The Latezonatus & Clarkii Groups

by | Jul 19, 2020 | 0 comments

The latezonatus Group

The Latezonatus or Blue-lip Clownfish (A. latezonatus) is an unfamiliar species to aquarists, rarely available and considerably more expensive than most of its relatives. It occurs across a very small geographic range, limited to the subtropical reefs of the Southwest Pacific, specifically those along the coast of New South Wales, Australia and east to Lord Howe Island, Middleton & Elizabeth Reefs, and more easterly still to Norfolk Island. There are even questionable reports of it from New Caledonia, though it seems to be rare there, if it occurs at all. Host anemones span all of the major genera—Heteractis crispa, Entacmaea quadricolor and Stichodactyla gigantea—and seems to be limited primarily by the limited species diversity of these cool waters.

Phylogenetically, A. latezonatus was previously thought to be most closely related to the Saddleback Clownfishes, on account of it having a similarly wide middle bar and a somewhat rounded profile to the body, but genetic data has consistently supported its placement as a distinct basal lineage amongst the larger Amphiprion clade (i.e. those exclusive of A. ocellaris/percula and P. biaculeatus). The isolated biogeography of A. latezonatus also supports this unique evolutionary position, as many other basal or otherwise isolated reef fish lineages show a similar distribution in the SW Pacific (e.g. Cirrhilabrus laboutei, Chaetodontoplus ballinae, Amphiprion akindynos… we’ll get to that one in a later article).

Morphologically, there isn’t all that much about A. latezonatus that is substantively different from others in this genus. It does have an unusually dingy coloration to the body—primarily black, with an olive hue beneath the spinous dorsal fin. The relative thickness of the middle band is certainly distinct and has given rise to this fish’s other common name, the Wide Band Clownfish. Perhaps the only truly unique feature is the thin blue band which runs just above the upper lip, giving this fish what amounts to a caerulean moustache. No other clownfish has this trait.

The clarkii Group

With a range that stretches from the Persian Gulf east to Tokyo and Australia, Amphiprion clarkii represents the single most widespread “species” of clownfish. Not surprisingly, this is almost certainly a gross underestimate of the true diversity present, as perhaps as many as fifteen distinct populations occur throughout different allopatric ranges of the Indo-Pacific. Adding to this confusion is an unusual phenomenon that appears to be prevalent in this group: anemone-induced melanism. When these fishes host in either Stichodactyla or Cryptodendrum anemones, they normally turn almost entirely black, for reasons that remain unstudied. Since this usually (but not always) obfuscates the diagnostic differences between populations, the following discussion will focus primarily on non-melanistic specimens.

The Indian Ocean is home to perhaps as many as six distinct populations. These can be distinguished from their neighboring Paciific counterparts by the bright yellow caudal fin found in both sexes, though this is untrue for more distant populations in Melanesia, where both sexes retain the yellow caudal fin. Three of these Indian Ocean populations are predominantly black-bodied, centered geographically in: 1) the Arabian Peninsula 2) the Maldives and India 3) the Andaman Sea and Greater Sunda Islands. Importantly, the black coloration of these clarkii is not caused by their host anemone; it is an innate quality of the species there.

The Arabian clarkii may be the most distinct, as it typically has the anterior bar attenuated dorsally, often resulting in it failing to connect with the opposite side, while the remaining populations usually have this bar much wider. Separating the Indian/Maldivian form from those to the east is far trickier, and the only useful diagnostic character relates to the relative thickness of the posterior bar. Specimens from the Maldives tend to have a thinner bar than those from the Andaman Sea, but enough variation exists to render this unreliable on its own. Note that Sri Lanka is the type locality for A. clarkii; the other two Indian Ocean populations lack associated names.

A seemingly unrecognized clarkii occurs in the Chagos Archipelago. This fish has been widely confused with another clownfish endemic there, A. chagosensis, but it differs in having a yellower body, with yellower fins and much thicker bars. It can be separated from the other Indian Ocean clarkii populations in lacking black along the body, even when it is found in melanism inducing anemones like Stichodactyla. Additionally, the posterior bar is completely hidden by the yellow of the caudal peduncle, whereas this remains partially visible in Maldivian clarkii. This Chagos clarkii has also been confused with a superficially similar species from the Red Sea, A. bicinctus, but their disparate ranges and differently shaped white bars strongly argue against these being conspecific. Based on available photographs, this Chagos species is known to associate with Macrodactyla doreensis, “Heteractis magnifica, Entacmaea quadricolor, and Stichodactyla mertensi. Note that A. chagosensis (and the related A. nigripes) are both specialists in “Heteractis magnifica.

