The Cleaner Fish Chronicles

Cleaner Fish Do Clean!

Summary

'Cleaner fish' are well known to biologists and tourists on the reef. Just what they do though is controversial. Through close observation of the biology of parasites and the cleaning behaviour of client fish, as well as the removal of cleaner fish from small reefs and counts of parasites and fish on these reefs compared to undisturbed reefs, we showed that cleaner fish do indeed have a dramatic effect on the numbers of fish parasites which likely benefit their fish clients. Furthermore, we showed that cleaning behaviour can involve complex cooperative and cognitive behaviours that had generally been assumed to occur only in primates and perhaps only in humans. Finally, we showed that cleaning interactions involve unusual behavioural and colour signals.

Below is a chronology of some of the work we have done involving cleaning behaviour by various cleaner fish and a cleaner shrimp.

Project description

For the past 19 years, we have been studying cleaning behaviours on the Great Barrier Reef. Fish cleaning behaviour involves cleaner fish pecking away at the bodies of fish (clients). Often, clients 'pose' motionless, spreading out their fins to give cleaners access. Some even allow cleaners to enter their mouths and gills - this is especially dramatic when the clients are large predators! Although cleaning interactions are extremely common, until recently there has been much controversy on why client fish seek the services of cleaners and whether parasites motivate this
behaviour.

Fish get cleaned a lot and cleaner fish eat many parasites (gnathiid isopods)

Between 1994 and 1996, we found that a single cleaner fish Labroides dimidiatus inspects more than 2,300 client fish a day from over 130 species and that amazingly, each cleaner fish eats about 1,200 parasites daily. Interestingly, cleaners preferentially eat gnathiid isopod larvae, parasites similar to ticks on land. These, we found, are one of the most common parasites of coral reef fish, but because they are so mobile they had been missed in most previous parasite surveys of fish. By following fish in the field, we determined that most fish are cleaned daily, with individuals of one species (a rabbitfish) seeking cleaners around 144 times a day. This works out to an individual being cleaned every 5 min!

Unrolling net to catch fish for parasite load estimates

Gnathiid isopod. Photo by Peter Parks

Does cleaning affect client parasite loads?

This of course raised the question of whether cleaning then reduces parasite loads on fish. Our early attempts in 1996-1997 to test whether removing cleaner fish from reefs for 6 months affected parasite loads and fish numbers found that these were not affected by cleaner fish presence. However, because in 1998 we had noticed that fish tended to have more gnathiid isopods at dawn than at sunset, we decided to use a different approach to look at this question.

Parasites attack fish day and night

In 1999, we placed fish that normally have relatively high loads of gnathiids in cages on coral reefs. This revealed that fish are attacked by gnathiids at a very rapid rate, but at a higher rate in the late afternoon and at night. Fish are therefore attacked by many of these parasites each day. However, since we had found that gnathiid abundance on wild fish declined between dawn and sunset we wondered whether this decline was due to the actions of cleaners.

Cleaning dramatically reduces parasite loads over 12 hours!!!

To test this, we placed caged fish on reefs with cleaner fish and on reefs with all cleaner fish removed and found that without cleaners, parasite numbers increased five-fold between dawn and sunset (12 hours!). This suggests that cleaners cause the daily decline of parasites we observed on wild fish. This is the first study to experimentally show that cleaners affect the abundance of parasites on fish and supports the idea that interactions between cleaner fish and clients are mutually beneficial.

Lexa feeding fish in cages

Parasitised fish seeking cleaning services from cleaner fish in aquarium

Parasite infection stimulates client cleaning behaviour

While previous studies suggested that clients sought cleaner fish for the rewarding tactile stimulation the cleaner fish provided (they often gently rub clients with the pelvic fins during cleaning interactions), in 2001 we found that it was parasite infection, not tactile stimulation, which motivated fish to seek cleaner fish.

Redouan Bshary getting ready to catch cleaner fish for experiments

Cleaner fish engage in complex behaviour

In 2002, in a series of experiments, we showed that cleaning behaviour can be used as a model system to understand the role that partner recognition, partner choice, and partner control play in cooperation among animals. We found that cleaner fish recognize familiar client fish, that clients which had been cheated by cleaners (i.e. bitten) choose to leave such cheaters, and clients which had been cheated controlled their cheating partners by punishing them with vigorous chases.

