ways in which gnathiids can be used to examine parasite-host-environment interactions. Captive-raised gnathiid juveniles are increasingly being used in parasitological research; however, the methodology for establishing gnathiid monocultures is still not widely known. Information to obtain in vivo monocultures on teleost fish is detailed for a Great Barrier Reef (Australia) and a Caribbean Sea (US Virgin Islands) gnathiid species, and gnathiid information gained over two decades of successfully maintaining continuous cultures is summarised. Providing a suitable benthic habitat for the predominantly benthic free-living stage of this parasite is paramount. Maintenance comprises provision of adequate benthic shelter, managing parasite populations, and sustaining host health. For the first time, we also measured gnathiids' apparent attack speed (maximum 24.5 cm sec-1; 6.9, 4.9/17.0, median, 25th/75th quantiles) and illustrate how to collect such fast moving ectoparasites in captivity for experiments. In addition to providing details pertaining to culture maintenance, we review research using gnathiid cultures that have enabled detailed scientific understanding of host and parasite biology, behaviour and ecology on coral reefs.

Aquatic Parasite Cultures and Their Applications.

In this era of unprecedented growth in aquaculture and trade, aquatic parasite cultures are essential to better understand emerging diseases and their implications for human and animal health. Yet culturing parasites presents multiple challenges, arising from their complex, often multihost life cycles, multiple developmental stages, variable generation times and reproductive modes. Furthermore, the essential environmental requirements of most parasites remain enigmatic. Despite these inherent difficulties, in vivo and in vitro cultures are being developed for a small but growing number of aquatic pathogens. Expanding this resource will facilitate diagnostic capabilities and treatment trials, thus supporting the growth of sustainable aquatic commodities and communities.

Cleaner shrimp are a sustainable option to treat parasitic disease in farmed fish.

Chemical use is widespread in aquaculture to treat parasitic diseases in farmed fish. Cleaner fish biocontrols are increasingly used in fish farming as an alternative to medicines. However, cleaner fish are susceptible to some of their clients' parasites and their supply is largely dependent on wild harvest. In comparison, cleaner shrimp are not susceptible to fish ectoparasites and they can be reliably bred in captivity. The effectiveness of shrimp in reducing parasites on farmed fish remained unexplored until now. We tested four cleaner shrimp species for their ability to reduce three harmful parasites (a monogenean fluke, a ciliate protozoan, and a leech) on a farmed grouper. All shrimp reduced parasites on fish and most reduced the free-living early-life environmental stages - a function not provided by cleaner fish. Cleaner shrimp are sustainable biocontrol candidates against parasites of farmed fish, with the peppermint cleaner shrimp reducing parasites by up to 98%.

Cleaner wrasse indirectly affect the cognitive performance of a damselfish through ectoparasite removal.

Cleaning organisms play a fundamental ecological role by removing ectoparasites and infected tissue from client surfaces. We used the well-studied cleaning mutualisms involving the cleaner wrasse, Labroides dimidiatus, to test how client cognition is affected by ectoparasites and whether these effects are mitigated by cleaners. Ambon damselfish (Pomacentrus amboinensis) collected from experimental reef patches without cleaner wrasse performed worse in a visual discrimination test than conspecifics from patches with cleaners. Endoparasite abundance also negatively influenced success in this test. Visual discrimination performance was also impaired in damselfish experimentally infected with gnathiid (Crustacea: Isopoda) ectoparasites. Neither cleaner absence nor gnathiid infection affected performance in spatial recognition or reversal learning tests. Injection with immune-stimulating lipopolysaccharide did not affect visual discrimination performance relative to saline-injected controls, suggesting that cognitive impairments are not due to an innate immune response. Our results highlight the complex, indirect role of cleaning organisms in promoting the health of their clients via ectoparasite removal and emphasize the negative impact of parasites on host's cognitive abilities.

The arginine-vasotocin and serotonergic systems affect interspecific social behaviour of client fish in marine cleaning mutualism.

Many species engage in mutualistic relationships with other species. The physiological mechanisms that affect the course of such social interactions are little understood. In the cleaning mutualism, cleaner fish Labroides dimidiatus do not always act cooperatively by eating ectoparasites, but sometimes cheat by taking bites of mucus from so-called "client" reef fish. The physiological mechanisms in these interspecific interactions, however, are little studied. Here, we focussed on three neuromodulator systems known to play important roles in intraspecific social behaviour of vertebrates to examine their role in clients' interspecific behaviour. We subjected the client fish Scolopsis bilineatus to ectoparasites and the exogenous manipulation of the vasotocin (AVT), isotocin (IT) and serotonin systems to test how this affects client willingness to seek cleaning and client aggression towards cleaners. We found that a single dose of AVT agonist and a selective antagonist caused clients to seek proximity to cleaners, independently of ectoparasite infection. In contrast, in a direct encounter task, the selective blocker of serotonin 5HT2A/2C receptors, Ketanserin (KET), made client reef fish more aggressive towards cleaners in the absence of cleaners' bites of mucus. IT did not yield any significant effects. Our results suggest that the AVT system plays a role in social affiliation towards an interspecific partner, while the serotonin system affects clients' acceptance of level of proximity to cleaner fish during interactions. These two systems, therefore, were apparently co-opted from intraspecific social interactions to affect the course of interspecific ones also.

