News / Species Spotlight / One Spot Foxface (Siganus unimaculatus) (05/21/18)

One Spot Foxface (Siganus unimaculatus)

Species Spotlight - One Spot Foxface (Siganus unimaculatus)
The Foxface Rabbitfish is sometimes taken for granted, perhaps overlooked a bit. Its an attractive, hearty, and useful aquarium fish thats just as at home in an SPS reef tank as it is in an aggressive fish-only. Its cheap and common and, perhaps for this reason, it tends to get glossed over. For instance, have you ever noticed that sometimes this fish has a big black spot on its sides? What causes this?

To find an answer, we need to examine the Foxface across its range in the Pacific. Specimens are found from Bali and Java in the west to Japan and Micronesia and Australia. Its only when we get to Fiji, Tonga, and Samoa that the bright yellow Siganus vulpinus that we know and love gives way to the half-black S. upsi and the all-black S. niger. And, rounding out this groups biodiversity, the Andaman Sea is home to a very different and beautiful variation, S. magnificus.

The spotted form turns out to have a limited distribution of its own. It is especially numerous in the Ryukyu Islands of Japan, and can be found in good abundance throughout the Philippines. It also occurs at reefs around the South China Sea and the Gulf of Thailand and, on rare occasion, around Northeastern Borneo. In these latter few localities, it is regularly seen swimming side by side (the species typically occurs in pairs) with the unspotted form.

This distribution is similar to other reef fishes in the region. For example, the Tomato Clownfish (Amphiprion frenatus) shows the same general pattern, being replaced by the more widespread Cinnamon Clownfish (A. melanopus) elsewhere in the Pacific. There are even a couple other unique species found around Fiji and the Andaman Sea, which really does illustrate nicely how the biodiversity of completely different groups can be influenced by the same sorts of evolutionary pressures.

An even more interesting comparison is the Blue-sided Fairy Wrasse (Cirrhilabrus cyanopleura). It too occurs across the West Pacific, before giving way in the Philippines to the similar Yellow-flanked Fairy Wrasse (C. ryukyuensis). These two fishes do, however, still co-occur throughout the Philippines and Japan, though the yellow variety is generally the more abundant in these regions. And this is precisely what we see with the spotted and unspotted varieties of the Foxface (though, interestingly enough, thats not the pattern observed with the Tomato and Cinnamon clownfishes, whose ranges have no apparent overlap).

So what causes this discrepancy? Why do some groups show a sharp break between closely related species in the Philippines while others intermingle? Well, the unsatisfying answer is that we dont really know yet. For the moment, the One Spot Foxface is treated by most references as a true species, Siganus unimaculatus (literally, one spot). But will it continue to stay a distinct species if it regularly interbreeds with its immaculate cousin? Only time will tell...