Cleaner fish presence affects client fish abundance and diversity

In 2003, we discovered that cleaner fish affect the abundance and diversity of reef fish. We found that in the absence of cleaner fish (all cleaner fish removed from reefs for 18 months), fish abundance and diversity was one-fourth and one-half, respectively, compared to that on reefs with cleaner fish. But only mobile fishes were affected with resident fishes not affected at all. Thus many fish appear to choose reefs based on the presence of cleaner fish and may leave if there are no cleaner fish.

Lagoon site showing experimental reefs (small reefs along bottom)

Casuarina Beach site showing experimental reefs (circle of small reefs in centre)

Parasitic gnathiid isopods feeding rates explain frequent client cleaning

A surprising finding that same year was that gnathiids fill up on fish blood very rapidly and only remain on fish for less than an hour. These, combined with other findings that they infect fish 24 hours a day, may explain why client fish seek cleaner repeatedly and at such short intervals. By going to cleaners often, it is more likely that the client's gnathiids will be removed, and that this will occur before gnathiids remove too much blood from the client.

Cleaner fish prefer fish mucus over gnathiid ispods

A discovery in 2004 that changed the way we viewed cleaning behaviour was the finding that, when given a choice of gnathiids or mucus (offered on plates), trained cleaner fish surprisingly preferred the mucus. Since mucus provides valuable benefits to the client, this suggested that such a preference by the cleaner was in conflict with the client's needs, and supports observations emphasizing the importance of partner control in keeping cleaning interactions mutualistic. Another study showed they preferred parrotfish over snapper mucus, suggesting the degree of conflict between cleaners and clients may vary among client species.

Cleaner fish choosing mucus over ganthiids

Cleaner fish choosing parrotfish mucus over snapper mucus

Tactile stimulation while dancing prevents conflicts with predators

In cleaning interactions, the classical question asked is why cleaner fish can clean piscivorous client-fish without being eaten. In 2004, we showed that cleaner-fish tactically stimulate clients while swimming in an oscillating ‘dancing' manner (tactile dancing) more when exposed to hungry piscivorous clients than satiated ones, regardless of the client's parasite load. Tactile dancing thus may function as a pre-conflict management strategy that enables cleaner fish to avoid conflict with potentially ‘dangerous' clients. How cleaner fish can tell a client is hungry, however, remains a mystery.

Fabian cleaning piscivorous fish holding tanks

Rocking dance in cleaner advertises cleaning services

The following year, we found that a “rocking dance” is used by cleaner shrimp to advertise cleaning services. Shrimp “rock dance” when approaching potential client fish and do so more when they are hungry. When given a choice, clients preferred hungry, rocking shrimp. The rocking dance therefore influenced client behaviour and thus appears to function as a signal to advertise the presence of cleaner shrimp to potential clients.

Rock dancing cleaner shrimp

Justine observing cleaner shrimp and client fish

The Coral Reef Ecology Lab on Lizard Island, summer of 2005

Client chasing makes cleaner fish behave

Cleaner fish sometimes cheat and eat client mucus or skin. Field observations suggest that clients control such cheating by using punishment (chasing the cleaner) or by switching partners (fleeing from the cleaner). Therefore, in 2005, we tested experimentally whether such client behaviours result in cooperative cleaner fish. Cleaners were allowed to feed from 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, this suggested that the same behaviours in clients improve the service quality of cleaners under natural conditions.

Eavesdropping pays off in cleaning interactions

In 2006, we found the first experimental evidence for ‘simple' indirect reciprocity in animals. We found that eavesdropping clients spent more time next to ‘cooperative' compared with ‘non-cooperative' cleaners which shows clients engage in image scoring behaviour. Furthermore, trained cleaners learned to feed against their preference – which corresponds to cooperatively eating ectoparasites rather than uncooperatively eating client mucus in the wild - in an ‘image scoring' context.

Gnathiid isopods implicated as vectors of blood parasites

That same year, we also found that some coral reef fish have blood parasites (haemogregarines), and that gnathiid isopods pick up these infections when feeding on the blood of these fishes. This suggested gnathiids might transmit these infections between fishes, much like mosquitoes transmit malaria. This is currently being examined Lynda Curtis.

Blood parasites from a triggerfish

Lynda, Ange, Rachel, Lexa, Conor and Nico working on isopods

Maxi doing cleaner fish sunscreen experiments

More clients means more cleaning

In 2007, using a meta-analysis of client-cleaner interactions involving 11 cleaner organisms from Brazil, the Caribbean, the Mediterranean and Australia, we found that there was a strong, positive effect of client abundance on cleaning frequency, but only a weak, negative effect of client body size. These effects were modulated by client trophic group and social behaviour. This study adds to a growing body of evidence suggesting a central role of species abundance in structuring species interactions.