Dopamine disruption increases negotiation for cooperative interactions in a fish.

Humans and other animals use previous experiences to make behavioural decisions, balancing the probabilities of receiving rewards or punishments with alternative actions. The dopaminergic system plays a key role in this assessment: for instance, a decrease in dopamine transmission, which is signalled by the failure of an expected reward, may elicit a distinct behavioural response. Here, we tested the effect of exogenously administered dopaminergic compounds on a cooperative vertebrate's decision-making process, in a natural setting. We show, in the Indo-Pacific bluestreak cleaner wrasse Labroides dimidiatus, that blocking dopamine receptors in the wild induces cleaners to initiate more interactions with and to provide greater amounts of physical contact to their client fish partners. This costly form of tactile stimulation using their fins is typically used to prolong interactions and to reconcile with clients after cheating. Interestingly, client jolt rate, a correlate of cheating by cleaners, remained unaffected. Thus, in low effective dopaminergic transmission conditions cleaners may renegotiate the occurrence and duration of the interaction with a costly offer. Our results provide first evidence for a prominent role of the dopaminergic system in decision-making in the context of cooperation in fish.

Presence of cleaner wrasse increases the recruitment of damselfishes to coral reefs.

Mutualisms affect the biodiversity, distribution and abundance of biological communities. However, ecological processes that drive mutualism-related shifts in population structure are often unclear and must be examined to elucidate how complex, multi-species mutualistic networks are formed and structured. In this study, we investigated how the presence of key marine mutualistic partners can drive the organisation of local communities on coral reefs. The cleaner wrasse, Labroides dimidiatus, removes ectoparasites and reduces stress hormones for multiple reef fish species, and their presence on coral reefs increases fish abundance and diversity. Such changes in population structure could be driven by increased recruitment of larval fish at settlement, or by post-settlement processes such as modified levels of migration or predation. We conducted a controlled field experiment to examine the effect of cleaners on recruitment processes of a common group of reef fishes, and showed that small patch reefs (61-285 m(2)) with cleaner wrasse had higher abundances of damselfish recruits than reefs from which cleaner wrasse had been removed over a 12-year period. However, the presence of cleaner wrasse did not affect species diversity of damselfish recruits. Our study provides evidence of the ecological processes that underpin changes in local population structure in the presence of a key mutualistic partner.

Forebrain neuropeptide regulation of pair association and behavior in cooperating cleaner fish.

Animals establish privileged relationships with specific partners, which are treated differently from other conspecifics, and contribute to behavioral variation. However, there is limited information on the underlying physiological mechanisms involved in the establishment of these privileged ties and their relationship to individual cooperation levels. The Indo-Pacific bluestreak cleaner wrasse Labroides dimidiatus often forages in mixed-sex pairs when cleaning fish clients. Intra-couple conflicts often arise during a joint client inspection, which may alter the overall quality of cleaning service provided. Here we tested two hypotheses: a) whether intra-pair association (i.e. association index), measured with joint interspecific cleaning and intraspecific behavior, is correlated with neuroendocrine mechanisms involving forebrain neuropeptides arginine vasotocin (AVT) and isotocin (IT) and b) whether these neuropeptide level shifts relate to an individual's interspecific service quality. We found that partner support (number of cleaning interactions and tactile stimulation) received by male cleaners increased with association index. When cleaners inspected clients alone, cleaners' cheating decreased with association index for females but not males. AVT levels did not differ according to sex or association level. Forebrain IT levels increased with association index for males, whereas no relationship was found for females. Finally, cleaner cheating varied between sex and forebrain IT levels. Findings indicate that variation in pairs' relationships influences male and female cleaner fish differently and contributes to the variation of brain neuropeptide levels, which is linked to distinct cooperative outcomes.

Cortisol mediates cleaner wrasse switch from cooperation to cheating and tactical deception.

