Redouan Bshary is best known for studying cleaner wrasse—tiny underwater hygienists that pick parasites from much larger fish, like the roving coral grouper. In 2006, Bshary decided to follow one of the groupers to see whether it sought the services of several cleaners in a row. Instead, he saw something wholly unexpected. The groupers repeatedly swam up to giant moray eels and made a vigorous head-shaking signal. It was a call to arms—a signal that meant “Hunt with me”.
The eels respond by swimming off with the groupers. They can slink through crevices and flush out hidden prey, while the groupers are lethal in open water. When they hunt together, little fish have nowhere to flee.