This brown and yellow damselfish depends on the cleaner wrasse removing parasites for a healthy life. But the relationship is complicated. Credit R Smith/Coral Reef Institute

Cleaners, fish and shrimp species that feed on parasites attached to larger fish are some of the strangest features of coral reefs. A series of recent studies on the Great Barrier Reef reveal both the importance of their role to reef ecosystems and the curious dynamic between cleaners and their "clients."

Cleaner wrasse (Labroides dimidiatus) mostly consume the gnathiid parasites that otherwise suck the blood of their clients. This can be a matter of life or death for the clients. Dr. Alexandra Grutter of the University of Queensland told IFLScience that for smaller fish, a parasite can end up “as large relatively as if a human was carrying a blood sucking cat around on them.”

Grutter has led a long-running study where cleaner wrasse are continuously removed from a number of small reefs in order to study its long-term impact on reef ecology. The small wrasse are carried on ocean currents in their larval stage, forcing Grutter’s team to remove them from the test reefs every three months and, as she says, “more often in summer” when most larvae settle. 

Previously, Grutter’s team found that reefs without wrasse slowly became depleted in damselfish over a 12-year period. Now, they have a possible explanation for why. First author Derek Sun showed in the Biology Letters paper that damselfish are less likely to settle where cleaners are absent. This is despite the fact that the team did not remove cleaner shrimps, who perform a similar purpose, and Grutter says that some non-specialist fish are known to provide occasional parasite-removal services as well.

Grutter says that the team have not yet established whether the failure to settle is the whole story, or if some fish eventually become refugees from reefs where they cannot get a good clean, despite the hazards of moving elsewhere.

However, as important as cleaner fish are, they sometimes irritate their clients by biting off the mucus the clients produce, rather than the parasites. In Coral Reefs, Grutter and Dr. Maxi Eckes have helped explain why they do this, even though it displeases their clients so much that they sometimes decide to get their parasites removed elsewhere.

Besides being rich in energy, the mucus contains sunscreens that protect the fish from getting lesions. Unable to produce these chemicals themselves, and getting none from the parasites, the cleaner fish obtain them by “cheating,” collecting mucus instead of parasites. The cleaners value the mucus so much that they will risk their relationship with clients to get some.

However, this is not the case when there is a queue of clients: Grutter has previously shown that cleaners value their reputation, to the extent that they will usually not take mucus over parasites when potential clients are queued up to be serviced, and might see them.

Yet, once again, this is not the full story, according to a paper Grutter co-authored in Ethology. She found that cleaner fish did not always learn of the damage to their reputation that such cheating produced. Further investigation revealed that the fish that never learned were from smaller and poorer reefs. Grutter says the investigation of why this might be the case is still in its infancy, telling IFLScience, “All we can say is that the capacity for learning depends on the environment they come from.”

Whether the cheating fish are genetically different or a product of bad upbringing remains unclear. “It could be that these are the fish that were booted out of all the good spots, or that something else is going on that affects their ability to learn,” Grutter said.

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