Moving into the Pacific, there is a sharp demarcation along the Sunda Shelf, where the dark-bodied, yellow-tailed forms of the Indian Ocean give way to a lighter-bodied, white-tailed population limited to eastern portions of Indonesia. This white caudal fin varies in its color saturation and can often appear somewhat yellow, but it is never as deeply colored as those in the Indian Ocean or Melanesia. Also of note is the sexually dichromatic nature of its caudal fin coloration, with males having bright yellow margins dorsally and ventrally. Additionally, the orange base coloration of the body extends above the pectoral fin base to cover a significant portion of the fish, though this can vary dramatically, with some specimens being almost entirely orange while others appear mostly black (without any apparent anemone-induced melanism).

The precise range for this population is difficult to assess and has to be inferred from comparison with other groups. It likely extends from the Java Sea east to West Papua and north to the Celebes Sea. North of this region, across a major biogeographic border known as Wallace’s Line, the Indonesian Clarki is presumably replaced by a genetically distinct population with a range encompassing the Gulf of Thailand east to the Philippines and north through the Ryukyu Islands. Without genetic data to go by, this “splitting” is somewhat speculative, as these two populations are essentially identical in their coloration. At least one of these will likely need to be treated as a distinct species or subspecies at some point, and the oldest available name is apparently Amphiprion chrysargyrus Richardson 1846, described from “China” and largely unused since.

Moving north into mainland Japan, another distinctive population can be found, the only species of clownfish to occur in this region. A comparative study by Moyer 1976 found several differences relative to the Philippines/Ryukyu form: 1) dominant males have a solid yellow caudal fin 2) the white bars are noticeably thinner, sometimes failing to converge dorsally in large females 3) the pelvic fins of the largest specimens turn black (in non-melanistic specimens) 4) a larger average size (88 vs. 80mm) for the Japanese form. There are photographs of specimens which seem to blur some of the distinction between these two, especially when it comes to the relative width of their bars, but there is undeniably something distinctive about this Japanese clarkii, subtle though it may be. Amphiprion japonicus Temminck & Schlegel 1843 is a synonymized name which might apply to this fish, though the type locality is ambiguously listed only as “Japan”.

We don’t have to travel far to find another unique variant, as the nearby Ogasawara Islands south of Tokyo has its own clarkii. In this small island group, evocatively dubbed the Galapagos of the Orient, the local population is permanently melanistic, irrespective of which host anemone is used. Even the juveniles are primarily black, though these do initially begin life with an orange coloration. Also, the sexual dichromatism of the mainland form is nowhere to be found, as both sexes have solid white caudal fins. The name A. snyderi Ishikawa 1904 has been used for this fish, and, given how different it is from those in nearby regions, there is a strong argument for resurrecting this taxonomy. With such a limited distribution, this small population is in significant need of conservation attention. A single widespread bleaching event could potentially exterminate much of this species’ numbers.

A non-melanistic pair of the Yellowtip Anemonefish, seen at Saipan. Credit: Goodtime

Heading south from here, we enter another archipelago prone to fostering endemic speciation, the Mariana Islands. Not surprisingly, we find yet another distinctive population here. These have a white caudal fin in the mature female, but, uniquely, the males have their caudal fin posteriorly margined in yellow. The bars of this fish are also noteworthy for their extraordinary width relative to neighboring populations. In other respects, the Mariana clarkii is similar in coloration to those from Indonesia and the Philippines.

Again traveling south from here, the clarkii population from the Federated States of Micronesia appears to deserve recognition. In many ways, this fish is identical to those found to the west in Indonesia and the Philippines, especially with respect to the pale yellow caudal fin. But recall that for other groups of reef fishes Palau generally marks the furthest eastward edge of their range. These Micronesian reefs tend to have a closer ichthyofaunal relationship to those of the Marshall Islands, and we can see this difference more acutely in some of the other groups of Amphiprion (particularly A. chrysopterus). Perhaps the most outstanding difference in this fish is the greater width of its bars compared to those from the Indo-Philippines, though this trait varies enough to be unreliable on its own. The precise limits of this form are particularly hard to nail down with any certainty, especially in the absence of genetic data, but it is likely to at least include Palau and the Caroline Islands.

