Cleaner fish

The following is a selection of few of the many marine cleaner species.

Commonly studied cleaner fish are the cleaner wrasse of the genus Labroides found on coral reefs in the Indian Ocean and Pacific Ocean.[12]

Neon gobies of the genera Gobiosoma and Elacatinus provide a cleaning service similar to the cleaner wrasse, though this time on reefs in the Western Atlantic, providing a good example of convergent evolution[13] of the cleaning behaviour.

Lumpfish are utilized as salmonid cleaner fish in aquaculture, but it is unknown if they display cleaning behaviour on salmon in the wild.[14]

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  A disruptively patterned white-spotted puffer being cleaned by a conspicuously coloured Hawaiian cleaner wrasse.
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  Lumpfish (Cyclopterus lumpus), a cleaner fish employed in salmon farming in Atlantic Canada, Scotland, Iceland and Norway[15]
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  A neon goby from the Western Atlantic.

Brackish water refers to aquatic environments that have a salinity in between salt and fresh water systems. Cleaning symbiosis has also been observed in these areas between two brackish water cichlids of the genus Etroplus from South Asia. The small species Etroplus maculatus is the cleaner fish, and the much larger Etroplus suratensis is the host that receives the cleaning service.[16]

Cleaning has been observed infrequently in fresh waters compared to marine waters. This is possibly related to fewer observers (such as divers) in freshwater compared to saltwater.[17] One of the few known examples of freshwater cleaning is juvenile striped Raphael catfish cleaning the piscivorous Hoplias cf. malabaricus. In public aquariums, Synaptolaemus headstanders have been seen cleaning larger fish.[18][19]

A facultative cleaner fish does not rely solely on specialized cleaning behaviour for nutritious food.[20] Facultative cleaners can be further divided by stationary vs. wandering facultative cleaners.[21] Facultative cleaners may display cleaning behaviour through their whole life history or solely as juveniles for additional nutrients during rapid growth.[21][20] Examples of facultative cleaners are commonly wrasse species such as the blue headed wrasse, noronha wrasse (Thalassoma noronhanum) and goldsinny wrasse (Ctenolabrus rupestris), sharp nose sea perch in Californian waters,[20] and the lumpfish (Cyclopterus lumpus).

Using the example of the blue wrasse from Caribbean waters, their alternative feeding strategy is described as being a generalist forager, meaning they eat a wide variety of smaller aquatic organisms based on availability.[21] When displaying cleaning behaviour, it has been noted that the blue wrasse inspects potential clients and only feeds on some, implying that the wrasse is seeking out a particular type of parasite as a diet supplement. It has also been quantified that the blue wrasse foraging behaviour does not change in proportion to cleaning opportunities, again suggesting that the cleaning behaviour in this facultative fish is for diet supplementation and not out of necessity.[21]

An obligate cleaner fish relies solely on specialized cleaning behaviour for its food.[20] Therefore, obligate cleaners have a higher output of cleaning on a wider range of parasites in comparison to facultative fish. To maximize nutrient consumption, obligate cleaners utilize a higher proportion of cleaning stations.[21] Obligate cleaner fish may also be divided by stationary and wandering. These life history choice are made based on the amount of interspecific competition from other obligate cleaners in the area.[22] An example of an obligate cleaner is the shark nose goby (Elacatinus evelynae) in the Caribbean Reef, where it has been observed to perform up to 110 cleanings per day.[21]

Client Mulloidichthys flavolineatus at a cleaning station.

Cleaning stations are a strategy used by some cleaner fish where clients congregate and perform specific movements to attract the attention of the cleaner fish. Cleaning stations are usually associated with unique topological features, such as those seen in coral reefs[21] and allow a space where cleaners have no risk of predation from larger predatory fishes, due to the mutual benefit from the cleaners' service.[23]

Interactions are begun by the client and ended by the cleaner, implying that the client is seeking out the service where the cleaner has control.[20]

Cheating parasitism occurs when the cleaner eats mucus or healthy tissue from the client. This can be harmful to the client as mucus is essential to prevent UV damage, and open wounds can increase the risk of infection.[20] Cleaner fish maintain a balance between eating ectoparasites and mucus or tissue because of the respective nutritional benefits, sometimes despite the risk to the client.[24] For example, the Caribbean cleaning goby (Elacatinus evelynae) will eat scales and mucus from the host during times of ectoparasite scarcity to supplement its diet. The symbiosis relationship between client and host does not break down because the abundance of these parasites varies significantly seasonally and spatially, and the overall benefit to the larger fish outweighs any cheating on by the smaller cleaner.[25]