Cleaner fish have super sunscreens

Coral reef fishes were recently discovered to have ultraviolet radiation (UV) screening compounds, most commonly known as mycosporine-like amino acids (MAAs), in their external body mucus. However, little is known about the identity and quantity of MAAs in the mucus of reef fish or what factors affect their abundance and distribution. Therefore, in 2008, we examined these using 7 coral fishes, including the cleaner fish Labroides dimidiatus. MAAs were found in the mucus of all the fishes. Interestingly, in comparison to most of the other species, cleaner fish had a relatively high concentration of all MAAs. Since fishes cannot produce their own MAAs but must obtain them via their diet, it raised the question of the source of MAAs in L. dimidiatus. This is currently being examined.

Cleaning stations are safe havens

Cleaner fish are thought to benefit from immunity to predation and use tactile stimulation as a pre-conflict management strategy to manipulate partners' decisions and to avoid being eaten by piscivorous client fish. In 2008, we showed that the presence of cleaner fish resulted in nearby fish not involved in the cleaner–client mutualism experiencing less aggression (chases) from predatory clients. In addition, the rate that predatory clients chased prey was negatively correlated with the amount of tactile stimulation given to the predator by the cleaner. These data suggest that, in the laboratory, the risk of aggression from predators toward nearby prey fish was greatly reduced as a by-product of cleaner fish presence and tactile stimulation of predators by cleaner fish. These results raise the question of whether cleaning stations act as safe havens from predator aggression.

Karen Cheney observing cleaning behaviour in tubs

Cleaner fish mimics can change their colours

Facultative mimicry, the ability to switch between mimic and non-mimic colours at will, is uncommon in the animal kingdom, but has been shown in a cephalopod, and recently by us in a marine fish, the bluestriped fangblenny Plagiotremus rhinorhynchos, an aggressive mimic of the juvenile cleaner fish Labroides dimidiatus. In 2008, we demonstrated for the first time that fangblennies adopted mimic colours in the presence of juvenile cleaner fish; however, this only occurred in smaller individuals. Field data indicated that when juvenile cleaner fish were abundant, the proportion of mimic to non-mimic fangblennies was greater, suggesting that fangblennies adopt their mimic disguise depending on the availability of cleaner fish.

Couples are better cleaners

In 2008, we presented a game-theory model based on the marginal value theorem, which predicted that as long as the client determines the duration, and the providers cooperate towards mutual gain, service quality will increase in the pair situation. This was demonstrated using cleaning mutualism, in which stable male–female pairs of the cleaner fish Labroides dimidiatus repeatedly inspect client fish jointly. Because clients often leave in response to cleaner fish cheating (feeding on mucus), the benefits of cheating can be gained by only one cleaner during a pair inspection. We found increased service quality during pair inspections. This was mainly due to the smaller females behaving significantly more cooperatively than their larger male partners.

Lexa collecting gnathiids using an emergence trap

A ton of gnathiids are gobbled up daily

In 2008, we examined the large scale interactions between gnathiid isopods, cleaner fish, and other fish. We calculated that the abundance of gnathiids emerging from the reef in search of hosts (gnathiids only are on fish while sucking fish blood, returning to the reef to digest and moult to the next stage) was 42 per metre squared per day or 4552 per reef (approximately 100 metres squared area) per day. This works out to about 5 emerging gnathiids per fish, but excluding the rarely infested pomacentrid fishes, this works out to 21 gnathiids per fish per day. Overall, the abundance of emerging gnathiids per patch reef was 66% of the number of gnathiids that all adult cleaner fish per reef eat daily while engaged in cleaning behaviour. That L. dimidiatus eat more gnathiids per reef daily than were sampled with emergence traps suggests that cleaner fish are an important source of mortality for gnathiids.

