The pretty little anemonefish shown above might not strike one as particularly noteworthy based on looks alone—the orange fins, the white bar absent from the tail, and the tall body shape allow us to readily identify this as Amphiprion chrysopterus, the Orangefin Anemonefish. Rather, what makes this find so intriguing is where this fish was discovered and what it tells us about how populations of reef fishes stay connected across the vast distances of the Pacific. Our understanding of these basic concepts of biogeography, such as how far a given fish can disperse to interbreed across neighboring coral reefs, remains poorly known. Only recently has the study of population genetics advanced enough for us to begin to answer these intractable questions, but, as of yet, relatively few studies have examined the issue in any real depth. Another approach is to analyze the occasional appearance of a species outside its known range, as is the case with the anemonefish at hand. 
The map shown above is from my upcoming phylogenetic review of Amphiprion and illustrates the many unrecognized geographic variations presently treated as the single species A. chrysopterus (as well as the related species A. akindynos & A. mccullochi). Careful examination shows that differences in fin coloration, the width of the white bars, and, to some extent, the coloration of the body all correlate to the informal boundaries separating the major ecoregions of the Pacific. For instance, those from Polynesia have thinner bars and lighter bodies, those from parts of Melanesia have black pelvic fins, etc. etc. etc. The single specimen of A. chrysopterus shown here was collected at Pagbilao, situated on the southern shores of Luzon in the Northern Philippines. It’s important to note that, while this species has been recorded from here before (Allen & Erdmann, 2012), there doesn’t appear to be a real breeding population present, merely occasional waifs. The next question to ask is which of the known phenotypic variations does this fish represent.
Compare the differences in pelvic fin coloration between these two neighboring populations. Note also the white caudal fin in both. Credit: Steve Gillespie & unknown
We can assume this fish came from a neighboring region, as it isn’t likely to have swam over from Tahiti of Fiji. Fortunately for us, there are only three contiguous populations this could be and each is highly distinct in its coloration.… More:
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