The Chrysopterus & Akindynos Groups

by | Jul 24, 2020 | 0 comments

The chrysopterus Group

Amphiprion chrysopterus, commonly known as the Bluestripe or Orangefin Clownfish, has traditionally been regarded as part of a diverse group, allied with the ubiquitous A. clarkii and several geographically restricted species from the Western Indian Ocean. This is a fair assumption, as these clownfishes all share certain superficial similarities in their appearance and behavior: large size, (usually) 3 stripes, catholic taste in anemones. But the available genetic studies now indicates that these three lineages are only distantly related, with their physical similarities perhaps only reflecting what the primitive Amphiprion bodyplan was like, a phenotype that would later go on to evolve in different directions with the some of the more derived lineages (e.g. the distinctive tomato, skunk, and saddleback clownfishes).

Comparing the respective biogeographies of A. clarkii and A. chrysopterus, we can see that these two groups occupy mostly separate portions of the Indo-Pacific. The range of A. clarkii begins in the Persian Gulf and extends easterly to Japan, Micronesia, and as far as Vanuatu. The notable absence of A. clarkii in Fijian and Polynesian waters is instead filled by A. chrysopterus. But rather than having entirely allopatric distributions, A. chrysopterus also extends into Micronesia and Melanesia, indicating that this is a species which has evolved throughout the Pacific Plate as a specialist on oceanic reefs. On the other hand, A. clarkii is perhaps a species which originated in the West Pacific and only more recently spread into the relatively young islands of Micronesia. Recall how at its easternmost edge (and its deepest incursion into A. chrysopterus territory), the clarkii group deviates wildly to form the distinctive A. tricinctus of the Marshall Islands. So, a combination of competitive exclusion, ecological specialization and past geographic barriers could be the reason why there are no Polynesian clarkii or Indo-Philippines chrysopterus.

For the uninitiated, it isn’t always clear how to reliably recognize chrysopterus from clarkii. There are relatively few diagnostic traits that can be relied upon, and even these tend to vary with some regularity. The orange fins alluded to in the common and scientific names of A. chrysopterus is usually the most useful trait to look for, but this trait is also present in the Melanesian and Australian clarkii populations. What’s more, small A. chrysopterus often have a brown coloration that could easily be confused with A. clarkii or, in the Southeast Pacific, with A. akindynos.

The shape and number of the white bars can also be useful for identifying this group, but this can prove to be unreliable on its own. In general, A. chrysopterus has the posterior bar entirely obfuscated by the yellow of the caudal fin (versus the usual presence of this bar, at least partially, in nearly all clarkii populations). It is the Vanuatu+New Caledonia clarkii which bucks this rule, as do some specimens from elsewhere in Melanesia—could this be construed as evidence for past hybridization between these two groups?. Differences in the shape of the anterior and middle bars follow a similar pattern. Most A. chrysopterus specimens have thin bars which often remain unconnected dorsally, particularly true for specimens from Polynesia. On the other hand, A. clarkii more often than not have wide, thick bars that connect dorsally above the head. But, again, in areas where both occur (Melanesia & Micronesia), this trait is less useful, with the bars of both groups being fairly robust. Recall also that A. tricinctus, while clearly belonging within the clarkii group, has very chrysopterus-like bars, perhaps relating to the hybridization documented between these species.

Much as we saw in A. clarkii, there are a number of distinct populations of A. chrysopterus which correlate to the established ecoregions of the Pacific. The type locality of the taxon is Palau, and it is here that we find a fish which has a white caudal fin and yellow pelvic fins. The shape of the anterior bar is unusually variable, from thin and broken to wide and dorsally connected. This form stretches east throughout the Caroline and Marshall Islands, and likely the Gilbert Islands as well. It gets replaced in the Mariana Islands by a nearly identical phenotype which differs in having a yellow caudal fin.

Perhaps the truest form of A. chrysopterus is that found in Polynesia, where the group is furthest removed from the genetic influence of A. clarkii. Here, the species has greatly reduced bars with little or no connection dorsally. The caudal and pelvic fins are all yellow, and the body has a brown, rather than black, cast to it. Specimens are well-documented from the Cook, Society and Tuamotu Islands, but those further north at the Phoenix Islands appear to have a noticeably darker look, more in keeping with the population from Fiji. This black Fijian fish is also seen at Neiu and Tonga, and can be expected at Samoa and other reefs associated with the Fijian Plate. Aside from its darker color in mature specimens and a tendency for slightly wider bars, there is otherwise minimal difference when compared to its Polynesian cousin.