Cleaner fish standout on the reef and tend to be blue

A common question in cleaning behaviour is how clients recognize cleaners and decide not to eat them. A longheld belief is that cleaner fish display a blue ‘‘guild'' coloration. In 2009, via colour analytical techniques and phylogenetic comparisons, we showed that cleaner fish are more likely to display a blue coloration, in addition to a yellow coloration, compared to noncleaner fish. Via theoretical vision models, we show that, from the perspective of potential signal receivers, blue is the most spectrally contrasting colour against coral reef backgrounds, whereas yellow is most contrasting against blue water backgrounds or against black lateral stripes. Finally, behavioural experiments confirmed that blue within the cleaner fish pattern attracted more client reef fish to cleaning stations. Cleaner fish thus have evolved some of the most conspicuous combinations of colours and patterns in the marine environment, and this is likely to underpin the success of the cleaner-client relationship on the reef.

Punishment promotes cooperation

Why do humans punish others even if they themselves are not the victim? Such behaviour has long been interpreted as one that benefits the group and not the individual that is doing the actual punishing. But theory predicts it can also benefit the individual. To test this, in 2010 we offered cleaner fish preferred and non preferred food (fish flakes) on a plate. If the female cheated and ate the preferred food (prawn) we removed the plate immediately. The male then chased the female as punishment for her cheating behaviour. After a number of punishments, the female became more cooperative and would then continue to eat from the non-preferred plate allowing the male to obtain more food. Thus punishment meted out by male cleaner fish toward female cleaner fish promoted cooperation and as a result rewarded the male with more food because it assured the client did not leave the cleaning station. This showed that understanding the behaviour of self-interested cleaner fish in response to personal loss may be a key step toward understanding why humans find it necessary to punish a third-party when they receive no direct benefit.

Stressed fish love a good massage

Experimental set-up testing the effect of physical stimulation by a (fake) cleaner fish on cortisol levels of surgeonfish [Reformatted from Soares et al 2011]

Previously, we had shown that cleaner fish manipulate client decisions by physically touching clients with its pectoral and pelvic fins, a behaviour known as tactile stimulation, but why clients would tolerate this behaviour remained unclear. The classic view has been that cleaners exploit the properties of the clients' sensory system and that clients gain little from tactile stimulation. In humans, physical stimulation, such as massage therapy, reduces stress and has demonstrable health benefits. In 2011, we found that tactile stimulation reduces stress in a surgeonfish fish. We simulated this behaviour by exposing surgeonfish to mechanically moving cleaner fish models. Surgeonfish had lower levels of cortisol when stimulated by moving models compared to controls with access to stationary models (Photo above). Thus we finally identified the elusive benefit to fish of receiving tactile stimulation from cleaners. Furthermore, it appears that physical contact alone, without a social aspect, is enough to produce fitness-enhancing benefits, a situation so far only demonstrated in humans.

Cleaner fish males punish females for eating too much

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. Cleaner fish feed in male-female pairs by removing parasites from larger ‘client' fish. While providing this cleaning service, cleaners may get greedy and bite clients rather than the parasites. This cheating by cleaners causes meal times to come to an abrupt end as the disgruntled client fish swims off. In 2012, we found that 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.

Photo by João Paulo Krajewski

The cleaner male fish stopped females from eating too much and then challenging his position as the dominant male. So telling your partner to watch her weight is not recommended-unless you're a male cleaner fish! This is because the male cleaner fish lose more than just a meal from their partner's big appetite – they also risk the female becoming so large that she will turn into a rival male and then challenge his position as the dominant male on the reef. This shows that male cleaner fish are aware of their female partner's size.

This is the first non-human example of where punishment fits the crime and results in the offender adjusting their behaviour according to the potential penalties.

What happens to reefs without cleaner fish?

A lot of bad things happen! Using a long-term experiment discussed below, we demonstrate the many benefits of cleaner fish to the reef. This approach measures the net outcome of cleaner fish on the reef. We clearly demonstrate the mutualistic nature of the association between Labroides dimidiatus and clients, and the reef.

Map of Lizard Island, Great Barrier Reef, showing experimental reefs. White reefs have been kept free of cleaner fish since 2000 and black reefs have been left undisturbed

Longest running “cleaner fish removal experiment” to date

In earlier studies (1999, 2000) we showed that cleaner fish presence can reduce parasite loads of caged fish within 12 h; however, the longer term benefits remained unknown. To examine these benefits, we currently have the longest running experiment to date involving the removal of cleaner fish L. dimidiatus from several patch reefs at Lizard Island, Great Barrier Reef. This is one of the most field-intensive experimental studies done to date involving coral reef fish. The goal of this experiment is to test the effect that cleaner fish presence has, both directly and indirectly, on the reef community. Since, 2000, we have been surveying 16 patch reefs for cleaner fish and removing them from 7 patch reefs (removals) and leaving them undisturbed on 9 reefs (controls), this has been done about every 3 months for a total of 50 + field trips! For this we thank the many staff, students, and volunteers involved. Our first result came out in 2003, where we showed that cleaner fish presence increases the abundance and diversity of non-resident fish within 18 months.

