Have you heard about pearlfish? They live in the bums of sea cucumbers.

No, it’s not the start of a terrible joke—it’s just one of many examples of how bizarre life in the ocean can be. But the weirdness doesn’t stop there, according to marine biologist Professor Maria Byrne. She was examining a giant sea cucumber specimen that had been sent for classification as a potential new species when she noticed something odd: there was nothing inside it.

“Then I saw there were two massive pearlfish inside this big sea cucumber,” Byrne says. “They’d eaten everything! Because sea cucumbers are so good at regenerating their own body parts … the pearlfish were having a great old time, sitting there with their food source, which would just regenerate its guts and gonads any time the pearlfish nibbled away at it.”

Read the full article at the Australian Academy of Science.