The Sandaracinos and Perideraion Groups

by | Aug 14, 2020 | 0 comments

Most Amphiprion are aesthetically quite similar, but the so-called Skunk Clownfishes represent an exception to this rule. Nearly all of the fishes in this group have a white stripe running dorsally along the body. The three-barred pattern so ubiquitous in other groups has been almost entirely lost, with only vestiges of the anterior bar present in a few of these species. The shape of the body also stands out, being smaller (~11 cm vs. 13–17 cm), with a more angular profile to the head and a more rounded caudal fin.

Given how different this group appears when compared to other clownfishes, it comes as a bit of a surprise to find them branching from well within the Amphiprion phylogeny. Differences between mitochondrial and nuclear data make it difficult to state precisely when these fishes split off into their own clade, but both datasets agree this occurred after the clarkii lineage had formed. So, in spite of its morphological eccentricities, the perideraion Group may just be a highly derived offshoot in clownfish evolution and certainly undeserving of its current status as a separate subgenus, Phalerebus.

When we examine these species biogeographically, we find evidence for two separate lineages. While the group occurs widely throughout the Indo-West Pacific, most regions have only a single species present, with one notable exception in the Coral Triangle. This suggests that what we are looking at here is one widespread clade spanning the Indo-Pacific and a second smaller clade limited to the Coral Triangle. However, this is in contrast to the narrative presented in the available genetic studies, wherein Pacific taxa are lumped together as a single nonsensical lineage, sister to the Indian Ocean populations.

The most straightforward taxon to discuss is the yellow-finned fish with a restricted range in the Coral Triangle, the Orange Skunk Clownfish A. sandaracinos. The unusual scientific name is derived from the Greek σανδαράκινος (sandarakinos), an adjective referring to the orange color of a specific chemical, arsenic trisulfide. The yellow dorsal and caudal fins immediately stand out with respect to the white fins of other Skunk Clownfishes, which makes it all the more puzzling that this fish escaped the notice of taxonomists for as long as it did, not being scientifically described until 1972! Another salient trait can be seen in the relatively thick white stripe running dorsally from the upper lip to the caudal peduncle. The thickness is roughly twice that seen in other skunk clownfishes, and this is the only species in which it extends onto the upper lip, rather than just near to it. While these differences in fin color and stripe shape may seem minor, they allow us to readily identify this species and its hybrids.

The other species occurring in the West Pacific is Amphiprion perideraion, known as the Pink Skunk Clownfish. The scientific name comes from the Greek adjective περιδέραιος (perideraion), meaning “passed around the neck”, alluding to the distinctive white necklace-like bar of this species. The common name alludes to the peachy (rather than orange) coloration of the fish, but it’s important to note that not all populations are identically colored. With a biogeographic range that extends into Micronesia, Australia and the Fijian Plate, it should come as no surprise that this fish is variable, and that these varieties correspond geographically with patterns of endemism seen elsewhere in the genus.

Likely because of the distinctive vertical bar of this species, researchers have tended to treat it as a more distant relative compared to those taxa possessing only the dorsal stripe. But, as evidenced by species like A. mccullochi and A. ephippium, the loss of these bars does not connote any great phylogenetic significance, and we have reason to believe that A. perideraion is the geminate sister to the Indian Ocean members of this group. The most obvious trait these share is the white dorsal and caudal fins, while the thin dorsal stripe, which extends anteriorly towards the mouth, is another important characteristic. Even more convincing is the preference for “Heteractis” magnifica as their host (versus Stichodactyla mertensi in A. sandaracinos). These species are largely anemone specialists, and the vast majority of in situ photographs show that the sandariconos and perideraion clades occupy mutually exclusive host anemones—this ecological niche partitioning is how they managed to sympatrically speciate in the West Pacific.

Amphiprion perideraion may be concealing further layers of biodiversity. While specimens from Indonesia and the Philippines have a characteristically peachy coloration, those from peripheral regions, like the Mariana Arc and Fiji, show a vibrant orange color, not far-removed from that seen in A. sandaracinos. The Fijian region is especially interesting as it is home to an endemic species in this group, A. pacificus, described in 2010. Its limited range includes the Melanesian islands that surround Fiji, areas like Tonga, Samoa, American Samoa, and Wallis & Fatuna. Endemism here is not unheard of among reef fishes, and can be seen among the Siganus foxfaces and the Meiacanthus fangblennies. This also strongly suggests that the orange-colored A. perideraion from Fiji is an undescribed species.