Cleaner fish (especially facultative cleaners) assess the value of possible clients when deciding whether to invest in a client or cheat and eat mucus or tissue.[21][26] Observations of cleaner and client interactions have found that cleaners may provide the client with tactile stimulation as a way to establish a relationship and gain the client's 'trust'. This interaction costs the cleaner as it is time not spent feeding.[26] This physical interaction demonstrates a cleaner fish's tradeoff. The cleaner minimizes feeding time to establish a memorable relationship with the client that also contributes to conflict management with a possibly predatory client.[26]

An example of an outdoor aquaculture facility in Chile.

Aquaculture is the farming of aquatic organisms, where salmon farming is growing in the North Atlantic.[33] Cleaner fish are used to eat parasitic sea lice from salmon to reduce outbreaks which cause disease in populations. The two most commonly used cleaner fish are the lumpfish, Cyclopterus lumpus, and the ballan wrasse Labrus bergeylta.[34] Lumpfish are distributed across the Atlantic ocean, ranging from Greenland to France, Hudson's Bay to New Jersey, and in high concentrations in the Bay of Fundy and St. Pierre Coast, near Newfoundland.[35] Ballan wrasse are distributed widely across the Northeast Atlantic Ocean.[36] The switch towards lumpfish has been preferred as wrasse are less active feeders during winter months.[37]

Cleaner fish are commercially cultured and introduced into salmonid sea cages. Salmon and lumpfish are able to coexist, where the lumpfish spend a certain amount of time foraging for supplemented food and only a portion of their time delousing salmon. With significant ratios of cleaner to client, the efforts are sufficient to minimize louse outbreaks.[37][34] Sea cages are designed with additional substrate for lumpfish to attach to during periods of inactivity to minimize stress levels in the cleaner fish and maximize delousing abilities.[37]

Sea lice causing morphological damage on cultured salmon in New Brunswick, Canada.

North Atlantic Aquaculture facilities use facultative cleaner fish (Cyclopterus lumpus, and Labrus bergeylta) in order to control the nutrients they receive during culturing, before their use in aquaculture. One of the challenges that comes along with using facultative cleaners is that parasite removal from salmon must be maximized while also balancing additional nutrients from supplemented feed to ensure the health of the cleaner fish and the safety of the salmonid clients.[38] Another challenge that arises in management of cleaner fish behaviour is balancing the number of cleaners to the number of clients. With a low cleaner-to-client ratio, the risk of lice infestation increases. With a high cleaner-to-client ratio, competition among cleaners increases and there is a higher risk of cheating and consumption of salmonid mucus and flesh thereby increasing their risk of infection.[38][34]

Minimizing disease in commercial lumpfish stocks is critical for the continuation of their usage in aquaculture. Vaccine development for the lumpfish is a current area of research as lumpfish demand is increasing in the aquaculture industry.[37] In an effort to minimize disease in the cleaner fish, commercial lumpfish stocks are supplemented with wild individuals during the breeding season to minimize inbreeding depression. The lumpfish genome has not yet been fully sequenced so subtle details between populations are not yet appreciated.[37]

Another consideration in using cleaner fish in aquaculture is minimizing escapees from sea cages. If escaped cleaner fish spawn with natural populations in the environment it may decrease the wild fishes' natural survival abilities.[37]

Cleaner fish have taken over lice-reduction strategies, which were based upon chemical delousers in the past. This decreases the amount of effluent waste affecting the surrounding wild habitats in outdoor aquaculture.[34] Introducing cleaner fish into salmonid aquaculture cages has also been found to be less stressful on salmonids than medical intervention for sea lice outbreaks.[37]

Cleaner fish in the wild contribute to the overall health of aquatic communities by reducing morphological and physiological injuries by parasites to other species of fish. Maintenance of these populations of fish help the complex web of interactions remain stable.[39]

Sea lice outbreaks are detrimental to the survival of cultured salmonids and cause the majority of revenue loss in the aquaculture business. By employing the cleaner fish instead of medical intervention for sea louse management, aquaculture farmers save money.[37]