Very cool! Rabbitfish may be the first fish observed exhibiting "watchdog" behavior
Take a look at the photos above from the Great Barrier Reef. Siganus vulpinus, S. corallinus and S. puellus pairs are cruising the reef grazing algae. While one eats, the other appears to "adopt a posture of vigilance." In other words, one rabbitfish is looking out for his/her companion. This is the first time this behavior - once believed reserved for higher ordered animals - has been documented in fish.
For anyone with pairs of con-specific grazing fish like tangs, rabbitfish, angels, or butterflies, have you observed this type of phenomenon in your aquarium? It would not surprise us if this behavior occurs with regularity in our aquariums but goes unnoticed.