News / Species Spotlight / Making Sense of the Cocopeel Angelfish (07/26/17)

Making Sense of the Cocopeel Angelfish

Species Spotlight - Making Sense of the Cocopeel Angelfish
The Central Pacific Ocean is home to a vibrant yellow dwarf angelfish named Centropyge flavissimathe Lemonpeel Angelfish. This is a common inhabitant of shallow reefs throughout the region, from French Polynesia and the Marquesas in the east to Fiji and Vanuatu in the west and up north into Micronesia, the Mariana Islands and, at its furthest extent, the Ogasawara Islands. But, strangely, there is another very similar yellow fish found far, far away in the Eastern Indian Ocean (specifically, Christmas Island and the Cocos-Keeling Islands), and, though they are separated by a vast stretch of ocean, these two disconnected populations have for years been treated as the same species.

But this never really made any sense. How could C. flavissima have colonized all of the Central Pacific, as well as a few tiny islands in the Eastern Indian Ocean, but somehow not establish itself anywhere else in the middle?! Some researchers suggested the possibility that the Indian Ocean population was a relic from a time when the Lemonpeel was more widespread. The West Pacific reefs that separates these two disconnected ranges are currently occupied by a closely related species with a very different colorationthe Half-black or Pearscale Angelfish (Centropyge vrolikii)so it was hypothesized that maybe the arrival of this species pushed the Lemonpeels out?

And this is where the Lemonpeels story starts to get especially confusing, for this lustful is not content to breed only amongst itself. Rather, everywhere that Lemonpeels and Pearlscales intermingle in the wild, the two form hybrids. These interspecies offspring show, to varying degrees, traits associated with both parent species sometimes dramatically so, as with some extreme examples of Lemonpeel-like specimens covered head to toe in bright blue markings. Of course, neither parent has such a patterning, so this really illustrates some of the unusual genetic consequences from mixed-species mating. Hybrid offspring do not necessarily need to look like either of the parents involved.

The main areas of overlap are in places like Vanuatu and the Ogasawara Islands south of Japan, though the northernmost Philippines, Palau and the Great Barrier Reef are also host to both species. Researchers refer to these as biogeographic suture zones, and its the confluence of species which normally dont encounter one another in the wild that allows for hybridization to occur in relative abundance. The same holds true in the Eastern Indian Ocean, as both Christmas Island, the Cocos-Keeling Islands and nearby Bali act as an intersection where the two oceanic faunasIndian and Pacificmeet and mate.

And now we can introduce the third party in this pomacanthid triumvirateCentropyge eibli, known variably as the Red-stripe, Blacktail, or Eibls Angelfish. This beautiful species occurs in the Andaman Sea and down the western coastline of Sumatra and Java, all the way south to the reefs of Northwestern Australia, where the species if relatively uncommon. In areas where it overlaps with the Lemonpeels of Cocos and Christmas, the two produce a stunning hybrid known as the Tigerpyge. This is one of the rarest and most desirable angelfishes in the aquarium trade, complete with a price tag worthy of its immense beauty.

There has been a burst of genetic research into what these various angelfishes are doing at Cocos. All three species occur here, though in different relative abundance, and just about every sort of hybrid and backcross can be encountered between them. But there is still no consensus when it comes to the yellow angelfish found here. Is it just a disjunct population of C. flavissima is it a unique species is it a hybrid or is it maybe just a xanthic color variation of either C. eibli or C. vrolikii?

One group of researchers who examined the problem came to the conclusion that the Cocopeel was actually a distinct species, which they named C. cocosensis. They pointed out a unique feature of this fish to justify their stancethe blue coloration of its eye (versus the blue ring which encircles the eye in the C. flavissima of the Central Pacific). However, when they looked into this fishs DNA, they were unable to find any meaningful difference between it and C. eibli. And this raises an interesting question: is the Cocopeel just a yellow variation of the Red-stripe Angelfish?

In another recently published study, a different lab took a stab at solving this mystery, and they arrived upon essentially the same results, but with a radically different interpretation. The Cocopeel is NOT genetically distinct from the main dwarf angelfish population in the Eastern Indian Ocean, C. eibli, and, in the opinion of these authors, is probably not deserving of species recognition. So what is making the fishes from Cocos and Christmas yellow?

In all likelihood, it probably has something to do with the rampant mixed-species breeding going on in this portion of the world. Perhaps one possible result of mixing a Pearlscale Angelfish and a Red-stripe Angelfish is a yellow hybrid, and if these xanthic progeny happen to be genetically dominant, it wouldnt be long before they became fairly common on these isolated oceanic islands. At least, this is what the genetic data seems to suggest. But does that make the Cocopeel a truly distinct species? Some would say yes, pointing out its unique coloration and relative abundance, while others would argue no, due to the lack of genetic differentiation and its willingness to breed with both C. eibli and C. vrolikii.

Its hard to know which side is correct here. Evolution can be a messy process, and its possible we are witnessing the recent birth of what will eventually become a new yellow dwarf angelfish species in the Eastern Indian Ocean. Or perhaps not.