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CLIENT SELECTIVITY IN THE BLUESTREAK CLEANER WRASSE LABROIDES DIMIDIATU
Introduction:
Bluestreak cleaner wrasse participate in a cleaning symbiosis with client species, receiving nutrition from the removal of ectoparasites, mucus and dead tissue from the surface of the fish. Although this has previously been assumed to be a mutualistic relationship, with the reduction in parasites increasing client fitness, this has yet to be proved conclusively; it is possible that this interaction may actually represent behavioural parasitism on the part of the cleaners (Grutter, 1999).
The cleaner station comprises a focal area, where the majority of the activity takes place, surrounded by a larger domain, through which the cleaner wrasse may swim to attract clients. This territory may be patrolled either singly or as a pair; occasionally multiple wrasse occupy the same area.
Their distinctive colouration is thought to play a role in client enticement, together with a characteristic dance. Client fish swim into the station and will often pose to solicit cleaning, typically adopting an immobile head or tail stand position which may be species specific. This may result in a cleaning event, recognised as oral contact between the cleaner and the surface of the client, the outcome of which is assumed to be removal of ectoparasites and other material.
There is much inter-specific variation in tendency of fish to pose; however, this is unrelated to the probability of being cleaned. Likelihood of posing also appears to be unrelated to body size of trophic type, refuting the hypothesis that posing may be a form of appeasement by piscivorous fish that represent a threat to the wrasse. Intra-specific variation in tendency to pose seems to indicate that for a particular species, posing increases the probability of being cleaned (Cote et al, 1998).
It is possible that tendency to pose may instead be related to attractiveness of the client to the cleaner fish. It is thought that attractiveness is determined by several factors, and is the basis for the observed selectivity of the cleaner wrasse. The most crucial element appears to be ectoparasite load, which in turn will depend on both host surface area and mean parasite intensity harboured by a particular species; the latter may be associated with particular lifestyles. Attractiveness may be determined to some degree by the nutritional value of the client's surface mucus, and may also incorporate behavioural features of the client, such as tendency to pose and aggression.
This study sets out to establish whether the cleaner wrasse Labroides dimidiatus demonstrates a preference for certain clients, and attempts to explain some of the differences therein.
Aims:
To investigate whether the bluestreak cleaner wrasse demonstrates a significant preference in client choice based on size and species of fish, indicating whether a ranking system exists where clients are chosen on the magnitude of their attractiveness.
Objectives:
Methods:
All results were obtained using Scuba equipment.
The study was carried out at Pulau Hoga, Wakatobi National Marine Park. Two sites were selected (Kaledupa, Buoy 1 and Home Reef, Buoy 3) with three cleaner stations chosen at each, located within a depth range of 13-18m. Four replicate sessions were taken at each of these stations, a total of 24 surveys. Several variables were noted: the station number, location and depth; the number and size of cleaner wrasse patrolling; and the time of initiation/completion of the study period.
Each survey involved a 30-40 minute period of observation by two divers whilst hovering approximately two metres from the station. After a delay of approximately a minute to allow the fish to become accustomed to our presence, each fish actively visiting the cleaner station was identified and its size estimated, using the size of the wrasse itself as a guide. Any fish that posed but failed to be cleaned were also recorded; it is possible that the percentage of successful solicitations may also be used as a measure of attractiveness. Any preferences exhibited by the cleaner wrasse were recorded; a preference was recognised if the wrasse selected one client over another when multiple clients were present at the station.
Analysis of results:
The clients will be analysed at the family level, allowing a ranking system of attractiveness to be constructed using the relative number of times each family is the first choice of the cleaner. A Chi-squared test will then be used to determine whether any of the apparent preferences demonstrated are significant. Ranking statistical techniques can be used to compare preference between the two sites; if there is no significant difference, the two sets of results can be combined and analysed together. A Kolmogrov-Smirnov hardline non-parametric regression can be employed to test whether there is a relationship between client size and their selection by cleaner wrasse.
Expected results:
It is not expected that there will be any difference in preference for particular fish species between the two sites, or between the different stations within each site; if this is the case, all results can be combined to produce a larger dataset.
Since variation in parasite load exists between different species, it is anticipated that the cleaner wrasse should demonstrate preferences for certain species relative to others, displaying selectivity in their choice of client. Species that carry a higher parasite load should be preferred if size is constant because they represent more efficient feeding for the wrasse. Since larger fish of a certain species should harbour more parasites, they would be expected to be preferred over a smaller client of the same species. However, a larger fish should not necessarily be selected over a smaller fish of a different species, although this should be the general trend. The outcome will depend on the relative attractiveness of the two species, and the relative importance of size and species in explaining the variation in cleaning preference.
References:
Arnal C, Cote I. M (1998). Interactions between cleaning gobies and territorial damselfish on coral reefs. Animal Behaviour 55, p1429-1442.
Arnal C, Cote I. M (2000), Diet of broadstripe cleaning gobies on a Barbadian reef. Journal of Fish Biology 57, p1075-1082.
Arnal C, Morand S, Kulbicki M (1999). Patterns of cleaner wrasse density among three regions of the pacific. Marine Ecology Progress Series 177, p213-220.
Cote I. M, Arnal C, Reynolds J. D (1998). Variation in posing behaviour among fish species visiting cleaning stations. Journal of Fish Biology 53, p256-266.
Grutter A (1995). Relationship between cleaning rates and ectoparasite loads in coral reef fishes. Marine Ecology Progress Series 118, p51-58.
Grutter A (1999). Cleaner fish really do clean. Nature 398, p672-673.
Report
A report entitled Client selectivity in the Bluestreak Cleaner Wrasse Labroides dimidiatus will be produced by Isabelle Sheridan, University of Oxford, by March 2002.