The pattern combines two elements. A vertical bar runs through the true eye, breaking its outline against the banded body, while a conspicuous round black spot ringed in white sits far back on the dorsal fin. Many butterflyfish share this arrangement, and it appears repeatedly across unrelated reef fish families.
The standard explanation is deflection: a predator aims at what it takes to be the head, the fish accelerates in the opposite direction, and the attack lands on expendable fin tissue. Damage patterns on wild fish fins are sometimes cited in support, and experiments with model prey have shown that conspicuous rear markings can shift strike location.
The evidence is not uniform. Some studies find no relationship between eyespot presence and fin damage, and an alternative reading is that the spot intimidates smaller predators by mimicking a large eye, or functions in species recognition among butterflyfish themselves. The deflection story is plausible and partially supported rather than demonstrated for every species that carries the mark.