News / Species Spotlight / Species Spotlight - Geminate Species Across The Indo-Pacific (01/03/18)

Species Spotlight - Geminate Species Across The Indo-Pacific

Geminate Species Across The Indo-Pacific
The geographic distribution of a coral reef fish can tell us a lot about its evolutionary history and even hint at its larval development. Species with especially broad ranges in the Indo-Pacific, such as the Clown Triggerfish (Balistoides conspicillum) or the Zebra Moray (Gymnomuraena zebra), are virtually identical morphologically and genetically, whether collected from Africa in the west or Tahiti in the east. And the reason is simple enough, these are groups whose planktonic larval stage is particularly lengthy, taking months to complete. This gives the juveniles a chance to spread far beyond the place they were born, resulting in a single, homogenous population across this vast geographic region.

On the other side of the spectrum are those fishes which exhibit a finer level of endemism. For instance, Cirrhilabrus naokoae, the Sharkfin or Naokos Fairy Wrasse, is only known from a handful of locations on the western side of Sumatra. Go any farther north, and youll run into a different species, C. joanallenae, in the Andaman Sea, and heading south towards Bali youll instead encounter C. humanni. Among the fairy wrasses, some combination of a shorter larval stage and rampant sexual selection has encouraged an unusual amount of speciation. And we see this in many other groups as well: dottybacks, damselfishes and anemonefishes, fangblennies, gobies, etc.

And then there are those fishes which fall somewhere in the middle. For many widespread Indo-Pacific fishes, a significant barrier to dispersal emerged during the Pleistocene ice ages, when the straits separating the main islands of Indonesia dried up, creating one vast peninsula of land dividing the Indian and Pacific Oceans. This repeatedly isolated the populations in these regions over the course of many hundreds of thousands of years, eventually leading to new species forming. Where they meet today in places that straddle this oceanic divide (like Bali and Christmas Island), we often see them hybridizing. Such species pairs are referred to as geminate species, after the mythological Gemini twins. Lets take a moment to examine some of the more prominent and enlightening examples.

The Sailfin Tangs (Zebrasoma desjardinii & Z. veliferum)
The unique qualities of the Indian Ocean Sailfin Tang has been known since it was first described in 1836, but it has really only been in the last twenty years that this fish has been consistently treated as a distinct species. If you open up some of the field guides written in the 1980s youll see a mix of opinions. Some listed it as a geographic variant of a single widespread species, while others leaned towards a formal designation as a subspecies, but, today, researchers better appreciate the true evolutionary distinctiveness that such seemingly similar fishes can possess. For the Sailfin Tangs, which are beastly creatures that can grow to the size of a dinner plate, these differences are glaringly obvious. In the Indian Ocean Z. desjardinii, the caudal fin is dark and the belly is heavily spotted, while the Pacific Z. veliferum has an orange tail and a more stripes than spots. Other morphological and genetic differences abound, indicating that these two have been going their separate ways for quite some time, even if they do still look alike.

Zebrasoma desjardinii

Z. veliferum

The Regal Angelfish (Pygoplites diacanthus)
Aquarists have long appreciated the differences between the Red Sea and Pacific varieties of Regal Angelfish. The former has noticeably brighter colors along its breast, while the latter is a rather dull grey. And this has concordantly been reflected in the relative desirability and price of these two fishes in the aquarium trade. But it wasnt until 2016 that the scientific community finally took efforts to recognize the differences inherent within Pygoplites. A genetic study confirmed what we already knew, but added a few interesting details to the evolutionary story. It appears that the two Regal Angelfishes have been separate from one another for somewhere around 1.4 million years! And there is evidence that much of the Indian Ocean was absent this species until around 0.4-0.6 million years ago, when the Red Sea Regals migrated to fill this void. Still, despite their long time apart, the authors of this study chose to treat these two as subspecies, rather than full species though such a decision is really fairly arbitrary when you think about.

Pygoplites diacanthus

The Lyretail Anthias (Pseudanthias squamipinnis)
This ubiquitous anthias is a fine example of a fish with clear differences across its range, and which has at times had separate names applied to its distinctive populations. Still, there is no consensus among taxonomists, as it simply hasnt been properly investigated. The Indian Ocean population is where the first specimen was described from, making this the true Lyretail Anthias. It has a redder coloration than those in the Pacific, and the caudal fin is both more lyre-shaped and differently colored, having a yellow margin. In the Pacific, this fish is typically purple or mave, with a more truncated tail shape that is bordered in blue. But even here there may be multiple species, as the populations from Fiji and Japan appear quite different when compared to those from the Coral Triangle. Clearly more work is needed.

Pseudanthias squamipinnis

The Exquisite Wrasse (Cirrhilabrus exquisitus)
And then there are the known unknowns the fishes which we know have visible differences across their range, but which pose particular challenges for study. The Exquisite Wrasse is a monumentally variable fish, even for those found within a single region. No two males ever look quite alike, and the relative level of sexual excitement a specimen feels can drastically alter its appearance. Still, their are some generalities that stand out across the Indo-Pacific which allow us to say with some certainty that there are multiple species in this group. The main difference is again between the Indian and Pacific oceans. In the former, there are prominent red markings around the pectoral fin and head, and, also, the dorsal fin is mostly yellow, with a pair of dark blotches. When we look at the population in Indonesia, we find a mostly green fish with red fins. But, again the story get more complex, as several other variations exist. The Exquisite Wrasse isnt so much a species as it is a whole complex of regional species, all in need of study and naming.

Cirrhilabrus exquisitus