Butterflyfish: The Reef Fish That Sleeps in 'Pajamas' and Signals Coral Decline

Butterflyfish are among the most striking fish on coral reefs, with flat bodies, bright colors and bold stripes. The article explains that the family, Chaetodontidae, includes more than 120 species found mainly in tropical oceans. Jordan Ratner, a master’s student in Prof. Roi Holzman’s lab at Tel Aviv University and the Interuniversity Institute in Eilat, describes their unusual behavior, feeding strategy and ecological role.

In the Red Sea, including Eilat Bay, butterflyfish live throughout tropical regions of the Indo-Pacific, the Red Sea and the Atlantic, with examples such as Australia, Indonesia, Hawaii, Egypt, Sudan, Brazil, Mexico, Belize and Florida. Many species live as long-term monogamous pairs, and if one partner disappears, the other may stop eating or show distress. They are also highly territorial, patrolling their range several times a day and threatening intruders by fanning their spiny dorsal fins and swimming aggressively side by side.

At night, butterflyfish change from their bright daytime colors to dark, blurred camouflage patterns that help them disappear among rock crevices while sleeping. This “pajama” coloration protects them from nocturnal predators such as moray eels and lionfish. Their most unusual trait is a second jaw joint, which lets them bite and pull prey, such as coral or sea anemone polyps, with the jaws still projecting forward, creating a strong and efficient tug without using the whole body.

Because many butterflyfish species feed directly on living coral tissue, they are considered bio-indicators. Their disappearance from an area is often one of the first signs that a reef is deteriorating or bleaching. Ratner says warming seas are the main threat, since coral bleaching and widespread reef death directly reduce their food supply. Her research, carried out on natural reefs with high-speed cameras running at 500 frames per second and a force sensor, examines how butterflyfish detach prey of different sizes and whether jaw mechanics or flexible feeding strategies drive success. She says the hardest part is earning the fish’s trust, noting that one individual they have studied since winter 2022 now approaches the setup with no fear.