Cleaner wrasse prefer client mucus: support for partner control mechanisms in cleaning interactions

Recent studies on cleaning behaviour suggest that there are conflicts between cleaners and their clients over what cleaners eat. The diet of cleaners usually contains ectoparasites and some client tissue. It is unclear, however, whether cleaners prefer client tissue over ectoparasites or whether they include client tissue in their diet only when searching for parasites alone is not profitable. To distinguish between these two hypotheses, we trained cleaner fish Labroides dimidiatus to feed from plates and offered them client mucus from the parrotfish Chlorurus sordidus, parasitic monogenean flatworms, parasitic gnathiid isopods and boiled flour glue as a control. We found that cleaners ate more mucus and monogeneans than gnathiids, with gnathiids eaten slightly more often than the control substance. Because gnathiids are the most abundant ectoparasites, our results suggest a potential for conflict between cleaners and clients over what the cleaner should eat, and support studies emphasizing the importance of partner control in keeping cleaning interactions mutualistic.     

Punishment and partner switching cause cooperative behaviour in a cleaning mutualism

What are the mechanisms that prevent partners from cheating in potentially cooperative interactions between unrelated individuals? The cleaner fish Labroides dimidiatus and client reef fish both benefit from an interaction as long as the cleaner eats ectoparasites. However, the cleaner fish prefers some client mucus, which constitutes cheating. Field observations suggested that clients control such cheating by using punishment (chasing the cleaner) or by switching partners (fleeing from the cleaner). Here, we tested experimentally whether such client behaviours result in cooperative cleaner fish. Cleaners were allowed to feed from Plexiglas plates containing prawn items and fish flake items. A lever attached to the plates allowed us to mimic the behaviours of clients. As cleaners showed a strong preference for prawn over flakes, we taught them that eating their preferred food would cause the plate to either chase them or to flee, while feeding on flakes had no negative consequences. We found a significant shift in cleaner fish foraging behaviour towards flake feeding after six learning trials. As punishment and terminating an interaction resulted in the cleaners feeding against their preferences in our experiment, we propose that the same behaviours in clients improve the service quality of cleaners under natural conditions.     

Degrees of honesty: cleaning by the redlip cleaner wrasse Labroides rubrolabiatus

Cleaning symbioses among coral reef fishes are highly variable. Cleanerfishes vary in how much they cooperate with (i.e. remove only ectoparasites) or cheat (i.e. bite healthy tissue, scales or mucus) on their fish clients. As a result, clients use various strategies to enforce cooperation by cleaners (e.g. punishment or partner choice), and cleaners use tactile stimulation to manipulate cheated client behaviour. We provide the first detailed observations of cleaning behaviour of the redlip cleaner wrasse Labroides rubrolabiatus and ask where interactions with this cleanerfish lie on the continuum of cleanerfish honesty, client control, and cleanerfish manipulation. Ninety per cent of redlip cleaner wrasses took jolt-inducing cheating bites from their clients, but they did so at a very low rate (~ 2 jolts per 100 s inspection). Retaliatory chases by clients were uncommon. Three-quarters (30 of 40) of cleaner wrasses used tactile stimulation on their clients, but rarely did so to reconcile with cheated clients. Instead, the majority (70%) of tactile stimulation events targeted a passing client that then stopped for inspection. The relationship between redlip cleaner wrasses and their clients appears to be less conflictual than those documented in other Labroides cleanerfishes. Future studies should test whether this low level of conflict is consistent across space and time and is underpinned by a preference for ectoparasites over other client-gleaned items. As an active cleaner that appears to take few cheating bites from their clients, L. rubrolabiatus has the potential to be as important a driver of fish health and community structure on coral reefs as its better-known relatives.

