The subtropical reefs of Japan’s main islands are home to an interesting mix of fishes. Many of the species found here are widespread across the West or Central Pacific, but, in addition, there are also quite a few fishes which occur here and nowhere else. Some prominent examples include the Japanese Butterflyfish (Chaetodon nippon) and the Wrought Iron Butterflyfish (Chaetodon daedelmus) and the beautiful Japanese Angelfish (Centropyge interrupta), though this last example is also known from some of the northernmost reefs of the Hawaiian Islands.
So it should come as little surprise that this region is home to its own member of the highly speciose Amphiprion clarkii group. Unlike so much of the biodiversity in this poorly studied complex, the unique Japanese population has been scientifically described and studied in a fair bit of detail. The name Amphiprion japonicus dates back to the mid-nineteenth century and was used for decades, but it was sunk into synonymy by Gerry Allen’s revisional work on the group in the 1970’s.
However, there are considerable and consistent differences between the anemonefishes of the main Japanese islands and those found nearby in the Ryukyu Arc and the Ogasawara Islands. The most salient traits to look for when making an identification are the fully yellow caudal fin of males and the relatively thin stripes in both sexes. This was investigated in considerable detail by Moyer 1976 (pdf link), but, despite correctly diagnosing the color variations across the region, he chose not to recognize this diversity taxonomically.
And so for the last forty years, Japan’s unique anemonefish has gone unappreciated… but isn’t it about time for a reappraisal of this population’s importance? The chilly, algae-dominated habitats that it favors are especially imperiled by any potential warming brought about through climate change, meaning that the entire population of Amphiprion japonicus might rightfully be considered at an existential risk when it comes to extinction. Will warmer Japanese waters result in an expansion of the Ryukyu population into the main islands, or will A. japonicus ultimately learn to adapt to the ecological shifts that a warmer ocean brings?