When presented with a free lunch of chopped up Pederson cleaner shrimp, predatory reef fish don’t hesitate: they eat it. If they knew about that, the shrimp would probably feel a lot more nervous. These fish are some of their best clients.
Cleaner shrimp make a living by plucking and eating the parasites and dead skin off fish, whose hygiene gets a welcome boost. Particularly enthusiastic – or perhaps optimistic -- shrimp even climb into fish mouths in pursuit of a thorough cleaning that's also a hearty meal.
Yet in spite of the fact that many such fish eat crustaceans as a matter of course, cleaner shrimp do not seem to end up on the menu. Perhaps not coincidentally, people have noticed certain odd behaviors happening around the same time as cleaning, such as antenna waving (human vision on left, fish vision on right),
and skin darkening, seen here in a spotted goatfish (this time "shrimp vision" on left, human vision on right):
Yet no one had actually tested to see if such behaviors were really involved in preventing unfortunate incidents – or reassuring nervous shrimp.
A team of scientists from Duke decided to investigate, and published their results last week in the Proceedings of the Royal Society B. They observed via strategically located camera the precise sequence of events between cleaners and their clients at natural cleaning stations. They recorded 199 interactions between 18 shrimp and 10 different species of fish at 10 cleaning stations on the reef of the Caribbean island of Curaçao.
When shrimp waved their unusually long, white antennae at potential clients, in 80% of cases the nearby fish assumed a cleaning posture. The shrimp were flagging down a meal while warning the meal delivery kit not to eat them.
On the other hand, fish that darkened themselves were able to persuade a non-whipping shrimp to clean them about half the time, tripling their chances of getting cleaned.
The scientists also employed what they referred to as “synthetic clients”(!), which were images of various geometric shapes in black or white on a gray ground played on an iPad next to a shrimp tank back in the lab. Black “synthetic clients” were the only ones that effectively led shrimp to flag them down, regardless of shape or movement. Simple screen darkening was not nearly as effective as a dark shape on a lighter ground.
The authors posit this system is a good example of a naturally evolved interspecies signaling system because the benefits are mutual and the signals reliable. Antenna waving reliably led to cleaning. Darkening reliably interested ambivalent shrimp. And no one but parasites gets eaten. In the 199 interactions observed, cleaning only happened four times when neither the shrimp whipped its antennae or the fish darkened. In zero of 199 interactions did anyone become scampi.
Eleanor M. Caves, Patrick A. Green, Sönke Johnsen. Mutual visual signalling between the cleaner shrimp Ancylomenes pedersoni and its client fish. Proceedings of the Royal Society B: Biological Sciences, 2018; 285 (1881): 20180800 DOI: 10.1098/rspb.2018.0800