Recent empirical research, mostly done on humans, recognizes that individuals' physiological state affects levels of cooperation. An individual's internal state may affect the payoffs of behavioural alternatives, which in turn could influence the decision to either cooperate or to defect. However, little is known about the physiology underlying condition dependent cooperation. Here, we demonstrate that shifts in cortisol levels affect levels of cooperation in wild cleaner wrasse Labroides dimidiatus. These cleaners cooperate by removing ectoparasites from visiting 'client' reef fishes but prefer to eat client mucus, which constitutes cheating. We exogenously administrated one of three different compounds to adults, that is, (a) cortisol, (b) glucocorticoid receptor antagonist mifepristone RU486 or (c) sham (saline), and observed their cleaning behaviour during the following 45min. The effects of cortisol match an earlier observational study that first described the existence of "cheating" cleaners: such cleaners provide small clients with more tactile stimulation with their pectoral and pelvic fins, a behaviour that attracts larger clients that are then bitten to obtain mucus. Blocking glucocorticoid receptors led to more tactile stimulation to large clients. As energy demands and associated cortisol concentration level shifts affect cleaner wrasse behavioural patterns, cortisol potentially offers a general mechanism for condition dependent cooperation in vertebrates.

Ultraviolet-B wavelengths regulate changes in UV absorption of cleaner fish Labroides dimidiatus mucus.

High-energy wavelengths in the ultraviolet-B (UVB, 280-315 nm) and the UVA (315-400-nm) portion of the spectrum are harmful to terrestrial and aquatic organisms. Interestingly, UVA is also involved in the repair of UV induced damage. Organisms living in shallow coral reef environments possess UV absorbing compounds, such as mycosporine-like amino acids, to protect them from UV radiation. While it has been demonstrated that exposure to UV (280-400 nm) affects the UV absorbance of fish mucus, whether the effects of UV exposure vary between UVB and UVA wavelengths is not known. Therefore, we investigated whether the UVB, UVA, or photosynthetically active radiation (PAR, 400-700 nm) portions of the spectrum affected the UV absorbance of epithelial mucus and Fulton's body condition index of the cleaner fish Labroides dimidiatus. We also compared field-measured UV absorbance with laboratory based high-performance liquid chromatography measurements of mycosporine-like amino acid concentrations. After 1 week, we found that the UV absorbance of epithelial mucus was higher in the UVB+UVA+PAR treatment compared with the UVA+PAR and PAR only treatments; after 2 and 3 weeks, however, differences between treatments were not detected. After 3 weeks, Fulton's body condition index was lower for fish in the UVB+UVA+PAR compared with PAR and UVA+PAR treatments; furthermore, all experimentally treated fish had a lower Fulton's body condition index than did freshly caught fish. Finally, we found a decrease with depth in the UV absorbance of mucus of wild-caught fish. This study suggests that the increase in UV absorbance of fish mucus in response to increased overall UV levels is a function of the UVB portion of the spectrum. This has important implications for the ability of cleaner fish and other fishes to adjust their mucus UV protection in response to variations in environmental UV exposure.

Power and temptation cause shifts between exploitation and cooperation in a cleaner wrasse mutualism.

In many instances of cooperation, only one individual has both the potential and the incentive to 'cheat' and exploit its partner. Under these asymmetric conditions, a simple model predicts that variation in the temptation to cheat and in the potential victim's capacity for partner control leads to shifts between exploitation and cooperation. Here, we show that the threat of early termination of an interaction was sufficient to induce cleaner wrasse Labroides dimidiatus to feed selectively against their preference (which corresponds to cooperatively eating client fish ectoparasites), provided that their preference for alternative food was weak. Under opposite conditions, cleaners fed selectively according to their own preference (which corresponds to cheating by eating client mucus). By contrast, a non-cleaning fish species, Halichoeres melanurus, failed to adjust its foraging behaviour under these same conditions. Thus, cleaners appear to have evolved the power to strategically adjust their levels of cooperation according to the circumstances.

Gnathia aureamaculosa, a likely definitive host of Haemogregarina balistapi and potential vector for Haemogregarina bigemina between fishes of the Great Barrier Reef, Australia.

Investigations to determine whether juvenile gnathiid isopods are vectors of haemogregarines between coral reef fishes were undertaken at Lizard Island, Australia. Haemogregarina balistapi parasitaemias in triggerfish, Rhinecanthus aculeatus, decreased under gnathiid-free, laboratory conditions, compared with those in tagged R. aculeatus returned to the reef. Gnathia aureamaculosa juveniles were common ectoparasites of reef R. aculeatus and laboratory reared specimens of this gnathiid were fed on R. aculeatus infected with H. balistapi. Subsequent squashes of this gnathiid contained haemogregarine gamonts similar to those seen in blood films of R. aculeatus, and haemogregarine developmental stages, including oocysts, sporozoites, meronts and merozoites. Biological transmission of H. balistapi and a second haemogregarine species, Haemogregarina bigemina, using laboratory reared gnathiids to several species of triggerfishes and surgeonfishes raised from larvae was then attempted. Investigations involved recipient fish ingesting, or being bitten by, G. aureamaculosa juveniles fed on donor fish with haemogregarines; control fish were exposed to gnathiids fed on uninfected donor fish. Subsequently, no haemogregarines were detected in recipient triggerfishes and controls were negative. However, a recipient surgeonfish, Acanthurus xanthopterus, which had ingested gnathiids likely infected with donor fish H. balistapi, carried H. bigemina-like stages. A second recipient surgeonfish, which had ingested gnathiids presumed to be infected with H. bigemina, also carried haemogregarine stages. Finally, a third surgeonfish apparently carried haemogregarines after gnathiids presumed to be infected with H. bigemina had bitten this fish, although not all gnathiids were recovered during the trials and the third infected surgeonfish may have also ingested gnathiids. The study provides strong evidence that G. aureamaculosa is the definitive host of H. balistapi, to our knowledge the first such observation from a coral reef environment. Although transmission of H. balistapi has not yet been demonstrated, laboratory trials tend to support the view that G. aureamaculosa is also a potential vector of H. bigemina between surgeonfish.

