Communicating with mutualistic partners: sensory systems and signal active space in cleaner shrimp-client fish interactions

Background/Question/Methods

In some mutualistic systems, engaging in mutualistic interactions influences movement ecology via one partner being motile, while the other occupies a set location. In cleaner shrimp-client fish mutualisms, cleaner shrimp occupy fixed locations called cleaning stations, at which they remove and consume ectoparasites and dead skin from “client” fish. Over 50% of client visits are by species that are crustacean predators, but cleaner shrimp are almost never consumed by clients. Evidence suggests that cleaners and clients have each evolved visual signals that identify cleaners as beneficial partners, rather than a food item, and clients as seeking cleaning, rather than a meal. Relevant to movement ecology, however, is the idea that signals are only effective within their active space, the volume within which a signal can elicit a receiver response. Active space in turn is influenced by the receiver’s sensory capabilities, which determine what a receiver can perceive. In this talk, I discuss the sensory capabilities of both cleaners and clients; how these capabilities constrain the distance at which cleaners and clients use signals to identify one another; and preliminary evidence that cleaner signals have evolved to be detectable by a wide range of reef fish visual systems.

Results/Conclusions

We used electroretinography to quantify color vision, and morphological and behavioral methods to quantify spatial vision, in three species of cleaner shrimp from three genera. To examine client fish vision, we used a literature synthesis approach. We then combined these measures of visual capability with in situ observations of cleaner-client interactions to determine (1) which behaviors serve as signals and (2) which species serve as clients. Among cleaner shrimps, we found evidence of a single spectral sensitivity peak—implying monochromatic, or colorblind, vision—and very low visual acuity. This constrains cleaners to identify clients as partners based on large-scale changes in brightness at close range, which aligns with field observations that clients undergo a brightness change at distances of < 10cm to elicit cleaning. Client fish, by contrast, possess a range of color vision capabilities and visual acuity at least an order of magnitude higher than cleaners. Thus, they can discern cleaner signals—which involve vigorously moving white legs or antennae—from at least 30cm. Lastly, I discuss recent Scanning Electron Microscopy data which show that cleaner signaling traits possess specialized nanostructures that increase broadband reflectance, perhaps maximizing their conspicuousness to a range of reef fish visual systems.