The family Acanthuridae boasts of charismatic, charming and iconic reef fishes that are immediately recognizable both in the field and in the trade. All eighty or so species spanning the six various genera have a single unifying trait, and that is the possession of razor sharp caudal peduncular spines. The family is diverse and well represented across all major oceans, spanning a cosmopolitan distribution wherever suitable habitat is provided.
In accordance to the family’s diversity, Acanthuridae are unsurprisingly heterogeneous in their form, with different genera or species groups adopting their own characteristic charm. The genus Ctenochaetus for example, have hinged jaws which differ greatly from those of Acanthurus. Naso and Prionurus have unretractable caudal scalpels, and members of Zebrasoma are well known for their elongated rostrum and sail like median fins. Although Acanthurus by far and large remains to be the most populous and cosmopolitan genus, it is Zebrasoma that proves to be the blue eyed boy of the family. Like the nigricans group complex in the genus Acanthurus, Zebrasoma will be receiving the same treatment, and in this article we discuss the speciation, biogeography and hybridization of this well loved genus.
The seven species of Zebrasoma (Z. scopas, Z. flavescens, Z. xanthurum, Z. gemmatum, Z. veliferum, Z. desjardinii and Z. rostratum) form a monophyletic group, with various members within forming their own cladistic distinctions. It has been hypothesized by Guiasu and Winterbottom that Z. veliferum (and subsequently Z. desjardinii) was the first species to diverge away from the Zebrasoma lineage, followed by Z. gemmatum. The remaining species forms a monophyletic clade, in which Z. rostratum is either the sister group of Z. xanthurum, or the sister species to Z. scopas and Z. flavescens.
We feel that Z. rostratum (with its unusually long snout and silver-streaked dorsum) is perhaps best left as a basal branch for the remaining four species, in which Z. gemmatum forms a cladistic pair with Z. xanthurum, and Z. scopas with Z. flacescens. This may be a little contentious compared to the published phylogeny hypothesised by Guiasu and Winterbottom, but the biogeography of both Z. gemmatum and Z. xanthurum (as well as both species possessing yellow caudal fins and spots on the body) may suggest a closer relationship than previously mentioned. The wide geographic overlap of Z. rostratum with Z. scopas also suggests their dissimilarities in cladistics, seeing as sister species are unlikely to be so widely sympatric. Again, these are just casual observations from a different perspective, and seeing as any number of varying characteristics can be used in consideration for a phylogenetic analysis, we might very well be wrong.
Phylogenetics aside, the genus Zebrasoma have evolved certain characteristics that set them apart from the rest of Acanthuridae. For one, all the members appear taller than they are wide, with very distinct sail-like dorsal and anal fins. These are particularly evident in their juvenile forms, in which they look remarkably disproportionate. As the juveniles reach adult hood, the elongation of their bodies compensate for the height of their sails, and the fish attains a more homogenous dimension. Their rostrums are elongated and pointed, with terminally placed mouths that are lined with small teeth adapted for picking at filamentous algae. In at least one species, this feature is highly prominent and diagnostic, lending the usage of its specific epithet “rostratum” to be very appropriate.
Their single, retractable peduncular spines are cloaked in a thin sheath, and in at least four species (Z. scopas, Z. flavescens, Z. xanthurum and Z. rostratum), a patch of fine setae (stiff bristles) is present anterior to this spine as a raised, oval mark. It has been suggested that the presence of this setae is sex linked, appearing only in large males, serving as a sexually dimorphic trait. However, not much is known about this curious morphological feature, and what function it serves or whether or not all species possess it remains unclear.
Zebrasoma veliferum is the most recognisable of the genus, and is the only one of two striped members. The species is hazelnut to chocolate, with five custard stripes running parallel along the body (reminiscent of a giant kinder bueno). The face is heavily spotted in large adults, and the caudal fin is yellow with a white perimeter. The darker body coloration between the bars are striated in orange which breaks into shorter dashes and spots nearing each terminal end.
The juveniles are washed in a strong yellow overtone, and the bars (except the ones on the face) are highly diffused and indistinct. Like many Zebrasoma, Z. veliferum is disproportionately tall in its juvenile phase, but the body elongates with age to compensate for the height of its median fins. Z. veliferum is widespread in the Pacific Ocean, and can be found in Vietnam eastward to the Pitcairn group, Hawaiian Islands, northward to southern Japan, southward to Rottnest Is., and New South Wales, Australia and Rapa, French Polynesia except the Marquesas. It is most often collected out of Indonesia and the Philippines. Z. veliferum strays to the Christmas Islands, where it is at its Indian Ocean limit. In the rest of the Indian Ocean, this species is replaced by its allopatric geminate sister, Z. desjardinii. This species is likely to hybridize with Z. desjardinii in the overlapping region of the Christmas Islands.
