Fairy wrasses are some of the most ubiquitous fishes in the marine aquarium trade, which is why it may come as a surprise to learn that the group has never received a serious taxonomic review. The scientific literature is filled with scattered species descriptions, but there is typically nothing more than a cursory sentence suggesting possible evolutionary relationships within the genus. And so Cirrhilabrus, despite being the most species rich of the wrasses, is very much a phylogenetic cipher. In an effort to fill this void, Lemon TYK and I will be presenting a series of articles covering this genus in all its baroque detail.
Cirrhilabrus currently stands at 49 described species, but this number is likely to be far lower than it should be—my best estimate is that there could potentially be greater than 60 distinct taxa. I use the word “taxa” because much of this argument centers on how we define the term “species”. The arbitrariness of its application within fairy wrasse classification has much to do with the current state of affairs. For instance, should a widespread and phenotypically diverse “species” like the Exquisite Wrasse (C. exquisitus) be treated in the same manner as the rubriventralis Group, whose members are equally diverse but are split into multiple “species” with separate ranges throughout the Indian Ocean? There’s clearly little argument to support this inconsistent usage, as the only real difference here may be the relative ages of these two taxa.
Throughout the numerous species groups we can see a consistent pattern of biogeographic endemism. Species from the Red Sea are likely to have a nearly identical sister taxa along the East Coast of Africa and another in the Maldives. Moving eastward, the waters of the Andaman Sea, Western Sumatra and Java typically has its own species distinct from the rest of the “coral triangle”. This hints at some of the historical geography of the region, when lower sea levels and different oceanic currents limited dispersal between the Indian and Pacific Oceans. To this day, there is still only a small region centered on Bali where the different oceanic populations converge.
Examining the Pacific groups, we find a recurrent theme of speciation forming an arc of similar taxa from Japan, through Indonesia and New Guinea, and eastward into the Melanesian islands of Fiji, Tonga and Samoa. Again, we find inconsistencies in how taxa are treated. In some groups (lanceolatus/roseafascia or lunatus/brunneus), Japanese and “coral triangle” populations are treated as separate species. In others (rubrimarginatus and temminckii), a diverse series of related populations are treated as a single taxon. But, why is one group more species-worthy than the other? This forces us to address one of the most vexing questions in biology: what defines a species?
There are of course no easy answers here. However we choose to define our species concept, exceptions will occur. If we look at a group like the fairy wrasses, we see a series of presumably isolated populations which share genes amongst themselves by interbreeding, and which are likely to have limited genetic exchange with neighboring populations. This is the classic model of allopatric speciation. But this raises an important question, how long must two populations be in isolation before speciation can be said to occur? Should the various populations of temminckii be treated as a single species based on the assumption that this taxon is young and hasn’t been isolated enough for “true species” to have formed? There is simply no data to confirm this one way or the other.
And what about species with wide ranges and limited phenotypic differences? The various populations of rubrimarginatus have little to recommend them as distinct taxa, but presumably there is genetic regionalism here waiting to be discovered. How much genetic divergence would be required to split this single species into its three major subpopulations? Is a 2% difference in its genetic code enough? How about 1%? Is it enough to say that populations with limited interbreeding are separate species even if the DNA is identical?
Sympatric speciation, which is the creation of new species within the same geographic range, seems to be a rare phenomenon in Cirrhilabrus. The two most obvious examples are poorly studied and somewhat contentious. Cirrhilabrus brunneus and the undescribed “cf lunatus” are both found in overlapping ranges in Indonesia and the Philippines, but as both of these are known from relatively few specimens, it may turn out that these are merely different sexes of a single species. And what of Cirrhilabrus lubbocki and flavidorsalis, another pair with completely overlapping ranges? There are already two color forms of lubbocki, couldn’t flavidorsalis be a third? Without any published research on whether these variants interbreed or what genetic divergence can be found here, how are we to know if there is a single heterochromatic taxon, three sympatric species or the status quo of two “species”? And then there is C. condei, which may have an allopatric sister species in Fiji and a sympatric species in New Guinea. To summarise the current state of affairs, confusion reigns.
While we may not yet have a firm grasp at the species level, examining the relationships between species groups appears more straightforward. The phylogenetic tree presented in this article is the first comprehensive treatment I’m aware of, but it comes with the major limitation that it is based on little more than similarities of coloration and body shape. Morphologically, Cirrhilabrus shows little differentiation. Commonly used characters like fin counts, scale counts and gill counts show a confusing amount of overlap. Juvenile coloration is often diagnostic within a species group, but seems less informative between them. While I expect much of this tree to stand up to more serious study, there are almost certainly erroneous relationships here. Fortunately, several researchers are studying this group’s genetics, and it is only a matter of time before a proper study appears. Until then, you’ll have to make do with this preliminary effort.
A few points concerning this tree merit discussion. There are three major lineages: one is a basal branching of the lubbocki group, while the other two lineages can be divided by the relative size of the pelvic fins, as well as a tendency for the caudal fin to flash during nuptial displays. This latter character is developed most fully in the temminckii and rubriventralis/filamentosus Groups, which can turn the entire caudal fin a reflective blue or gold during displays. With the exception of the cyanopleura Group, all of the major species groups in this lineage also have at least one species with a light girdle present beneath the anterior dorsal fin in nuptial males.
A few groups are particularly difficult to place into a coherent phylogeny. Most troublesome is the lubbocki group, whose morphology and pattern (particularly the unstriped females) seem different enough to suggest their basal placement, but which could equally belong alongside the other groups with short pelvic fins. Some authors have suggested a possible relationship with C.bathyphilus, but the differences in head shape and body pattern seem to argue against this. The bathyphilus group is also of uncertain placement, as it seems best diagnosed by a lack of characters rather than the presence of its own unique apomorphies. Its isolated range suggests it may be a phylogenetic relic, but perhaps more species in this deep-dwelling group await discovery.
And then there’s Cirrhilabrus laboutei, whose bizarre anal fin extensions and unusually rounded caudal fin suggest a highly isolated lineage. In fact, my initial tree used these differences to place this as the basal-most clade, but the great similarities in body patterning with species like C. exquisitus and C. lunatus ultimately led me to include it alongside them. An alternative would be to regard the prominent dorsal and medial stripes as ancestral in the genus, with groups like lubbocki and bathyphilus being derived, miniaturized taxa which have lost this ancestral feature. This may in fact be a more parsimonious way of looking at things, but only genetic study can clear this matter up.
Future articles in this series will focus on each of the species groups, of which there are around eleven. That is to say, this has been no small undertaking. Rather than my dry taxonomic drivel, you’ll have the pleasure of being guided through this genus’ immense diversity by the flowery prose of Lemon TYK. Hopefully we can do justice to the fascinating story these fishes have to tell…
Photography credits for the collage photo include Kevin Kohen, Hiroyuki Tanaka, Rudie H. Kuiter, Gerry Allen, John Randall and Kazu.