by B. N. Sullivan
At first glance, all the fishes in the photo look alike. They're similarly shaped, their bodies are white with lines, and they all have yellow fins and black trim, including a black bar over the eye. But if you look more carefully you will notice that there actually are two species of Butterflyfishes swimming together.
The two fishes in the foreground of the photo -- the ones with the black spots near the trailing edge of their dorsal fins and the diagonal lines on their bodies -- are Threadfin Butterflyfish (Chaetodon auriga).
The larger fish in the upper part of the photo is a Lined Butterflyfish (Chaetodon lineolatus). Note that it has a broad black arc on its back, instead of a spot, and the lines on its body are vertical rather than diagonal. (And yes, that's another Lined Butterflyfish behind the two Threadfins.)
Both of these are Indo-Pacific species, and they are widely distributed. We have seen them -- usually in pairs -- in many locations from the Red Sea all the way to Hawaii. The ones here were photographed in Honaunau Bay, Hawaii.
Earlier we posted close-up images of each of these species. If you take a look at those earlier photos of the Threadfin Butterflyfish and the Lined Butterflyfish, you will be better able to appreciate the differences in their markings.
So, if you see a pretty Butterflyfish -- white, with lines on its body, and with yellow and black trim -- now you know which details to look for that will distinguish the Threadfin Butterflyfish from the Lined Butterflyfish.