Fish growth is higher on reefs with cleaner fish

Since then, in 2011, we showed that after 8 years of cleaner fish absence, growth was reduced and parasitic copepod abundance was higher on lemon damselfish from removal reefs compared with controls, but only in larger individuals. Behavioural observations revealed that lemon damselfish cleaned by cleaner fish were 27 % larger than nearby damselfish of the same species. The selective cleaning by this cleaner probably explains why only larger individuals benefited from cleaning. This is the first demonstration that cleaners affect the growth rate of client individuals; a greater size for a given age should result in increased fecundity at a given time. The effect of the removal of so few small fish on the growth of another fish species is unprecedented on coral reefs.

Fish are larger on reefs with cleaner fish

Also in 2011, we showed that after 8.5 years, individuals of two site-attached client damselfishes were smaller on reefs without compared to those with cleaner fish. Furthermore, resident fishes were 37% less abundant and 23% less species rich per reef, compared to control reefs. Such changes in site-attached fish may reflect lower fish growth rates and/or survivorship. Additionally, juveniles of non-residents were 65% less abundant on removal reefs suggesting cleaners may also affect recruitment. This may, in part, explain the 23% lower abundance and 33% lower species richness of visitor fishes, and 66% lower abundance of visitor herbivores (surgeonfishes) on removal reefs that we also observed. This is the first study to demonstrate a benefit of cleaning behaviour to client individuals, in the form of increased size. Many of the fish groups affected may also indirectly affect other reef organisms, thus further impacting the reef community.

Settlement of cleaner fish juveniles is enhanced on reefs with cleaner fish

Photo by Derek Sun - Lemon damselfish posing for juvenile cleaner fish

In this experiment, we also examined the effect resident adult cleaner fish presence has on juveniles of the same species (conspecifics). In 2012, we found that 1) new settling juveniles abundance was negatively related with adult abundance with each additional adult resulting in a 75% reduction in settler abundance. And 2) on reefs with all conspecifics experimentally removed, settler abundance was reduced by 72% compared with controls . This suggests a deleterious effect of adults on settlers, such as competition which is, however, outweighed by the enhanced settlement of juveniles associated with conspecific (mostly adult) presence. This combination may explain the observed relative permanence of fish cleaning stations. This enhanced settlement of cleaner fish juveniles via apparent conspecific attraction also ultimately affects the wider fish community.

UVB wavelengths regulate changes in UV absorption of cleaner fish mucus

Organisms living in shallow coral reef environments possess UV absorbing compounds in their mucus. Cleaner fish have been shown to have very high levels of these compounds in their skin mucus. While it has been demonstrated that exposure to UV affects the UV absorbance of fish mucus, whether the effects of UV exposure vary between UVB and UVA wavelengths is not known. Therefore, in 2013, we investigated whether the UVB, UVA, or photosynthetically active radiation (PAR) portions of the spectrum affected the UV absorbance of skin mucus and the body condition of cleaner fish. We found that the UV absorbance of epithelial mucus and body condition differed among lab treatments and decreased with depth of wild-caught fish. These suggests that the increase in UV absorbance of fish mucus in response to increased overall UV levels is due to UVB. 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 cleaner fish

In cooperation, often 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. In 2013, we found that the threat of early termination of an interaction was sufficient to induce cleaner fish 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 more on preferred food (which corresponds to cheating by eating client mucus). A non-cleaning fish species, however, 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.

The most common food of cleaner fish is likely the vector of a fish blood parasite

Cleaner fish ingest 1200 blood-sucking gnathiid isopods a day. Gnathiids are presumed to be a possible vector of blood parasites that occur in fish. In 2013, we examined whether gnathiids are vectors of haemogregarine fish blood parasites. Blood infections in a triggerﬁsh decreased under gnathiid-free laboratory conditions, compared with tagged fish returned to the reef. Laboratory reared uninfected gnathiids allowed to feed on the blood of fish infected with blood parasites picked up the parasites. We also tested biological transmission of blood parasites using reared gnathiids and uninfected fish raised from larvae. Recipient ﬁsh ingested, or were bitten by, gnathiids that had fed on donor ﬁsh infected with blood parasites. Two surgeonﬁsh, which had ingested gnathiids infected with blood parasites, had blood parasites. A third surgeonﬁsh also had blood parasites after infected gnathiids had bitten it. This is the strongest evidence to date that gnathiids may transmit blood parasites. Intriguingly, cleaner fish have never been found with blood parasites, despite eating so many gnathiids.