     

Asymmetric cheating opportunities and partner control in a cleaner fish mutualism

How can cooperation persist if, for one partner, cheating is more profitable than cooperation in each round, while the other partner has no option to cheat? Our laboratory experiments suggest that such a situation exists between the cleaner fish Labroides dimidiatus and its nonpredatory client reef fish species, which actively seek cleaners to have their ectoparasites removed. Clients Ctenochaetus striatus regularly jolted in response to cleaner mouth contact, and these jolts were not linked to the removal of parasites. In addition, cleaners did not search for parasites but fed on mucus when exposed to anaesthetized clients, which could not control the cleaners' behaviour. Field data showed that clients often terminated an interaction immediately after a jolt. Client species with access to only one cleaning station, owing to their small territories or home ranges, terminated interactions mainly by chasing cleaners while clients with access to two or more cleaning stations mainly swam away. Thus, the chasing of cleaners appeared to be a form of punishment, imposing costs on the cleaner at the client's (momentary) expense. Chasing yields future benefits, as jolts were on average less frequent during interactions between cleaners and individuals that had terminated their previous interaction by aggressive chasing.     

The Meaning of Jolts by Fish Clients of Cleaning Gobies

Cooperative interactions offer the inherent possibility of cheating by each of the interacting partners. A key challenge to behavioural observers is to recognize these conflicts, and find means to measure reliably cheating in natural interactions. Cleanerfish Labroides dimidiatus cheat by taking scales and mucus from their fish clients and such dishonest cleaning has been previously recognized in the form of whole‐body jolts by clients in response to cleaner mouth contact. In this study, we test whether jolts may be a general client response to cheating by cleaners. We experimentally varied the ectoparasite loads of yellowtail damselfish (Microspathodon chrysurus), a common client of the cleaning goby Elacantinus evelynae, and compared the rates of jolts on parasitized and deparasitized clients. As predicted if jolts represent cleaner cheating, deparasitized clients jolted more often than parasitized clients, and overall jolt rates increased over time as client parasite load was presumably reduced by cleaning activity. Yellowtail damselfish in the wild jolted significantly less frequently than those in captivity, which is consistent with a loss of ectoparasites during capture. Our results suggest that jolts by clients of cleaning gobies are not related to the removal of ectoparasites. Client jolts may therefore be a generally accurate measure of cheating by cleanerfish.     

Temporal Variation in Cleanerfish and Client Behaviour: Does It Reflect Ectoparasite Availability?

We tested the importance of ectoparasites as the proximate cause of cleaning interactions by comparing the activity of Caribbean cleaning gobies (Elacatinus evelynae) and of their clients during three daily periods (early morning, midday, and late afternoon) in which ectoparasite availability varied naturally. Emergence from the benthos of gnathiid isopod larvae, the main target of cleaning goby predation, was higher at night, when cleaners were inactive, than during the day. As a result, overall ectoparasite loads on client fish tended to be higher in the morning. Inspection bouts by cleaning gobies were longest in the morning, but also at midday when ectoparasite availability on clients was lower. Client fish were observed at cleaning stations most often in the afternoon, when they harboured few ectoparasites, but they were more likely to adopt incitation poses, which increase the likelihood of being cleaned, in the morning than later in the day. Most cleaner and client behaviours therefore did not change predictably in response to natural diurnal variation in ectoparasite availability. Our study suggests that the ultimate and proximate causes of cleaning behaviour need not necessarily coincide.     

Roving and Service Quality in the Cleaner Wrasse Labroides bicolor

The cleaner wrasse Labroides dimidiatus occupies fixed ‘cleaning stations’ on coral reefs, which ‘client’ reef fish visit repeatedly to have parasites removed. Conflict arises because cleaners prefer to cheat by feeding on client mucus instead of parasites. Clients can prevent L. dimidiatus from always cheating using control mechanisms such as chasing and partner switching, which depend on repeated interactions. These control mechanisms would be undermined in the absence of frequent repeated interactions, if cleaners roved over large areas. Roving behaviour has been anecdotally described for the closely related cleaner wrasse Labroides bicolor. Here we report field data comparing these two species in Moorea, French Polynesia. Our results confirmed that L. bicolor home ranges are much larger than L. dimidiatus home ranges, and showed that cleaning interactions occurred all over the L. bicolor home range: home range of cleaning interactions increased with total home range size. Moreover, we found that cleaner initiation of interactions increased with home range size in L. bicolor, which would give L. bicolor with large home ranges additional leverage to increase cheating. In line with these results, we found that client jolt rate (used as a measure of cheating) was higher among clients of cleaners with large home ranges. Our results emphasise the importance of game structure and control over initiating interactions as parameters in determining the nature of interactions in mutualisms.     

