Pederson's shrimp advertising to a coral grouper
On July 17th, 2018 we learned about

Cleaner shrimp communicate with their client fish via a set of specific visual signals

Cleaner shrimp have been found to stake a lot of their survival on their eyesight, which is impressive because their eyesight is terrible. Tiny crustaceans like Ancylomenes pedersoni were long thought to depend on their sense of smell to figure when they were safe to go out and forage, but new experimental data suggests that they get by with visual cues alone. It might help that these cues are somewhat interactive, and so the source of the shrimp’s food helps inform them when it’s time to eat, or when they might be eaten themselves.

A. pedersoni forage on the parasites and debris found on the gills, scales and even in the mouths of predatory fish in coral reefs. Blue tangs, parrotfish and snappers that could happily gobble up the shrimp themselves have learned that A. pedersoni provide bigger benefits if left to their work, and thus make a point to cooperate with the crustaceans’ foraging. Cleaning sessions start when a shrimp waves its white antennae in the water, signalling that it is ‘open for business’ to nearby fish. The fish then indicate their intent by darkening their body color as they approach, letting the shrimp know that they’re there to be serviced instead of posing a threat. Both parties stick to the script quite consistently, leading to successful partnerships over 80 percent of the time.

Which signals are sufficient?

To pin down exactly which signals mattered, researchers showed captive shrimp a variety of images on a tablet computer outside their aquarium. Thanks to the shrimps’ poor eyesight, these images didn’t need to closely resemble fish as long as they followed the expected protocol of a potential cleaning client. So as long as a collection of circles and triangles wiggled and then darkened, the shrimp were happy to go to work, wiggling their antennae and even attempting to hop on top of the the imaginary fish outside the tank. It may not seem like the shrimp were being especially discerning in their response, but it does prove that they’re looking for visual signals from cooperative fish instead of olfactory or auditory signals.

As a final step, researchers modeled the visual perception of both fish and A. pedersoni using a simulator. Software called AcuityView confirmed that the shrimp can’t make out shapes or colors clearly, which is probably why fish who want to be cleaned need to announce themselves with a light-to-dark color change. The fish, on the other hand, have slightly better vision, and can likely spot a shrimp’s waving antennae from a few feet away in the reef’s shallow water. So on both sides of this partnership, visual signals alone are coordinating how these two species manage their cooperative behavior.

My kindergartner said: Oh, it’s like the guys waving signs at the car wash. A shrimp car wash!

Source: When cozying up with would-be predators, cleaner shrimp follow a dependable script by Duke University, EurekAlert!