Haemogregarine blood parasites (Photo: Copyright A. Grutter)

Geographical variation in the beneﬁts obtained by a coral reef ﬁsh mimic

In 2014, we examined a contentious classic textbook example of mimicry between the cleaner fish and its mimic, the blenny Aspidontus taeniatus. We found that the beneﬁt obtained by the mimic varied between four geographical locations. At the Great Barrier Reef, in Indonesia and in the Red Sea, it rarely attacked ﬁsh victims, but relied on other food such as substrate items, damselﬁsh eggs and tubeworms. Here, the main function of the mimicry system could be to protect the mimic from predation (protective mimicry). However, in French Polynesia, the mimic aggressively frequently attacked ﬁsh, and potential victims were more likely to pose to solicit a cleaning interaction. Diet analysis from such individuals indicated they ate material off ﬁsh, including large pieces of ﬁn, implying an increase in beneﬁts obtained from attacking ﬁsh (aggressive mimicry). Thus, the beneﬁts obtained by the mimic vary between different environmental conditions and/or geographical locations.

Mimic cleaner fish Aspidontus teaniatus

Cleaning up the biogeography of cleaner fish using phylogenetics and morphometrics

Cleaner fishes are some of the most conspicuous organisms on coral reefs due to their lateral stripe and blue/yellow colouration. However, variability in this cleaning signal of the cleaner fish has been documented. Therefore, in 2014, we investigated the geographic distribution of their cleaner signals and contrasted this to geographic variation in mitochondrial (mt) DNA. We used samples for genetic analyses from the Red Sea to Fiji. We also measured the body stripe stripes using photographs. We found that body stripe width was correlated with tail stripe shape and geographical location, with Indian Ocean populations differing in morphology from western Pacific populations. Cleaner haplotypes formed two reciprocally monophyletic clades, although in contrast to morphology, Japanese cleaner fish fell within the same clade as Indian Ocean ones and both clade types were sympatric in Papua New Guinea. Overall, the findings suggest the diversity within Labropides dimidiatus is underestimated.

Cleaner fish tails (Photo: Copyright A. Grutter)

Cortisol mediates cleaner wrasse switch from cooperation to cheating

Recent research, mostly done on humans, recognizes that individuals' physiological state affects levels of cooperation. In 2014, we found that shifts in cortisol affects cooperation in cleaner fish. We treated wild fish with three different compounds (cortisol, a cortisol blocker, and saline), and observed their cleaning behaviour for 45 min. Changes in cortisol caused changes in the cleaning service provided to clients. Cleaners treated with cortisol provided more tactile stimulation to small clients, indicating they were more cooperative towards them, whereas it caused more jolts in large clients, indicating that these cleaners were more dishonest. Blocking corticoid receptors led to more tactile stimulation to large clients. indicating they were more cooperative towards them. Cortisol thus potentially offers a general mechanism for condition dependent cooperation in vertebrates.

Variation in cleaner cooperation and cognition: inﬂuence of developmental environment?

Deviations from predictions of strategies leading to stable cooperation between unrelated individuals have raised considerable debate in regards to decision-making processes in humans. Cleaners vary their service quality based on a clients’ strategic behaviour. Cognitive tasks designed to replicate such behaviour have revealed a strong link between cooperative behaviour and theoretical predictions. However, in 2014, we show that individuals from a speciﬁc location within our study site repeatedly failed to conform to the published evidence. We found that failing individuals lived in a socially simple environment. Decision rules of these cleaners ignored existing information in their environment, in contrast to cleaners living in a socially complex area.

Location of study. Copyright: Wismer et al. 2014

Neuropeptide regulation of pair association and behavior in cleaner ﬁsh

Animals establish privileged relationships with speciﬁc partners, which are treated differently from other conspeciﬁcs, which contributes to behavioral variation. However, there is limited information on the underlying physiological mechanisms involved in the establishment of these privileged ties. The cleaner wrasse 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. In 2015, we found that variation in pairs' relationships inﬂuences male and female cleaner ﬁsh differently and contributes to the variation of brain neuro-peptide levels, which is linked to distinct cooperative outcomes.

