The living toothbrushes that keep coral reefs healthy

The most important fishes on a coral reef aren’t the impressive predatory ones or the flashy colourful ones – they’re just humble cleaners.

The cleaner fish Laborides dimidiatuscleaner fish Laborides dimidiatus is cross between a janitor and a medic. It runs special “cleaning stations”, which other fish and ocean animals visit for a regular scrub. The cleaners remove parasites from their clients, even swimming into the open jaws of predators like moray eels and groupers. They’re like living toothbrushes and scrubs. And they work hard – every day, a single cleaner inspects over two thousand clients, and some clients visit the stations more than a hundred times a day.

The cleaners, and their relationships with their clients, make a classic case study for biologists studying the evolution of cooperation. The tiny fish clearly get benefits in the form of a meal, and they enjoy a sort of diplomatic immunity from otherwise hungry hunters. On the face of it, the clients also benefit by getting scrubbed of harmful parasites. Now, Peter Waldie from the University of Queensland has shown how important this hygiene is.

Eight and a half years ago, Waldie removed all the cleaners from a few patch reefs at Lizard Island in the Great Barrier Reef. Now, these reefs have 23% fewer species than those where the cleaners are still around. And the populations of the fish that stayed around are 37% smaller.

Waldie also focused on two species that use the cleaners’ services – the lemon damselfish and the ambon damselfish. In the cleaners’ absence, these clients grew more solely, and were smaller as adults. Perhaps parasites sap energy that would otherwise be used for growth. Perhaps the damsels have to spend more time on the lookout for danger (the presence of cleaner fish tends to pacify nearby predators). Either way, this is the first time that anyone has shown that a cleaner animal affects the size of its clients, and it shows how troublesome parasites can be.

This shift in size will probably cascade through the generations, since smaller damselfishes lay fewer eggs and raise fewer young. And damselfishes don’t patronise cleaning stations very often, so more regular clients probably depend on the cleaners to an even more dramatic extent.

The absence of the cleaners could also ripple through the entire reef community. Many reef fish graze on algae that would otherwise impede the growth of corals, while others control the populations of starfish that eat corals. Fewer cleaners lead to fewer fish, which could mean weaker reefs.

These a remarkable changes, when you consider that the cleaners are small and relatively rare. So far, scientists have looked at what happens to coral reefs when you remove common reef fish or big predators through overfishing. These studies clearly show that such removals can change the surrounding reef communities, but Waldie has found that displacing this single species of tiny cleaners has the same effect as massive overfishing.

Waldie writes, “The large-scale effect of the presence of the relatively small and uncommon fish, Labroides dimidiadus, on other fishes is unparalleled on coral reefs.” He recommends that conservationists pay more attention to this unassuming fish, especially since it is one of the top ten most exported aquarium fish to the USA and the UK.

Reference: Waldie, Blomberg, Cheney, Goldizen & Grutter. 2011. Long-Term Effects of the Cleaner Fish Labroides dimidiatus on Coral Reef Fish Communities. PLoS ONE http://dx.doi.org/10.1371/journal.pone.0021201

More on cleaner fish:

• Cleaner fish punish cheats who offend their customers
• Fake cleaner fish dons multiple disguises