The Indian Ocean representative of the striped Zebrasoma clade is Z. desjardinii. This species is highly similar to its Pacific Ocean counterpart, and is most easily discerned only in its adult form. Z. desjardinii is dark chocolate brown with cream colored stripes confined only to its face, which is otherwise heavily spotted. Instead of the typical custard stripes, Z. desjardinii is marked with a series of vertical orange stripes which run parallel along the body. The ventral region is heavily spotted in the same orange, and a series of fine striations trace the dorsal and anal fins in concentric semicircles. The caudal fin is black and spotted in white, and this serves as the most reliable distinction between the two sisters.
Juveniles and young adults adopt the more typical veliferum appearance, and are adorned in the usual custard stripes. The median fins are however, heavily reticulated in orange as with the adults, and the bars fade away with age. Z. desjardinii is found in the Red Sea, south to Natal, South Africa and east to India, Java and the Cocos-Keeling Islands. In the Pacific Ocean and the Christmas Islands it is replaced by Z. veliferum. Z. desjardinii is most often exported out of the Red Sea. This species is likely to hybridize with Z. veliferum in the overlapping region of the Christmas Islands. Together, Z. veliferum and Z. desjardinii represents the “striped clade” of the Zebrasoma group.
In Guiasu and Winterbottom’s cladogram, Zebrasoma gemmatum stands alone in the monophyletic group, unpaired with any other species. The spotted appearance is a rather unique trait for this genus, with Z. xanthurum being the only other species to show a trace amount of this characteristic on its head. In Z. gemmatum, the body coloration is asphalt grey to obsidian, with the facial region being a lighter, more chestnut-brown. A pair of glaucous, almost gelatinous appearing streaks are present behind the operculum, just above the pectoral fin. The body and median fins are heavily spotted in white or the faintest shade of alice blue, and in fully grown specimens, these may conjoin and realign to form a series of weakly noticeable stripes. The caudal fin is hyaline yellow and spotted as well.
Z. gemmatum is restricted to the Indian Ocean, where it can be found in Mauritius to Madagascar and Natal, South Africa. It has also been recorded from Mozambique. This species is often encountered singly, and never in any great amount. The geographical proximity of this species in relation to Zebrasoma xanthurum, and the minor phenotypic similarities of the two may suggest a cladistic relationship, as seen in our hypothesized phylogenetic tree. Z. gemmatum is most often exported out of Madagascar and Mauritius. It is rare in the trade and is regarded as the most prized and expensive of the surgeonfishes, although I find this to be remarkably unjust. It hybridizes with Z. scopas throughout its range.
The love child between the two species isn’t quite as dramatic as you’d expect, seeing as both parents are dark colored in nature. The influence of scopas DNA however, muddies the spot pattern in gemmatum, and the resulting hybrid loses the uniformity in its design. The median fins are vestigially spotted, and the caudal fin adopts a hyaline brown instead of yellow. Despite its rarity and affluent price tag, the hybrid is meager and rather unimpressive, and loses shine to a pedigree Z. gemmatum.
Despite its common name of “Purple Tang”, Z. xanthurum is more accurately a deep inky cobalt with a purple slate best seen under a side light. A constellation of spots and reticulated markings cover the face and a series of innocuous stripes run parallel horizontally across the body. The pectoral fins and tail are coloured in a rich yellow, and ironically it is this feature which earns the species its name xanthurum. Juveniles are coloured the same. Zebrasoma xanthurum has the most restricted geographical range of the genus, and is found strictly in the Red Sea and the Persian Gulf. Strays are sometimes recorded, although exceedingly rarely, in the Maldives.
Aberrations in Z. xanthurum occur very rarely, and is nowhere near as common as those seen in Z. scopas or Z. flavescens. Rare individuals such as the ones above sporting fully purple bodies and piebald patterns have been documented, but the homogeneity of this species is rather strongly represented. Z. xanthurum is mostly solitary or found in small groups in the wild, and is sometimes found in the company of Zebrasoma desjardinii. Despite its bellicose and pugnacious behaviour, this species remains to be one of the hobby’s most well loved acanthurids. This species is not known to form any hybrids.
Zebrasoma rostratum is morphologically peculiar with its highly pronounced rostrum. This species ranges in color from jet to liquorice black, but in adults, a greenish tinge may invade the face and intramembranous spaces along the dorsal and anal fins. Sub-adults and adults may develop a silvery-glacous streak that traces the dorsal fin base, starting at the nape and ending at the peduncular base. A patch of setae can be seen in very large specimens, and this presents itself as a raised, oval patch just anterior of the peduncular spine. It has been suggested that this is a sexually dimorphic trait only present in males, but not much is known about this unique trait. Z. scopas, Z. xanthurum and Z. flavescens also possess this peduncular setae, although it may not be apparent in photos.
Juveniles are entirely jet black save for the caudal peduncular spine, which is white. Again, as with as Zebrasoma, the juveniles are more prominently “sail-finned”, but this disproportion eases out into adulthood. Z. rostratum ranges across the central Pacific from Tuvalu to Pitcairn including all of French Polynesia, north to the Line Islands and south to Rapa. A single waif was found in Oahu, Hawaii. Z. rostratum is uncommon throughout its geographic distribution, and occurs singly. Its rarity translates into the aquarium industry as well, where it fetches for a rather high sum. This species is most often collected out of Kiritimati, the Christmas Island. It hybridizes with Z. scopas throughout its range.
