We might think of some reef fish as being rare but this video of a huge school of wrought iron butterflyfish shows how locally common some fish species can be. While most aquarist would be content to see even one Chaetodon daedalma here in the U.S., these lucky divers at have born witness to a what they call a “wrought iron ball” with hundreds of medium sized fish swimming together and converging and dispersing from grazing spots within seconds. The video was uploaded by a dive shop called Bientos that operates out of Hachijo-jima, a popular dive spot in the Izu Islands South of Japan but north of the Ogasawara Islands. We’ve posted our share of wrought iron butterflyfish videos in the past but this writhing mass of this species really takes the cake.