Fish mucus versus parasites as energy and sunscreen sources for cleaner ﬁsh

Cleaner fish feed mainly on blood-sucking gnathiid isopods and also on the epidermal mucus of client fish; the nutritional quality of these foods, however, is unknown. The epidermal mucus of reef fish contains ultraviolet (UV)-absorbing compounds (mycosporine-like amino acids, MAAs), which are only obtained via the diet; nevertheless, while cleaner fish haves high amounts of MAAs in its mucus, their source is unknown. In 2015, we found that the energetic value of mucus and gnathiids varied among fishes. Overall, various energetic measures were higher in the mucus of most client species compared to gnathiids. Mucus also had high MAA levels in mucus, whereas gnathiids had none. This suggests that cleaner fish obtain MAAs from mucus but not from gnathiids. This may explain why cleaner fish prefer to feed on mucus over gnathiids.

Gnathiid isopod (Photo: Copyright N. Smit)

The role of serotonin in the modulation of cooperative behaviour

The physiological pathways mediating cooperation remain relatively obscure. Here, we show that altering the activity of serotonin in wild cleaner wrasses has causal effects on both social and cooperative activities. We found that enhancing serotonin made cleaner wrasses more motivated to engage in cleaning behavior and more likely to provide physical contact to clients (tactile stimulation). Blocking serotonin-mediated response resulted in an apparent decrease in cleaners’ cheating levels and in an increase in cleaners’ aggressiveness toward smaller conspecifics.

Cleaner wrasse increases recruitment of damselfishes

In 2015, using a controlled field experiment to examine the effect of cleaners on recruitment processes of a common group of reef fishes, we showed that small patch reefs with cleaner wrasse had higher abundances of damselfish recruits than reefs from which cleaner wrasse had been removed over a 12-year period.

Lemon damselfish (Photo: Copyright Derek Sun)

Equivalent cleaning in a juvenile facultative and obligate cleaning wrasse
In 2015, we investigated the foraging ecology of the tubelip wrasse, a facultative cleaner as a juvenile and corallivore as an adult, and compared its juvenile ecology with that of juvenile blue-streak cleaner wrasse, an obligate cleaner. The number of client individuals and species that were cleaned and the proportion that posed did not differ between the two, nor did the number of ectoparasitic isopods in their guts. In contrast, adult yellowtail tubelip wrasse had fewer isopods and more coral mucus in their guts than juveniles. These data suggest juvenile cleaning acts as an evolutionary precursor to obligate cleaning.

Juvenile cleaner fish, juvenile tubeblip, and adult tubelip. (Photo: Copyright R. Smith)

A tropical cleaner wrasse finds new clients at the frontier

During two field trips to the Solitary Islands, we observed cleaning interactions between the cleaner wrasse and six other fish species that range broadly into temperate southeastern Australian waters. Our observations suggest that the cleaner wrasse’s array of potential clients is broader than previously thought. This generality in client choice may enable cleaner wrasse populations to permanently establish themselves in latitudes that are currently too cold but that may become thermally tolerable as a result of global climate change.

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 dopamine system plays a key role in this: for instance, a decrease in dopamine, signalled by the failure of an expected reward, may elicit a distinct response. In 2016, we found that blocking dopamine receptors in the wild induces cleaners to initiate more interactions and provide more physical contact to their client fish partners. Thus, in low dopamine conditions cleaners may renegotiate with a costly offer.

Cleaner fish using tactile stimulation (contacting with fins) (Photo: Copyright A. Grutter)

Cleaner wrasse influence habitat selection of young damselfish
The presence of Labroides dimidiatus on coral reefs increases total abundance and biodiversity of reef fishes. In 2016, we showed that young damselfish preferentially settle into microhabitats where cleaner wrasse are present, suggesting that cleaners serve as a positive cue. Young fish were rarely cleaned compared with larger damselfishes and the ones cleaned were larger than the ones in the surrounding area. The selection of a microhabitat adjacent to cleaner wrasse in the laboratory, despite only being rarely cleaned in the natural environment, suggests that even rare cleaning events and/or indirect benefits may drive their settlement choices. This behaviour may also explain the decreased abundance of young fishes on reefs from where cleaner wrasse had been experimentally removed.