Things are no less confused in the Indian Ocean, as we again find evidence suggesting that hybridization and cryptic speciation are running rampant. Four distinct populations exists, two of which are nearly identical and go by the name A. akallopisos. But, counterintuitively, these clownfish doppelgangers are situated on opposite ends of the Indian Ocean: one along the African coastline and the other in the Andaman Sea and Greater Sunda Islands. Sandwiched in between are two entirely different looking fishes, the Blackfoot Clownfish A. nigripes, which is known only from the Maldives and Sri Lanka, and the Chagos Anemonefish A. chagosensis, found just to the south at the Chagos Archipelago. Take a moment to ponder an evolutionary explanation for how this unusual scenario might have been brought about. 

The answer has been hinted at in some recent genetic research, though none of these authors have made mention of it. In both mitochondrial and nuclear phylogenies, A. nigripes, which is without question a member of the Skunk Clownfish species group, has been consistently found as sister to A. chagosensis. This is not terribly surprising, as the only major difference between them is the presence of a second bar. It’s important to note that both species also share with other members of this group a scaled interorbital (i.e. the dorsal region between the eyes). An important question to ask here is why the Skunk Clownfishes of the Chagos-Laccadive Ridge look so different from their relatives. Why do they have black pelvic fins, a barred patterning and no dorsal stripe?

The answer might lie in the seemingly anomalous conflation of nigripes+chagosensis with the Western Indian Ocean clade (i.e. the allardi Group)  in genetic studies. The most likely explanation for their distinctive appearance is a past hybridization event, and a likely culprit for this may be a neighboring species which also has black pelvic fins and a barred pattern, the Oman Clownfish A. omanensis. This species occupies a range just a bit to the north of where A. nigripes occurs. What’s more, there are known examples of fish from this region (e.g. Zebrasoma xanthura) appearing as waifs in the Maldives. It seems unlikely, at least in the present time, that A. omanensis would be able to make this journey, but perhaps past oceanic conditions made such vicariance more feasible. Alternatively, there is some evidence of a connection between the northern shoals of the Mascarene Plateau and nearby Chagos, suggesting A. chrysogaster could have a role to play in this story.

The African and Sumatran populations of A. akallopisos are seemingly identical in coloration and morphological counts, but there is one behavioral difference which might lend credence to the notion that these are distinct species. Parmentier et al 2005 took a novel approach to studying these two populations, utilizing recordings of the sounds these fishes make to essentially show that they speak different languages. Parameters such as the peak frequency, the number of pulses, and the number of peaks per pulse all differed between the two. Given that around 6,500 km separate them, it should come as no surprise that genetic data now supports these as being distinct, with further evidence that Northeast Madagascar may also harbor a cryptic population. Oddly, this widespread group has seemingly never made its way to the Mascarene Plateau, where it is instead replaced by the apparently unrelated A. chrysogaster

Amphiprion akallopisos was originally described, in 1853, from Sumatran specimens. This was the first of the Skunk Clownfishes to be published, and the unusual scientific name—from the Greek ἀκαλλώπιστος (akallopistos), meaning unadorned—alludes to the lack of vertical bars. It bears mention how truly idiosyncratic some of the nomenclature in this group is. No other creature, fish or otherwise, has ever been given the name “perideraion” or “akallopisos”. The author of both species, the inimitable 19th century Dutch ichthyologist Pieter Bleeker, seemed to have a predilection for making use of the most obscure Greek he could get his hands on. Nearly as uncommon is “sandaracinos”, which is a name shared only with an African cichlid and a long-horned beetle. Nor should we forget the equally abstruse “akindynos”. 

Before moving on, we need to briefly return to the Pacific, where a fish that looks more or less identical to A. akallopisos can be found at the easternmost edge of this group’s distribution, specifically, the reefs of Tonga, Samoa, and Wallis & Futuna. This population, described in 2006 as A. pacificus, has only rarely been available in the aquarium trade and generally remains little-known and poorly documented. It’s evolutionary origins are likewise misunderstood, as it has generally only been discussed in relation to the similarly unbarred A. sandaracinos, despite it lacking all of the other defining traits of that clade, both phenotypic and behavioral. Recall that A. sandaracinos is a specialist in Carpet Anemones, not the Giant Sea Anemone used by A. pacificus. The presence of this endemic species lends further credence to there being cryptic diversity present amongst the other peripheral perideraion populations.

