Reef fishes use sea anemones as visual cues for cleaning interactions with shrimp

Marine cleaners benefit diverse fish clients via removal of ectoparasites, yet little is known about how fishes locate small, inconspicuous cleaner shrimps on coral reefs. Pederson shrimp Ancylomenes pedersoni are effective cleaners in the Caribbean Sea, and additionally form obligate associations with corkscrew sea anemones Bartholomea annulata, which also serve as hosts to a variety of other crustacean symbionts. We examined the visual role of B. annulata to reef fishes during cleaning interactions with A. pedersoni by comparing anemone characteristics with fish visitation rates, and by manipulating the visibility of anemones and cleaner shrimp in field experiments using mesh covers. Rates of visitation by fishes to cleaning stations increased primarily with anemone body size and the total number of crustacean symbionts, but did not change consistently in response to covers. Fishes posed for cleaning at stations only where anemones remained visible, regardless of whether shrimp were visible. Shrimp at stations where anemones were covered performed fewer cleaning interactions with fishes, as fishes did not continue to pose when anemones were not visible. We conclude that anemones serve as visual cues to client fishes prior to cleaning by shrimp. These visual cues facilitate fish cleaning interactions with shrimp, and provide a previously unknown symbiotic benefit to cleaner shrimp from association with sea anemones.

► Sea anemones provide visual cues to fish during cleaning interactions with shrimp.
► Fish visitation rates increase with anemone size and crustacean abundance.
► Fish pose for cleaning only when anemones are visible at cleaning stations.
► Shrimp perform more cleans when anemones are visible than when they are not.
► These benefits link anemones, shrimp, fish, and parasites in multi-level symbioses.