Geographic Variation in the Behaviour of the Cleaner Fish Labroides dimidiatus (Labridae)

Geographical variation in the outcome of interspecific interactions has a range of proximate ecological causes. For instance, cleaning interactions between coral reef fishes can result in benefits for both the cleaner and its clients. However, because both parties can cheat and because the rewards of cheating may depend on the local abundance of ectoparasites on clients, the interaction might range from exploitative to mutualistic. In a comparative analysis of behavioural measures of the association between the cleaner fish Labroides dimidiatus and all its client species, we compared cleaning interactions between two sites on the Great Barrier Reef that differ with respect to mean ectoparasite abundance. At Heron Island, where client fish consistently harbour fewer ectoparasites, client species that tended to pose for cleaners were more likely to receive feeding bites by cleaners than client species that did not pose for cleaners. This was not the case at Lizard Island, where ectoparasites are significantly more abundant. Client fish generally spent more time posing for cleaners at Lizard Island than their conspecifics at Heron Island. However, fish at Heron Island were inspected longer on average by cleaners than conspecifics at Lizard Island, and they incurred more bites and swipes at their sides per unit time from cleaners. These and other differences between the two sites suggest that the local availability of ectoparasites as a food source for cleaners may determine whether clients will seek cleaning, and whether cleaners will feed on parasites or attempt to feed on client mucus. The results suggest that cleaning symbiosis is a mosaic of different outcomes driven by geographical differences in the benefits for both participants.     

Short‐Term Variation in the Level of Cooperation in the Cleaner Wrasse Labroides dimidiatus: Implications for the Role of Potential Stressors

There is a wealth of game theoretical approaches to the evolution and maintenance of cooperation between unrelated individuals and accumulating empirical tests of these models. This contrasts strongly with our lack of knowledge on proximate causes of cooperative behaviour. Marine cleaning mutualism has been used as a model system to address functional aspects of conflict resolution: client reef fish benefit from cleaning interactions through parasite removal, but cleaner fish Labroides dimidiatus prefer client mucus. Hence, feeding against their preference represents cooperative behaviour in cleaners. Cleaners regularly cheat non‐predatory clients while they rarely cheat predatory clients. Here, we asked how precisely cleaners can adjust service quality from one interaction to the next. We found that non‐predatory clients receive a better service if the previous client was a predator than if the previous client was a non‐predator. In a related laboratory experiment, a hand‐net used as a stressor resulted in cleaners feeding more against their preference in subsequent interactions. The combination of the cleaners’ behaviour in the two studies shows that the cleaners’ service quality for a given client species is not fixed, but it can be manipulated. The results suggest that short‐term stress is one factor that causes cleaners to increase their levels of cooperation, a hypothesis that is amenable to further experiments manipulating the endocrine system.     

Cleaner fish Labroides dimidiatus manipulate client reef fish by providing tactile stimulation.

The cleaner wrasse Labroides dimidiatus often touches 'client' reef fish dorsal fin areas with its pelvic and pectoral fins. The relative spatial positions of cleaner and client remain constant and the cleaner's head points away from the client's body. Therefore, this behaviour is not compatible with foraging and the removal of client ectoparasites. As clients seek such 'tactile stimulation', it can be classified as an interspecific socio-positive behaviour. Our field observations on 12 cleaners (observation time of 112h) suggest that cleaners use tactile stimulation in order to successfully (i) alter client decisions over how long to stay for an inspection, and (ii) stop clients from fleeing or aggressive chasing of the cleaner in response to a cleaner fish bite that made them jolt. Finally, predatory clients receive tactile stimulation more often than non-predatory clients, which might be interpreted as an extra service that cleaners give to specific partners as pre-conflict management, as these partners would be particularly dangerous if they started a conflict. We therefore propose that cleaner fish use interspecific social strategies, which have so far been reported only from mammals, particularly primates.     

