The butterflyfish of the Chaetodon genus is rife with many closely related species that are differentiated based on geography, and seemingly subtle color differences. While on the Fluval Sea Flores Expedition we had the chance to observe one such pair of species living sympatrically, Chaetodon melannotus and C. ocellicauda.
The black backed butterflyfish Chaetodon melannotus is by far the most commonly seen of the two species in captivity, in fact we’d never seen a living tail spot butterflyfish, Chaetodon ocellicauda until that dive. What was most surprising about our observation of the tail spot butterflyfish is that the few specimens we observed were living on a reef where the black butterflyfish was most abundant.
This setting allowed us to take note of the real differences between these similar looking butterflyfish which are predominantly separated into Indo-Pacific and Indian Ocean geographic distributions. The most obvious feature distinguishing Chaetodon melannotus and C. ocellicauda is the separated black tail-spot in the latter species which is connected to the continuous black markings in the former.
The other distinguishing feature which is not at all apparent in photographs is that Chaetodon ocellicauda‘s body-markings are actually a dark grey color whereas in Chaetodon melannotus the markings are black like the namesake. These minutiae of the differences between sympatric species will appeal primarily to the diehard fish keepers but it just goes to show that there is real value in making observations of little-known fish in the wild, books and pictures can only convey so much.