The remaining portion of Micronesia, the Marshall Islands, has its own unusual clarkii variant; however, this population has been recognized as a separate species, A. tricinctus. Their relatedness has been confirmed with genetic study, providing one of the more salient examples of the taxonomic inconsistencies that plague this genus. This fish has essentially the same orange belly & black back we see in most of the other clarkii forms, but in tricinctus the black back is somewhat limited in its extent (at least in non-melanistic specimens) and uniquely spreads throughout the caudal fin. The bars are far slimmer than in any other clarkii form, and hint at the likely admixture of another fish’s genome, A. chrysopterus, with its own. Recall that the Marshall Islands represent the biogeographical edge for both the clarkii and chrysopterus groups, meaning that both are likely to have relatively isolated populations. This is a recipe for hybrid speciation to occur, and we have ample evidence of it in this fish. There are well-documented pairings of tricinctus with chrysopterus, with at least one mixed-species pair having been photographed together for multiple years actively guarding eggs. Presumed hybrids have also been reported.

Melanesia has another unique take on clarkii. It can be immediately recognized here from its yellow caudal fin and the highly reduced (or even entirely absent) black along the body. Melanistic forms are especially useful when determining an identification, as this region’s population is unique in having the soft portion of the dorsal fin bright yellow. Lastly, note the exaggerated width of the middle bar, giving a polymnus-like quality to it—this has sometimes led to these being mistaken for the Indian Ocean’s A. sebae. Specimens are well-documented from Papua New Guinea and the Solomon Islands, likely extending west to the Raja Ampats. It is also present in northern portions of the Great Barrier Reef, though at very low abundance. Amphiprion papuensis Macleay 1883 is an available name.

A nearly identical fish occurs in Vanuatu and New Caledonia, perhaps differing only in that the posterior bar is always overlaid with yellow (whereas the New Guinean form usually has some white showing). While visually these two may seem identical, their biogeography suggests they are likely isolated from each other, especially when we compare their differences to those seen amongst the A. chrysopterus from this region. For reef fishes prone to endemism, Vanuatu+New Caledonia appears to be a distinct ecoregion from surrounding reefs. One of the most baffling aspects of the clarkii group is that no population exists at Fiji, Tonga or Samoa, where this otherwise ubiquitous group seems to have been replaced by the Central Pacific’s equally diverse and common chrysopterus group. There’s no good explanation for why this is, as it seems implausible that it couldn’t reach these remote islands, what with there being representative populations in equally remote parts of Micronesia. And even the competition theory falls flat when we consider that clarkii and chrysopterus live in harmony throughout Papua New Guinea and the Solomon Islands, generally utilizing different ecological niches: offshore reefs for chrysopterus and inshore for clarkii. So whither the Fijian clarkii?

Finally, let’s take a look at Australia’s clarkii diversity. As with most reef fishes from here, there are separate and distinct populations split by the Torres Strait. On the western side of the continent, from Darwin to Perth, we find a yellow-tailed fish which seems to take its cues from the Indian Ocean population. Relative to these, the medial bar is noticeably wider. Specimens also seem to exist in a permanent melanistic state, even when occupying Bubbletip Anemones. What little color is to be found in these non-melanistic specimens is limited to a faint dorsal swath of brown and sometimes an orange chest. It has been reported that a 2011 bleaching event may have substantially reduced the presence of this fish and its host anemone around the popular dive site of Ningaloo, with Entacmaea coverage on the reef dropping from an estimated 70% to none, zero, zilch (Thomas et al 2014).

An entirely different fish exists along the Great Barrier Reef, though its affinity with the clarkii Group is not without controversy, with most sources recognizing this as belonging to A. akindynos. In common with that species, the Great Barrier Reef population is devoid of a melanistic form, even when found in carpet anemones, and, rather than the traditional orange and black of other clarkii phenotypes, it has a mostly orange-brown body, which subtly lightens along the pectoral and ventral fins. All of these fins, as well as the dorsal fin, are a more vivid orange-brown, while the caudal fin contrasts in a stark white. The anterior and medial bars are useful when it comes to identification, being comparatively thick and well-connected dorsally.

The sixth and final Indian Ocean clarkii population to discuss is one limited to the offshore reefs of Northwestern Australia and likely eastwards into the Timor Sea. This is one of the most poorly documented forms, but it appears to combine traits of the Indian Ocean fauna (e.g. a yellow caudal fin) with traits of the neighboring Pacific Ocean fauna (e.g. an orange-brown body coloration). This is in keeping with this region’s status as a “biogeographic suture zone” and a hotspot for endemic speciation.

E pluribus clarkii unum.


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