Cleaning Activity of Juvenile Angelfish, Pomacanthus paru, on the Reefs of the Abrolhos Archipelago, Western South Atlantic

Studies on fish cleaning symbiosis in the tropical western Atlantic concentrate on specialized cleaner gobies and wrasses. On the reefs of the Abrolhos Archipelago, off the eastern Brazilian coast, juvenile french angelfish, Pomacanthus paru, clean a rich and varied community of fish clients. We recorded 31 reef fish species, including large predators such as groupers, jacks, and morays, being serviced by the french angelfish on cleaning stations situated mostly on seagrass flats. The angelfish performs a characteristic fluttering swimming at the station and, during cleaning events, touches the body of the clients with its pelvic fins. Frequency of encounters between the cleaner and its clients do not reflect the local abundance of client species; most of these move from the reefs to the sand flats to be cleaned. We found no correlation between client size and duration of cleaning events. The conspicuous black and yellow pattern, the fluttering swimming, the tenure of cleaning stations, the physical contact with the client, and the varied community of clients, qualify the juveniles of P. paru as specialized cleaners comparable to the gobies of the genus Elacatinus.     

Parasite infection rather than tactile stimulation is the proximate cause of cleaning behaviour in reef fish.

Cleaning behaviour is a popular example of non-kin cooperation. However, quantitative support for this is generally sparse and the alternative, that cleaners are parasitic, has also been proposed. Although the behaviour involves some of the most complex and highly developed interspecific communication signals known, the proximate causal factors for why clients seek cleaners are controversial. However, this information is essential to understanding the evolution of cleaning. I tested whether clients seek cleaners in response to parasite infection or whether clients seek cleaners for tactile stimulation regardless of parasite load. Parasite loads on client fish were manipulated and clients exposed to cleaner fish and control fish behind glass. I found that parasitized client fish spent more time than unparasitized fish next to a cleaner fish. In addition, parasitized clients spent more time next to cleaners than next to control fish, whereas unparasitized fish were not attracted to cleaners. This study shows, I believe for the first time, which is somewhat surprising, that parasite infection alone causes clients to seek cleaning by cleaners and provides insight into how this behaviour evolved.     

Crime and punishment in a roaming cleanerfish

Cheating is common in cooperative interactions, but its occurrence can be controlled by various means ranging from rewarding cooperators to active punishment of cheaters. Punishment occurs in the mutualism involving the cleanerfish Labroides dimidiatus and its reef fish clients. When L. dimidiatus cheats, by taking scales and mucus rather than ectoparasites, wronged clients either chase or withhold further visits to the dishonest cleaner, which leads to more cooperative future interactions. Punishment of cheating L. dimidiatus may be effective largely because these cleaners are strictly site-attached, increasing the potential for repeated interactions between individual cleaners and clients. Here, we contrast the patterns of cheating and punishment in L. dimidiatus with its close relative, the less site-attached Labroides bicolor. Overall, L. bicolor had larger home ranges, cheated more often and, contrary to our prediction, were punished by cheated clients as frequently as, and not less often than, L. dimidiatus. However, adult L. bicolor, which had the largest home ranges, did not cheat more than younger conspecifics, suggesting that roaming, and hence the frequency of repeated interactions, has little influence on cheating and retaliation in cleaner–client relationships. We suggest that roaming cleaners offer the only option available to many site-attached reef fish seeking a cleaning service. This asymmetry in scope for partner choice encourages dishonesty by the partner with more options (i.e. L. bicolor), but to be cleaned by a cleaner that sometimes cheats may be a better option than not to be cleaned at all.     

