By Shannon Moorhead, SRC Masters Student
If you’ve ever gone diving on a tropical coral reef, you may have noticed some of the fish seemed to be behaving rather strangely: a solitary fish hovering just above the reef while smaller fish pick at its skin and mouth. While this may appear bizarre compared to the other creatures zipping about the reef, this is a common behavior in the animal kingdom known as cleaning symbiosis. Cleaning symbiosis occurs when, after brief communication between the animals involved, a cleaner animal removes harmful materials from a client animal of a different species. This is a mutually beneficial act for both the cleaner and the client: the client is ridded of parasites and dead skin while the cleaner gets an easy meal!Many water-dwelling organisms have been observed cleaning others. Currently, researchers estimate there are about 208 species of marine and freshwater cleaner fish and 51 species of marine cleaner shrimp known to the scientific community. Some of these species are considered “dedicated cleaners”; for these fish and shrimp, cleaning is a major aspect of their lifestyle once they grow past the larval stage. Other, less committed cleaners, are designated as “facultative cleaners”. There are various levels of facultative cleaning. Some facultative cleaners are opportunistic, only partaking in cleaning when the opportunity presents itself. Others may be temporary, acting as cleaners during only a portion of their life cycle.
Interspecific communication between cleaner and client is a key aspect of cleaning symbiosis. Acts of assertion or submission, by client or cleaner or both, are the catalysts that initiate the cleaning process. Often, cleaners will perform a “dance” or touch the client to signal their intention to clean; the willing client then submits and poses itself in a way that indicates its acceptance of the cleaning. While visual cues are important to this process, in some cases tactile stimulation is just as, if not more important. By approaching and touching the client, cleaner fish make it clear that they are not prey items, as prey would not directly approach a potential predator. Cleaning interactions between cleaner shrimp and moray eels, who have poor vision and are primarily nocturnal, are most likely initiated solely by tactile stimulation. Shrimp have been observed touching eels with their antennae and legs, followed by morays submitting and opening their mouth, allowing the shrimp to begin cleaning.Though the cleaning relationship is usually mutually beneficial, sometimes the cleaner or client may take advantage of the other’s trust, referred to as “cheating”. Cheating occurs when the symbiotic relationship is disturbed by one of the partakers. For example, clients have been known to eat their cleaners and cleaners have been reported choosing to eat the mucus or healthy scales of their client fish instead of ectoparasites or dead or diseased tissue. Despite the threat this breach of contract poses to the health of cheated participants, it does not occur often enough to outweigh the benefits of cleaning symbiosis.
Though it is well known that the removal of ectoparasites is beneficial for the health of fishes, the ecological balance maintained by cleaner organisms is poorly understood. Many studies have attempted to quantify loss of reef fish abundance and diversity after the removal of one or multiple species of cleaners from a reef, with varying results. Some studies reported little change in the number of fish, while others reported drastic differences in the number and diversity of fish observed, as well as increases in the number of lesions on remaining fish. The large diversity and abundance of cleaners in marine ecosystems suggest they perform a critical ecological function; the discrepancies among study results make it clear that more research must be done to fully understand the intricacies and significance of cleaning symbiosis.
Vaughn DB, Grutter AS, Costello MJ, Hutson KS (2016). Cleaner fishes and shrimp diversity and a re-evaluation of cleaning symbiosis. Fish and Fisheries: 1-19. Doi: 10.1111/faf.12198