Life on the reef can be a complex balance for many of the creatures who live there. Survival depends on getting food and avoiding becoming food. Occasionally, survival depends on relationships between creatures, requiring them to work together. What could be more surprising than an understanding between huge fish and minute fish that allows the smaller ones to swim around inside a larger fish's mouth in complete safety.
Large creatures, such as this Nassau Grouper cruise over the coral in search of specific fish, cleaner Wrasses, who will provide him a service that keeps him healthy and free of parasites.
When the Grouper finds these fish congregated in one area, a "cleaning station", he must adopt an upward facing pose or a downward facing one. He must also open his mouth and gills and float motionless. This behavior is a signal to the cleaner fish that it is safe to approach and enter his mouth. Even though the Wrasses are just one chomp away from death, they are completely confident that they will not be eaten.
This is a symbiotic relationship that benefits the Wrasses, as they will get food from the Grouper. But it also benefits the Grouper because he will be free of food particles that could cause infection and discomfort and he will be cleansed of parasites that could threaten his health.
This relationship requires complete trust. It is an understanding that is almost as old as the reefs themselves and it exists between the cleaner fish and other animals such as eels, rays, turtles and sharks.
The cleaner fish will not engage in cleaning behavior until the larger creature gets into the right position. When they do, the cleaner fish emerge from the coral immediately and begin scouring all surfaces of the host. The delicate balance that exists in the ocean is extremely complex and fascinating.