Regal Angelfish biogeography. From Coleman et al 2016
Aquarists have long recognized the regional color differences inherent in the Regal Angelfish Pygoplites diacanthus. Those from the Indian Ocean have always been held in higher esteem, as their breast coloration is a vibrant yellow in comparison to the drab grey of the Pacific population. But, despite these readily apparent phenotypic characteristics, the scientific community has largely ignored the issue. But now, with the recent publication of an in-depth molecular study from across the species’ range, we finally are beginning to unravel the mystery of this Yellow Regal’s origins, and it’s turning out to be a bit more complex than we would have suspected. The concept of geminate species pairs should be familiar to aquarists. As it relates to Indo-Pacific marine fishes, it most often refers to two species which find themselves restricted to either the Indian or Pacific Ocean, implying that a single, formerly homogenous population was split into two distinct species by the formation of a land bridge across Indonesia during past periods of lowered sea level. Unfortunately, taxonomists have been highly inconsistent in how they treat populations which exhibit this phenomenon. Some recently described species (Nemateleotris decora/exquisita, Pseudojuloides cerasinus/kaleidos) have been split, while others (Pomacanthus imperator or Arothron hispidus) show just as much variation but, due to a lack of study, remain lumped as a single taxon.
Comparison of the Pacific and Indian Ocean populations of Pomacanthus imperator. Note the orange caudal fin and rounded dorsal of the Indian Ocean phenotype (right). Credit: Bernard DUPONT & Malcolm Browne
In the case of Pygoplites, a thorough examination of specimens from across much of its biogeographic range indicates what has long been suspected, those from the Indian Ocean show a genetic distinction with respect to the Pacific population. However, some of the finer details revealed by Coleman et al 2016 points to a curious evolutionary history for these fishes. The respective oceanic populations were found to differ by 1.2% – 1.5% in the genes analyzed for this study, which is a bit lower than the usual 2% cutoff used to recognize distinct “species”, but it should be noted that many recently described taxa (Pseudojuloides edwardi, Cirrhilabrus marinda, Paracheilinus alfiani) have been recognized despite sharing nearly identical genetic sequences with their respective sister species.
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