Australasian Science: Australia's authority on science since 1938

The Fish Cleaners

Tropical fish the sailfin tang visits a cleaner fish.

Tropical fish the sailfin tang visits a cleaner fish.

By Stephen Luntz

Not all cleaner fish are trustworthy, so why don’t more of them get eaten?

Cleaner fish provide a fascinating insight into the workings of evolution. Dr Lexa Grutter of the University of Queensland’s School of Biological Sciences has pioneered research into this vital component of coral reef ecosystems. She has now added these stunning images to her insights, many of which have been reported in Australasian Science in the past.

The cleaner fish occupy specific “stations” on the reef where other fish will come when they need to have parasites removed. Grutter has revealed that some fish visit these stations 150 times each day (AS, June 1999, p.9), and a single cleaner can eat 1200 parasites a day.

The parasites probably don’t kill fish directly, but Grutter has found that larvae infected by parasites have poorer swimming performance and oxygen consumption. In the competitive environment of the reef this can be enough to ensure that a fish does not survive. Some parasites have also been implicated in transmitting blood parasites, much like mosquitoes do with malaria.

While the benefits of being cleaned are large, ensuring that the operation goes smoothly for both sides can be tricky. After all, cleaner fish would make a tasty morsel for species that are used to eating anything they can get their jaws on. It is detrimental for reef fish in general if cleaners get eaten, but no one is quite sure why no species seems to take advantage of the benefits of an easy meal given that the cleaners are not hard to find.

Grutter has shown that cleaner fish use “tactile dances” to establish their credentials with fish that might be tempted to eat them, dancing more when they encounter hungrier fish (AS, September 2004, p.8). Some fish act as cleaners when they are young, but as they grow they take up other lifestyles, perhaps because they can’t get enough food from cleaning alone.

More recently Grutter demonstrated that some cleaner fish cheat, eating mucus rather than parasites off a client. Unsurprisingly, clients do not appreciate this, and seek out reliable cleaners by watching how cleaners behave with other clients. It seems that even though cleaner fish prefer mucus to parasites (demonstrated when Grutter offered them both on plates) they will usually stick to eating parasites in order to impress potential clients that may be watching (AS, August 2006, p.9)

The bright colours visible in these photographs are no coincidence. Grutter’s colleague, Dr Karen Cheney, showed that cleaner fish worldwide are usually blue and yellow, with the exceptions having a black horizontal stripe adjacent to one of those colours along their length. These colours stand out best against coral and water backgrounds, serving as a form of advertising. Cheney found that other fish are more likely to approach a fish-shaped object if it has these colours (AS, August 2009, p.11).

The parasites are intriguing in their own right. With hosts visiting cleaning stations frequently, parasites must avoid hanging on too long lest they be eaten. “The longest they stay on for is an hour, and in their entire lifetime they may be on a fish for a maximum of 3 hours,” Grutter says. “In some ways they are more like mosquitoes than ticks.” Clients may be almost constantly infected with parasites, she has found, but they are different individuals doing the infecting.

Female marine ticks carry their young in a body compartment similar to a marsupial’s pouch. “They have about 30 babies in this pouch and, when the mother releases them, all these tinier parasites shoot out and swim out into the ocean wilderness,” Grutter says.

Although these particular parasites are less of a problem in the open oceans, others exist, and Grutter says there are reports of cleaners setting up homes among floating debris, to which pelagic fish come for an occasional clean.