Little fish, big job: How the absence of a small cleaner wrasse causes parasite infestation to rise!!

Article: Grutter, A. S., De Brauwer, M., Bshary, R., Cheney, K. L., Cribb, T. H., Madin, E. M. P., … & Werminghausen, J. (2018). Parasite infestation increases on coral reefs without cleaner fish. Coral Reefs, 37(1), 15-24.

We all have that best friend that is always there when we need them and in turn we are always there for them. So, let’s say you needed to crash at their place for a week and in return you offer to  cook for them. Your friend would benefit from a home cooked meal and you would have a place to sleep. This is what in nature we call a mutualistic relationship or mutualism, where two individuals co-exist and benefit from one another. Now, let’s imagine that your friend’s friends decided to visit the whole time you are there because they want to have some of your amazing food. That is what we would call an indirect effect where your presence influenced the frequency of their visits. In nature, this means that a mutualistic relationship between two individuals may have indirect effects on a third individual.


Mutualistic relationships are important in shaping ecological communities. The cleaner wrasse (Labroides dimidiatus) for example has many fish with whom he shares a mutualism. These are called his client fish and they benefit from the wrasse by… you guessed it! Getting cleaned! The wrasse will remove and eat any ectoparasites (parasite that lives outside the host) the client fish may have and as a reward he goes home with a full stomach. The mutualism between a cleaner wrasse and his client fish prevents major parasite effects on the client fish and outbreaks on the reef. Recent studies have shown that reefs suffering from cleaner wrasse population declines also suffer from a decrease in size, growth, diversity, and abundance of client fish populations. This means that reefs without wrasses have smaller, less abundant, and less diverse client fish groups. Scientists have also found that this is true for those fish that are regularly cleaned and have many parasites (attractive species) and those who are rarely clean and rarely have any parasites (unattractive species).

Fig. 1 Cleaner wrasse (Labroides dimidiatus). Photograph: Lubomír Klátil

The Study

As a response to this finding on their long-term study (13 years) of cleaner wrasse removal on reefs, researchers Grutter et al., focused on sampling the indirect effect of cleaner wrasse on parasite infestation using an attractive and an unattractive species of client fish. The blackeye thicklip fish was used as the attractive fish species and the ambon damselfish as the unattractive fish species. Both species were placed in traps for 9 days in reefs with cleaner wrasse and reefs without (cleaner wrasse had been removed on those reefs for 13 years) to measure parasite infestation. They were sampled during the day and night to account for diurnal and nocturnal infestation rate, empty traps were used as control to see if parasites were found using the trap as shelter.

Fig. 2 Blackeye thicklip, attractive species. Photograph: Ahmed Abdul Rahman. (Top). Ambon damselfish, unattractive species. Resource: (Bottom)


Reefs without cleaner wrasse had higher parasite infestation with 2.5 parasites per trap while reefs with cleaner wrasse had 1.7 parasites per trap. Parasite infestation was higher during the daytime because parasites were observed to be more active than during the night. The attractive species was found to have more parasites than the unattractive species.


Fig. 3 Parasite (gnathiid) abundance in reef with wrasse presence (Grey) and reefs without wrasse (white) during daytime and nighttime. Black dots represent the outliers (data points outside of the observed pattern)


Why is it important?

All fish species play important roles in their ecosystems, their absence may shift ecosystem trophic dynamics (how the reef works) and change reef fish assemblages (how many different species and how many of each species). Cleaner wrasses and other reef fish populations are at risk of decline due to habitat loss, overfishing, and climate change. In this study, cleaner wrasses indirectly reduced client fish exposure to parasites suggesting that their absence might increase parasite infestation, thus affecting client fish health and growth. Remember, many of these client fish are commercial species that humans consume. So, in a big way, this tiny fish can cause quite a change in daily reef life and our lives too. 

Fig. 4 Cleaner wrasse doing their jobs on client fish. Photograph: Milan Kořínek


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Sandra Schleier

I recently completed a MSc. in Biological Environmental Sciences at the University of Rhode Island where I studied the effects of coral restoration on Caribbean reef communities. I am interested in helping out our planet through ecological restoration in degraded ecosystems. When I am not out doing research, I am a marine biologist guide in Puerto Rico doing guided and educational tours of our natural resources. I am a traveling junkie with the goal of visiting every country in the world.

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