The cost of cleanerfish mimics to their models

In aggressive mimicry, a 'predatory' species resembles a model that is harmless or beneficial to a third species, the 'dupe'. Perhaps the most extraordinary case of aggressive mimicry occurs in Indo‐Pacific cleaning symbioses, where cleaner wrasses (the models) remove ectoparasites from larger fish clients. Several species of fangblennies mimic cleaners in behaviour and coloration. Instead of removing ectoparasites, however, fangblennies tear off fins, skin and scales from unsuspecting clients (the dupes). There is some debate over the extent to which cleanerfish mimics are really mimics because in some populations, the contribution of fish tissue to fangblenny diet is limited. In this study, I examine the impact of the resemblance between bluestriped fangblennies ( Plagiotremus rhinorhynchus ) and its putative model, the juvenile bluestreak cleaner wrasse ( Labroides dimidiatus ), on the model's cleaning activity to test the theoretical prediction that mimics should decrease the fitness of their models. I show that the presence of a bluestripe fangblenny in the vicinity of cleaner wrasses results in significantly lower client visit rates and inspection times compared to cleaners without a fangblenny nearby, and discuss why cleaner wrasses tolerate mimics near cleaning stations.     

Treatment with the glucocorticoid antagonist RU486 reduces cooperative cleaning visits of a common reef fish, the lined bristletooth

Cooperation often involves a conflict of interest. This is particularly true in situations where one individual seeks out a service but cannot properly control the quality of the service given by the partner who would gain from defecting. An example is cleaning mutualism involving the bluestreak cleaner wrasse (Labroides dimidiatus) and its reef-fish 'clients'. These cleaners may reduce the stress experienced by their clients by removing parasites; however they occasionally cheat clients (i.e. defect) by eating mucus and other living tissues. Here we present experimental support for the hypothesis that stress responses increase the motivation for clients to seek out such risky asymmetric interactions. We manipulated the stress response by blocking glucocorticoid receptors with the antagonist RU486 in a species that is a regular visitor of cleaner fish, the lined bristletooth (Ctenochaetus striatus). Field observations 1 week after treatment with RU486 showed that antagonist treatment led to a reduction in cleaning duration compared to control treatment. This was not explained by a general effect on client behavior as intraspecific social behavior appeared unaffected. We propose that antagonist treatment reduced stress responses to the presence of ectoparasites, which in turn reduced the client's perception of benefits from seeking out cleaning interactions. The results demonstrate a hitherto overlooked variable role of stress and stress responses on cooperative behavior.     

Cleaner fish, Labroides dimidiatus, diet preferences for different types of mucus and parasitic gnathiid isopods

Cleaner fish, Labroides dimidiatus, prefer the mucus of the parrotfish, Chlorurus sordidus, to parasitic gnathiid isopods, the main items in their diet, indicating a major conflict between clients and cleaners over what the latter should eat during interactions. We tested whether the conflict varied with client species (and the quality of its mucus) and with the presence of blood in the gnathiids. First, we offered cleaners the choice between mucus of the parrotfish and that of the snapper, Lutjanus fulviflamma. When offered equal amounts of mucus on Plexiglas plates, cleaners readily developed a significant preference for the parrotfish mucus. Reducing the amount of parrotfish mucus by 75% made the preference disappear. In a second test, we offered the cleaners gnathiids that were or were not engorged with client fish blood. Cleaners showed no significant preference for either food item. Our results suggest that the degree of conflict between cleaners and clients may vary between species, depending on whether the latter have a preferred mucus. In contrast, the cleaners' lack of preference for engorged gnathiids benefits clients because it means that cleaners do not hesitate to eat unengorged gnathiids before the gnathiids harm the fish by removing blood or by transmitting blood parasites.     

Cleaning interactions at the only atoll in the South Atlantic

In marine ecosystems, cleaning is a mutualistic relationship in which so-called cleaners remove ectoparasites, diseased tissue, or mucus from the body of their clients, and thus help to maintain a healthy reef community. In spite of its importance in many marine habitats, this interaction remains poorly understood, particularly at oceanic islands. Here, we present the first comprehensive study of cleaning interactions in a reef fish assemblage at Rocas, the only atoll in the South Atlantic. We recorded 318 cleaning events, in which six fish species, including two endemic ones, and two shrimp species acted as cleaners. The clients serviced by these cleaners were 21 bony fish species, one shark and one sea turtle. The cleaner wrasse Thalassoma noronhanum and the cleaner goby Elacatinus phthirophagus were the cleaners with the greatest number of events and species richness of clients. Additionally, 82% of clients in the cleaning events were non-piscivores, and the abundance of both cleaners and clients positively influenced the number of cleaning events (R² = 0.4; p < 0.001). Our results indicate that Rocas atoll has a high species richness of cleaner species despite its small size and highlight the importance of studies of cleaning symbiosis, even in isolated places with low species richness, for a better comprehension of this association in reefs.     