Two additional white-tailed forms exist in the western parts of Melanesia, far-removed from those found to the north in Micronesia. The small ecoregion composed of Vanuatu and New Caledonia has a population that is essentially identical to those occurring at the type locality in Palau. It would be tempting to lump these together as a single wide-ranging species, but, as we saw with the clarkii group, these reef systems foster endemism and should be presumed to be distinct. This is further supported by the presence of another unique population situated between them—New Guinea, the Solomon Islands, and the Coral Sea—recognized by having black pelvic and anal fins. With how readily identifiable these various populations are, it’s rather surprising that taxonomists have not yet remedied the situation. These differences have been well-documented for many decades now.

The akindynos Group

A separate group, quite similar in its overall appearance to A. chrysopterus, can be carved out in the Southwest Pacific for A. akindynos and A. mccullochi, occurring only in the subtropical reefs of Australia and New Caledonia (and a few southerly parts of Vanuatu). Recall that this is the same area where endemic fishes like the Conspicuous Angelfish (Chaetodontoplus conspicillatus) and Feminine Wrasse (Anampses femininus<span style=”font-weight: 400;”>) cavort about, and it becomes easier to recognize the evolutionary novelty of these otherwise mundane clownfishes, a notion that has received further support from genetic studies.

Amphiprion akindynos is a dull brown fish with thin, dorsally broken stripes and a white caudal fin. It was described from Heron Island, part of a reef system called the Capricorn Group, situated south of the Great Barrier Reef. Here, the disparate marine faunas of Queensland and New South Wales intermingle, and so the thin-barred A. akindynos can be found alongside the similar looking A. cf clarkii population that occurs northwards. Not surprisingly, these hybridize regularly, such that the population here is nearly indecipherable, showing all manner of variation in the width of their stripes and an intermediate color that is not quite as dark as the southern populations of A. akindynos and not quite as light as the northern A. cf clarkii. Many biologists would be content to label this as clinal variation (i.e. phenotypic changes that slowly shift over geographic distance), but there is little indication that marine organisms speciate in this way, and the grander biogeographic patterns of this genus (and marine fishes in general) can be said to argue against this hypothesis. In retrospect, this was an awful place to describe a new species from, but the type specimens are, at least, a good match for the larger A. akindynos population, having the characteristic thin, broken stripes.

As a brief etymological aside, the epithet akindynos is entirely unique within the annals of scientific nomenclature and is derived from the Greek ἀκίνδυνος (akindynos), meaning “free from danger”, in reference to the safety this fish enjoys when fully ensconced within the stinging embrace of its actiniarian host. 

Amphiprion mccullochi is reported only from Lord Howe Island and the nearby Elizabeth and Middleton Reefs, with rare observations of its phenotype found in New South Wales. Morphologically it is nearly identical to A. akindynos, differing only in its darker, nearly black, coloration and in having a greater reduction to its bars, most often with just the anterior bar being present. Lord Howe is known for having a variety of endemic marine fishes, so it should come as no great surprise that a clownfish should speciate in these waters. A comprehensive study (van der Meer et al 2012) into the population structure of this species found evidence for past hybridization in the mitochondrial genome, as well as support from the nuclear genome suggesting limited contemporary gene flow between these two species.

East from here, in New Caledonia and the Tafea Province of Vanuatu, the akindynos phenotype miraculously reappears, though it is slightly darker in its coloration. Given the geographic separation of these regions and the tendency for endemism in this reef system, it can be presumed that this is yet another cryptic species. A somewhat analogous situation has arisen in the evolutionary history of Cirrhilabrus bathyphilus, whose population is centered on these same oceanic islands (though extending further north in the Coral Sea). It should be noted that distinguishing between the dull brown morph of small A. chrysopterus and the dull brown A. cf akindynos is… difficult. The relative darkness of the ventral fins may be of some diagnostic importance, but these species are still too poorly documented to be reliably distinguished.