Arginine vasotocin regulation of interspecific cooperative behaviour in a cleaner fish.

In an interspecific cooperative context, individuals must be prepared to tolerate close interactive proximity to other species but also need to be able to respond to relevant social stimuli in the most appropriate manner. The neuropeptides vasopressin and oxytocin and their non-mammalian homologues have been implicated in the evolution of sociality and in the regulation of social behaviour across vertebrates. However, little is known about the underlying physiological mechanisms of interspecific cooperative interactions. In interspecific cleaning mutualisms, interactions functionally resemble most intraspecific social interactions. Here we provide the first empirical evidence that arginine vasotocin (AVT), a non-mammalian homologue of arginine vasopressin (AVP), plays a critical role as moderator of interspecific behaviour in the best studied and ubiquitous marine cleaning mutualism involving the Indo-Pacific bluestreak cleaner wrasse Labroides dimidiatus. Exogenous administration of AVT caused a substantial decrease of most interspecific cleaning activities, without similarly affecting the expression of conspecific directed behaviour, which suggests a differential effect of AVT on cleaning behaviour and not a general effect on social behaviour. Furthermore, the AVP-V1a receptor antagonist (manning compound) induced a higher likelihood for cleaners to engage in cleaning interactions and also to increase their levels of dishonesty towards clients. The present findings extend the knowledge of neuropeptide effects on social interactions beyond the study of their influence on conspecific social behaviour. Our evidence demonstrates that AVT pathways might play a pivotal role in the regulation of interspecific cooperative behaviour and conspecific social behaviour among stabilized pairs of cleaner fish. Moreover, our results suggest that the role of AVT as a neurochemical regulator of social behaviour may have been co-opted in the evolution of cooperative behaviour in an interspecific context, a hypothesis that is amenable to further testing on the potential direct central mechanism involved.

Male cleaner wrasses adjust punishment of female partners according to the stakes.

Punishment is an important deterrent against cheating in cooperative interactions. In humans, the severity of cheating affects the strength of punishment which, in turn, affects the punished individual's future behaviour. Here, we show such flexible adjustments for the first time in a non-human species, the cleaner wrasse (Labroides dimidiatus), where males are known to punish female partners. We exposed pairs of cleaners to a model client offering two types of food, preferred 'prawn' items and less-preferred 'flake' items. Analogous to interactions with real clients, eating a preferred prawn item ('cheating') led to model client removal. We varied the extent to which female cheating caused pay-off reduction to the male and measured the corresponding severity of male punishment. Males punished females more severely when females cheated during interactions with high value, rather than low value, model clients; and when females were similar in size to the male. This pattern may arise because, in this protogynous hermaphrodite, cheating by similar-sized females may reduce size differences to the extent that females change sex and become reproductive competitors. In response to more severe punishment from males, females behaved more cooperatively. Our results show that punishment can be adjusted to circumstances and that such subtleties can have an important bearing on the outcome of cooperative interactions.

Tactile stimulation lowers stress in fish.

In humans, physical stimulation, such as massage therapy, reduces stress and has demonstrable health benefits. Grooming in primates may have similar effects but it remains unclear whether the positive effects are due to physical contact or to its social value. Here we show that physical stimulation reduces stress in a coral reef fish, the surgeonfish Ctenochaetus striatus. These fish regularly visit cleaner wrasses Labroides dimidiatus to have ectoparasites removed. The cleanerfish influences client decisions by physically touching the surgeonfish with its pectoral and pelvic fins, a behaviour known as tactile stimulation. We simulated this behaviour by exposing surgeonfish to mechanically moving cleanerfish models. Surgeonfish had significantly lower levels of cortisol when stimulated by moving models compared with controls with access to stationary models. Our results show that physical contact alone, without a social aspect, is enough to produce fitness-enhancing benefits, a situation so far only demonstrated in humans.