Multiple cleaner species provide simultaneous services to coral reef fish clients

Cleaning symbioses on tropical coral reefs are typically documented between two species: a single client fish and one or more conspecific cleaners. However, multiple cleaner species living sympatrically in the Caribbean have been anecdotally reported to simultaneously clean the same client. Nothing is known about the patterns and processes driving these interactions, which may differ from those involving a single cleaner species. Here, we used remote underwater videography on three reefs in Honduras to record simultaneous cleaning interactions involving Pederson''s cleaner shrimp (Ancylomenes pedersoni) and cleaner gobies (Elacatinus spp.). A pilot study on adjacent shrimp and goby stations found interactions were always initiated by shrimp. A larger, multi-year dataset shows cleaner gobies joined 28% of all interactions initiated at A. pedersoni cleaning stations with cleaner gobies residing nearby. Client body size significantly predicted simultaneous cleaning interactions, with 45% of interactions simultaneous for clients greater than 20 cm total body length compared with only 8% for clients less than 20 cm. We also found that simultaneous cleaning interactions lasted over twice as long as shrimp-only interactions. We propose these novel multi-species interactions to be an ideal model system to explore broader questions about coexistence, niche overlap and functional redundancy among sympatric cleaner species.     

Cleaner wrasses Labroides dimidiatus are more cooperative in the presence of an audience

Humans may help others even in?situations where the recipient will not reciprocate [1-5]. In some cases, such behavior can be explained by the helpers increasing their image score, which will increase the probability that bystanders will help them in the future [5-7]. For other animals, the notion that many interactions take place in an environment containing an audience of eavesdropping bystanders has also been proposed to have important consequences for social behavior, including levels of cooperation [8]. However, experimental evidence is currently restricted to the demonstration that cleaner fish Labroides dimidiatus can learn to solve a foraging task [9]. The cleaners learned to feed against their preference on artificial clients if that allowed them to access additional artificial clients, which would translate into cooperatively eating ectoparasites rather than cheating by eating client mucus under natural conditions [10]. Here we show that cleaners immediately increase current levels of cooperation in the presence of?bystander client reef fish. Furthermore, we find that bystanders respond to any occurrence of cleaners cheating their current client with avoidance. In conclusion, the results demonstrate, for the first time, that image scoring by an audience indeed leads to increased levels of cooperation in a nonhuman animal.     

Does Competition for Clients Increase Service Quality in Cleaning Gobies?

In a biological market, members of one trading class try to outbid each other to gain access to the most valuable partners. Competition within class can thus force individuals to trade goods or services more cheaply, ultimately resulting in conflict (e.g. cheating) over the value of commodities. Cleaning symbioses among fish appear to be good examples of biological markets. However, the existence and effect of outbidding competition among either types of traders (cleaners or clients) have never been tested. We examined whether increasing competition among cleaning gobies ( Elacatinus spp.) for access to clients results in outbidding in the form of provision of a better cleaning service. On reefs where fish clients visited cleaning stations less frequently, and thus competition among cleaners was higher, cleaning gobies ingested fewer scales relative to the number of ingested parasites, i.e. they cleaned more honestly. This shift in cleaner behaviour towards greater honesty is consistent with a greater market value of access to clients in the face of competition among cleaners. However, this pattern could have also arisen as a result of differences in ectoparasite availability across reefs and therefore in value of the commodity offered by clients. Experimental manipulations will be required to determine whether cleaning service quality by cleaning gobies was enhanced solely because of competitive outbidding.