Lastly, we must make mention of a couple hybrids from this group which are still inappropriately recognized as legitimate species. The most well-documented of these is A. leucokranos, reported from numerous locations around New Guinea. This form looks much like a duskier version of A. sandaracinos, but with an anterior bar and abbreviated “bonnet” rather than a full dorsal stripe. Specimens can actually be quite variable in this patterning, presumably as a result of backcrossing, with specimens showing dorsal stripes of intermediate length combined with an equally variable bar. 

The other parent in this mix is the New Guinean population of A. chrysopterus, which is attested to by the many in situ photographs showing A. leucokranos forming mixed species pairs with both parent species. Gainsford et al 2015 found that 35% of anemones at Kimbe Bay, New Guinea had mixed species pairs, and A. leucokranos pairs were only seen in 2% of same-species pairs. It was also found that the size disparity of the parent species results in F1 hybrids born of a female A. chrysopterus and male A. sandaracinos, and never the other way around. Recall that Melanesia is on the biogeographic periphery for both parents, providing a plausible mechanism encouraging their hybridization. Still, it’s a bit confounding why certain other mixed species pairs fail to form here (e.g. clarkii X chrysopterus or clarkii X perideraion).

Amphiprion thiellei is another presumed hybrid known from the Philippines and Borneo. It, too, has clear evidence of parentage from A. sandaracinos, as seen in its orange fins and broken white dorsal stripe. But unlike A. leucokranos, there are additional white markings corresponding to the position of middle and posterior bars. The dorsal fin also shows a highly convoluted margin, moreso than what we normally see in sandaracinos. This hints at an interesting hypothesis for the second parent, A. ocellaris. Remember that these two species usually occupy different anemones, but there are rare exceptions where A. sandaracinos can be found in “Heteractismagnifica and A. ocellaris in Stichodactyla mertensi. Such instances might be the result of an imbalance in the anemone:clownfish ratio at a particular location, encouraging mixed species pairs and hybridization. A similar situation is the likely cause of sandaracinos X perideraion hybrids.

It’s curious that the thiellei phenotype is only documented from the Philippines and Sabah, Borneo. The most likely explanation is that thiellei is found across the Coral Triangle where the two parent populations coöccur, but it’s rarity has caused it to go unnoticed. This hybrid phenotype is apparently much rarer than A. leucokranos, as only a handful of photos document it in the wild. Unlike with the leucokranos hybrid, the thiellei hybrid forms in regions where both parent species are abundant. In this instance, it is ecological rarity, rather than geographic rarity, which drives their miscegenation. Mixed-species pairs have been documented from Cebu, Philippines showing a female thiellei hybrid with a male A. ocellaris. Notice that it is the A. ocellaris which is in the “wrong” anemone. It’s quite likely the formation of a thiellei hybrid requires this specific parentage and results from male A. ocellaris being unable to find an open “Heteractismagnifica. The next best option for such a wandering male would be Stichodactyla, an anemone far more likely to have a resident A. sandaracinos than another A. ocellaris.

Hybridization also likely extends to A. perideraion X A. ocellaris, which appears to occur about as infrequently as the thiellei pairing, if not moreso. There are still many questions that remain. Does a female A. ocellaris and male A. sandaracinos result in this thiellei phenotype as well? Are all of the offspring from such a mixed-species pair identical, or is there a gradient of patterns from more ocellaris-like to more sandaracinos-like? Is it only the A. ocellaris population from the northernmost portions of the Coral Triangle which can create this hybrid, or can thiellei be expected in areas like Bali and Sulawesi as well. And, importantly, when will someone get around to officially synonymizing leucokranos and thiellei? There is no reason for these hybrids to continue to be recognized with scientific names.

O N E  S T R I P E

  • Joe Rowlett

    Joe is classically trained in the zoological arts and sciences, with a particular focus on the esoterica of invertebrate taxonomy and evolution. He’s written for several aquarium publications and for many years lorded over the marinelife at Chicago’s venerable Old Town Aquarium. He currently studies prairie insect ecology at the Field Museum of Natural History and fish phylogenetics at the University of Chicago.


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