The cleaner wrasse, Labroides dimidiatus, is a key organism for reef fish diversity at Ras Mohammed National Park, Egypt

Summary In marine cleaning symbioses, small fish or shrimps remove ectoparasites and other material from cooperating ‘client’ fish. Until now, there has been conflicting evidence with respect to the importance of cleaning for client health and in particular with respect to the impact of cleaning organisms on local fish diversity. Data are provided on the impact of the cleaner wrasse Labroides dimidiatus on reef fish distribution at Ras Mohammed National Park, Egypt. During four field seasons, I regularly sampled fish species composition in 46 reef patches. Natural fluctuations in cleaner fish presence or absence at these reef patches allowed me to assess how this variable correlates with reef fish diversity. In addition, cleaner fish were removed experimentally from some reef patches and added at others to test whether the presence or absence of cleaner fish causes changes in fish diversity. Both cleaner fish disappearance and cleaner fish removal had no immediate effects on fish abundance, i.e. within the first few weeks. A significant decline in fish diversity was only detectable after 4–20 months. In contrast, cleaner immigration or experimental addition led to a significant increase in fish diversity within the first few weeks. Generally, effects were most pronounced for client species that visit reef patches and soon move off again, and less pronounced (but still significant) for resident clients that do not move between reef patches. Surprisingly, the presence or absence of cleaner fish also affected the distribution of resident fish that hardly ever interact with them. This study demonstrates that cleaner fish are a key organism for local reef fish diversity.