As with Z. gemmatum x Z. scopas, the incorporation of Z. scopas DNA with Z. rostratum serves to nullify much of the jet black qualities of the former. Hybrids are often dark grey-granite with a lightening of the facial region. The most discerning characteristic is the presence of very fine striae in the form of meandering squiggles on the body. This is a characteristic seen in Z. scopas, but otherwise absent in the featureless Z. rostratum. Seeing as Z. rostratum is uncommon throughout its range while Z. scopas is a swathing plague, hybrids are sometimes more common than pedigree Z. rostratum, as the former species fornicates like a demonic weasel whenever opportunity arises. This lovechild is less expensive than Z. rostratum.
Zebrasoma scopas is literally one of the most widely distributed reef fishes and can be found in nearly all suitable habitats in the Pacific and Indian Ocean. Its distribution map is featured at the start of this article, and the species can be viewed as the piscine version of the common household rodent. This species is uniformly muted brown with a series of very fine spots on its face and rostrum. The body is adorned with very fine teal lines that run horizontally across the body.
Juveniles are heavily striped, but this feature is transient and quickly disappears as the fish grows into adulthood. The adults of Z. scopas are highly mutable, being found in a wide array of color forms and patterns. Aberrations are particularly commonplace for this species, and piebalds, xanthic and leucistic individuals have all been documented. No one is really sure why this happens, or how, but this species is very much more susceptible to variation than any other Zebrasoma. Aberrations may cause confusion on the specific identity of the fish, but the presence of fine spots and striae on the body is usually diagnostic of Z. scopas, as these are completely absent in Z. flavescens.
The series of photos above shows a small sampling of the infinite variability of this species. Z. scopas is common and relatively cheap in the aquarium trade, but all of its variations and aberrations are expensive. This species probably hybridizes with the very closely related, and sister, Zebrasoma flavescens. The two species overlaps significantly in Micronesia and the Marshall Islands, but very narrowly in certain parts of Japan and the Philippines, and this overlap in the latter cannot be considered a true representation of sympatry. The presence of Z. flavescens throughout this distribution is made possible only by exceedingly rare strays and waifs, which by no means indicate a strong resident population of the species. Hybrids with the promiscuous Z. scopas are therefore not unlikely, and are most often seen in the northern regions of the Philippines and Japan. However, the existence of a xanthic Z. scopas form makes these hybrids nearly impossible to diagnose. Genetic analysis would most likely prove futile as well, seeing as both species are very similar in genetic makeup. In an analysis of the cytochrome c oxidase subunit I (COI) gene, it was found that the two species differed by only less than 0.3%. This result will likely differ in a multi-locus study, but the relationships and likewise cladistic pairing of Z. scopas and Z. flavescens in the phylogenetic tree is highly irrefutable.
Zebrasoma flavescens, or affirmably, the Yellow Tang, is undeniably the most popular Zebrasoma of all time. This evergreen staple is uniformly bright yellow overall, with chartreuse accents on the edges of the fins where it is more translucent. A single glaucous stripe runs laterally across the body, and the fin membranes may be ever so slightly tinted in orange. Juveniles are similarly coloured, but as with Z. scopas, a series of very fine striations are seen, which quickly disappears as the fish grows. Z. flavescens is distributed in the central and western Pacific, from the Hawaiian Islands and Johnston Island to the Marshall Islands, Wake, Mariana Islands, Philippines, Ogasawara and Ryukyu Islands, Japan. It was also recorded from southern Taiwan. It is rare everywhere except Hawaii, and in areas such as Japan, Taiwan and Philippines, this species is most often seen as strays and rarely indicate a strong population.
Z. flavescens, as with Z. scopas, is afflicted by the exact same aberrations. These are mainly piebald and leucistic abnormalities that are less uncommon than you think. As usual, these aberrations are highly variable and below is just a small sampling of the numerous forms present. As mentioned above, this species probably hybridizes with Z. scopas wherever the two overlap, but hybrids are impossible to discern both phenotypically and genetically due to their extreme variations and similarities in genetic make up.
In conclusion, the members of Zebrasoma are fairy straightforward to differentiate phenotypically. Things get frightfully confusing in the closely related Z. scopas and Z. flavescens, where the two species blur the lines on their taxonomic distinctions. Z. veliferum and Z. desjardinii are distinct, fully speciated members of their striped clade and cannot be confused with any other species. Z. gemmatum and Z. xanthurum still remains unclear on their cladistic placement in the phylogenetic tree, but otherwise are easy enough to tell apart based on appearance. Z. rostratum and its highly elongated rostrum is most likely apomorphic to that species, and its unlikely that it forms a sister relationship with Z. scopas as previously mentioned. My thanks to Joe Rowlett for making the distribution maps and his insights in this topic.