A Bluespine Unicornfish (Naso unicornis) visits a cleaning station on a Maldivian reef where a few Bluestreak Cleaner Wrasses (Labroides dimidiatus) are on cleaning duty. These wrasses remove dead skin and tissue from the visiting fish, often called the “client,” and sometimes even remove parasites.
Although the cleaners are an enormous hygienic aid for the client, they are actually cheaters. Their main goal, however, is quite different from what one would expect. The cleaners who do a good job of keeping their clients free from infections and parasites are more interested in the protein-rich mucus that covers the client’s skin and/or scales. They attract their clients by performing a little wriggle, followed by gentle caresses, a much-appreciated act by the client fish (it has been observed that some fish species mainly visit these cleaning stations to get these short rubs or gentle caresses). This is followed by a few surgically precise removals of dead skin to show the client how serious the little cleaners are. Suddenly and without warning, the cleaners take a quick bite of the nutritious mucus covering the client’s skin or scales. Because it is only one painful pinch amongst so many other pain-free bites, the client may experience it as a mistake. If the cleaners would only go after the mucus, the client fish would learn very quickly to avoid the cleaner wrasse. The little bite marks are now little wounds and a new source of dead skin and tissue that would require the client to visit a cleaning station again in the near future. By doing so, the cleaner wrasse ensure the continued come and go of client fish, providing a rich and diverse alimentation to the wrasse.
It was believed that client fish changed colour to signal to the cleaners that they were ready to get cleaned. Client fish, like the Bluespine Unicornfish in this underwater video clip, do have the ability to change colour thanks to chromatophores (pigment cells) in their skin and scales. However, new German research suggests that client fish change colour not to signal that they are ready to be cleaned and will not harm the cleaner, but to signal to another individual from their own species that they have entered another's territory, but are just here to get a clean and are not looking for a territorial dispute. Unfortunately, the other Bluespine Unicornfish that swam over and triggered the colour change from light blue to dark green of the cleaning station’s client felt out of the frame of this underwater video clip.
Just like many other fish species, unicorn fish are quite territorial, and they can inflict severe lacerations and cuts with their scalpel-like modified scales situated on both sides of the tail base. The client fish in this short video clip bears a big scar on one of its sides. It is, however, impossible to tell the origin of this seemingly healed wound.
This footage was filmed without the use of video lights. It is perfectly possible to obtain natural colours with the aid of an orange filter. Orange filters are recommended for wide-angle scenes on sunny days and when filming deeper than 6m/20ft. It is however necessary to perform a white balance each time the underwater luminosity changes (by changing depths or by the appearance of clouds in the air, reducing the amount of sunlight entering the water).
The little bite sound was added in post-production to accentuate the painful pinch experienced by the client fish.
This short underwater videoclip has been filmed in the Maldives 🇲🇻
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