The Polymnus Group

by | Aug 3, 2020 | 0 comments

Saddleback clownfishes comprise the only Amphiprion group that occurs away from coral reefs, favoring silty lagoons and seagrass beds. In these habitats, the selection of host anemones is limited primarily to Stichodactyla haddoni, Heteractis aurora, and, less commonly, Macrodactyla doreensis. Currently, just two species are recognized, but the true biodiversity of this group is twice that.

The quintessential Saddleback Clownfish is the population found in the Philippines, north through the Ryukyu Islands and west to the Gulf of Thailand. This is the most frequently seen member of this lineage in the aquarium trade and has the most well-developed “saddleback”, referring to the greatly thickened and abbreviated middle bar. Specimens are variably black or orange, without any obvious correlation to variables like the host anemone, geography, or sex of the fish. Despite its distinctive appearance, this population has long been lumped in with the true A. polymnus, found south of Wallace’s Line in Indonesia. A more correct name would be A. annamensis Chevey 1932, a taxon described from Vietnam but which has largely gone unused; however, this necessary taxonomic change still awaits a proper revision of the genus, something which hasn’t happened since Gerry Allen’s seminal 1972 study of the group.

Elsewhere in the Coral Triangle, we find a permanently melanistic population in the waters around the Banda Arc, Sulawesi, and, rarely, north as far as Cebu, where it can sometimes be seen in mixed-population pairs with the northern phenotype. Linnaeus described this form as Perca polymna, listing the ambiguous “Indiis” as the type locality. It is readily identified by its black body and fins and the thin, nearly vertical middle bar. 

Finally, a third easily recognizable phenotype occurs along the northern New Guinean coastline and into the Solomon Islands—the prominent absence of it from Vanuatu exemplifies the major biogeographical break between these two regions, a pattern seen elsewhere in the clarkii and chrysopterus groups. The Melanesian Saddleback Clownfish is much closer in appearance to the geographically distant Philippines population, having the same shape to its middle bar. The main difference between the two is that this bar is typically longer and narrower in Melanesia, and the orange coloration is usually restricted to the region around the pectoral fins in mature specimens. A. laticlavius Cuvier 1830, described from New Guinea, matches this species closely, but recent works incorrectly treat it as a synonym.

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Lastly, the Indian Ocean is home to a closely related species, the Sebae Clownfish A. sebae, named after the 18th century Dutch naturalist Albertus Seba, who published the first illustration of this fish. It occurs from the Persian Gulf to Bali, tracing the same biogeographic range as the true A. clarkii in this region, and, like that species, it shows minimal phenotypic variation across this vast expanse. When compared to its Pacific cousins, differences in A. sebae can be seen in the yellow caudal fin and peduncle, as well as the relatively straight middle bar. Mixed species pairs are easily identified in areas of overlap (e.g. Bali) by examining the tail, and suspected hybrids with intermediate phenotypes are documented.

https://www.inaturalist.org/observations/15067932

The pattern of speciation among the Saddleback Clownfishes is analogous to what is encountered in the clarkii group, where high phenotypic diversity of the Pacific populations stands in stark contrast to a rather homogenous Indian Ocean fauna. This would imply one of two scenarios: 1) the Indian Ocean was only recently colonized and has yet to undergo any extensive evolutionary divergence. 2) the populations in the Indian Ocean are better able to maintain gene flow via dispersal from one region to another. If the calibrated dates of Litsios et al 2007 are to be believed, sebae and polymnus last shared a common ancestor over 2 MYA, which is old enough to imply the Indian Ocean might act to discourage speciation in Amphiprion. Still, this interpretation is argued against by the major differences seen in the Skunk Clownfishes from this region, as well as the limited Indian Ocean distributions for the biaculeatus and frenatus groups. There may not be a one-size-fits-all explanation for how clownfish diversify and speciate in this region.

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