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Signals transmit information to receivers about sender attributes, increase the fitness of both parties, and are selected for in cooperative interactions between species to reduce conflict [1, 2]. Marine cleaning interactions are known for stereotyped behaviors [3-6] that likely serve as signals. For example, "dancing" and "tactile dancing" in cleaner fish may serve to advertise cleaning services to client fish [7] and manipulate client behavior [8], respectively. Cleaner shrimp clean fish [9], yet are cryptic in comparison to cleaner fish. Signals, therefore, are likely essential for cleaner shrimp to attract clients. Here, we show that the yellow-beaked cleaner shrimp [10] Urocaridella sp. c [11] uses a stereotypical side-to-side movement, or "rocking dance," while approaching potential client fish in the water column. This dance was followed by a cleaning interaction with the client 100% of the time. Hungry cleaner shrimp, which are more willing to clean than satiated ones [12], spent more time rocking and in closer proximity to clients Cephalopholis cyanostigma than satiated ones, and when given a choice, clients preferred hungry, rocking shrimp. The rocking dance therefore influenced client behavior and, thus, appears to function as a signal to advertise the presence of cleaner shrimp to potential clients.

Aim We studied the underlying biotic and abiotic drivers of network patterns in marine cleaning mutualisms (species feeding upon ectoparasites and injured tissues of others) at large spatial scales. Location Eleven marine biogeographical provinces. Time period 1971–2018. Major taxa studied Reef fish and shrimps. Methods We combined field and literature data to test whether recurrent patterns in mutualistic networks (nestedness, modularity) describe the distributions of marine cleaning interactions. Nested network structures suggest that some cleaner species interact with many clients while the others clean fewer, predictable subsets of these clients; modular network structures suggest that cleaners and clients interact within defined, densely connected subsets of species. We used linear mixed models to evaluate whether the life‐history traits of cleaners contribute to the emergence of these patterns locally and whether environmental and geographical factors influence the network structures. Results Marine cleaning networks were more nested than modular. Nestedness was prevalent in communities with dedicated cleaners (ones that feed exclusively by cleaning), whereas communities with only facultative cleaners (ones that clean opportunistically) were generally unstructured. Cleaner type and taxa were the only traits shaping networks, with dedicated fish cleaners contributing disproportionally more than facultative cleaners and shrimps to the emergence of nestedness. Although cleaner species seem concentrated around the tropics and biodiversity centres, we did not detect an influence of environmental and geographical factors on network structures. Main conclusions Dedicated species are key in shaping the structure of marine cleaning mutualistic networks. By relying exclusively on cleaning to feed, dedicated cleaners interact with most of the available clients and form the network core, whereas the opportunistic facultative species tend to clean the most common clients. We hypothesize that trophic niche variation and phenotypic specialization are major drivers of this asymmetry in marine mutualisms. Our study strengthens the links between biotic interactions at the community level and the distribution of species and specializations at larger spatial scales.

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This study investigated the spatial distribution patterns of three shrimp species, Periclimenes holthuisi, P. brevicarpalis, and Thor amboinensis on the sea anemone Stichodactyla haddoni in the laboratory. Anemones were partitioned into ¢ve zones (mouth, inner tentacle, outer tentacle, upper column, and lower column), and shrimp distribution on these zones was determined. Regardless of species, signi¢cantly higher numbers of shrimps chose outer tentacles (440%) over other zones during daytime. Such distribution might be attributed to their feeding practices as these crustaceans clipped and ate parts of the outer tentacles. Periclimenes holthuisi also showed varying temporal distribution patterns on their hosts. At night when anemones contracted their tentacles, shrimp moved in signi¢cant numbers from the outer tentacle region either to the column or o¡ the anemones. Shrimps returned to the tentacles during daytime when anemones expanded their tentacles. Thus, spatial and temporal distribution of shrimps depend upon their feeding activities and degree of anemone expansion.