Effects of parasites on fish cortisol and hematocrit levels
Clients are known to adjust their visits to cleaners according to their ectoparasite load. Whether physiological changes due to the ectoparasitic infection, prior to engaging in cleaning interactions, might inform clients of their current need to visit cleaners remains unclear. In 2016, we found that gnathiid-exposed fish had higher cortisol compared with controls, suggesting gnathiids cause a stress response in fish. Also, hematocrit level within gnathiid-exposed fish was negatively related to attached gnathiid number, suggesting hematocrit may inform fish about gnathiid load. These suggest physiological mechanisms underlying client’s decisions involving cleaners.

Gnathiid isopod (Photo: copyright Nico Smit)

Cleaner fish use predators as social tools to reduce punishment

In 2016, we showed that cleaner fish uses generalized rule application in their use of predators as social tools against punishing reef ﬁsh clients. Punishment occurs as cleaners do not only remove ectoparasites from clients, but prefer to feed on client mucus (constituting cheating). During consecutive exposure to pairs of novel species, cleaners demonstrated generalization of the ‘predators-are-safe-havens’ rule by rapidly satisfying learning criteria.

Neuromodulatory systems affect social behaviour of client ﬁsh

Many species engage in mutualistic relationships with other species. The physiological mechanisms that affect the course of such social interactions are little understood. In 2017, we focussed on three neuromodulator systems to examine their role in clients' interspeciﬁc behaviour. Our results suggest that the vasotocine 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.

Cleaner fishes and shrimp diversity and a re-evaluation of cleaning symbioses: a review

Cleaning symbiosis has been documented extensively in the marine environment over the past 50 years. In 2017, we estimated global cleaner diversity comprises 208 fish species from 106 genera representing 36 families and 51 shrimp species from 11 genera rep-resenting six families. While marine cleaner fishes have dominated the cleaning symbiosis literature, comparatively little focus has been given to shrimp. Interest in cleaning organisms as biological controls in aquaculture, however, is increasing due to their value as an alternative to various chemical ectoparasite controls. Reports of the importance of cleaner organisms in maintaining a healthy reef ecosystem has also been increasing and we reviewed the current biological knowledge on cleaner organisms.

Cleaner wrasse indirectly make fish smarter

In 2018, we tested how client cognition is affected by ectoparasites and whether these effects are mitigated by cleaners. Damselfish from experimental reefs without cleaner wrasse performed worse in a visual discrimination test than ones from reefs with cleaners. This task was also impaired in damselfish experimentally infected with gnathiid parasites. Our results highlight the 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.

Parasite infestation increases on coral reefs without cleaner ﬁsh

Experimental manipulations of cleaner wrasse reveal declines in ﬁsh size and growth, and population abundance and diversity of client ﬁshes in the absence of cleaner wrasse. Fishes grow more slowly and are less abundant and diverse on reefs without cleaner wrasse, both for larger species that are regularly cleaned and have high ectoparasite loads (‘‘attractive species’’), and for those smaller species that are rarely cleaned and are rarely infested with parasites (‘‘unattractive species’’). In 2018, we showed infestation was higher on reefs without cleaners than on those with them. The effect was only detected during the daytime when cleaners are active and only on the attractive species. Thus, cleaner presence indirectly reduced ﬁsh exposure to parasites in a species that is highly susceptible to parasites, but not in one that is rarely infested with parasites.

Johanna and Derek placing sentinel trap on the reef.

Using a parasite culture to study cleaning interactions

As part of a broader review, in 2018, we summarised studies on how our gnathiid parasite culture at Lizard Island has been used, including increasing our understanding of fish-cleaning behaviour: Changes in gnathiid numbers affect the success of cleaner ﬁsh mimics. Infestation by, and physiological responses to gnathiids, such as host haematocrit and blood cortisol, suggest that several proximate mechanisms inﬂuence ﬁsh client’s cleaning behaviour. That cleaners prefer client mucus over gnathiids, which is more nutritious, suggests a cleaner–client conﬂict and the need for partner control in this mutualism. That cleaners showed no preference for fed over unfed gnathiids, however, showed that no cleaner–client conﬂict occurs over which gnathiids should be eaten. Clients exposed to gnathiids spent more time seeking cleaners, showing that parasite infection is a proximate cause of cleaning. Finally, that client visual discrimination was reduced in gnathiid-exposed ﬁsh, providing a mechanism for how long-term cleaner presence in the wild similarly affected clients.

Life cycle of gnathiids and the host used in their culture. Copyright: Hutson et al. 2018

All the above information is published. Please see publications list for more details.

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