Why do cleaner wrasse swim into larger fish’s mouths?

Cleaner wrasse operate at reef “cleaning stations,” where larger fish line up to be inspected. The wrasse picks off parasites, dead skin, and bits of damaged tissue, and the host fish often holds still to avoid interrupting the service. This is a classic example of mutualism: the cleaner gets food, and the larger fish gets a health check. Some cleaner wrasse species even enter the mouths and gill chambers of bigger fish without being eaten, which shows how strongly the behavior has evolved into a recognized interaction. Scientists have studied cleaner fish because they offer a striking example of cooperation in nature.