The fish seen in this video is named Antigonia, and it belongs to a small family, Caproidae, whose members are known commonly as boarfishes. More often than not, these fishes occur at considerable depths and are seldom seen in captivity—this one was recently collected at 280 meters (900 ft!) in the waters south of Taiwan. The question I’ll pose to you, dear reader, is this… what is Antigonia?
Observe how it swims… note the structure and shape of the fins… the profile of the body… the color patterning. What sort of fish does this remind you of? Where in the grand scheme of fish evolution might this genus fit? Who are its closest relatives? Let’s have a look at another video…
There’s something about Antigonia that seems both familiar and foreign. It vaguely resembles a number of common fish families, but always with enough discrepancy to exclude it from consideration. To my eye, it feels most like a deepwater version of Monodactylus, sharing the tall, rhomboid shape and the elongated dorsal and anal fins. The banded patterning brings to mind some of the butterflyfishes, particularly those found at greater depths, and perhaps we can lump angelfishes and surgeonfishes into this discussion, as they all share the same sort of laterally compressed body. But none of these are the correct answer.
Morphologists have bickered over the years about where to classify this family. Some have drawn comparison to the zeiforms, a relatively small and obscure group whose most familiar member is the ubiquitous John Dory (Zeus faber). Zeiforms certainly do share many superficial similarities to Antigonia when it comes to their general appearance, and many likewise occur in the same sort of deep, coastal habitats. For instance, the delightfully named oreos (Oresosomatidae) are another common rariphotic fish which looks a bit like a dark boarfish and even swim in a similar manner, but it’s now thought that the zeiforms are actually quite distant relatives, belonging closer to things like cod and oarfish.
Others have highlighted the Acanthuroidei as a possibility. This group notably contains the tangs and surgeonfishes, along with the Moorish Idol, the rabbitfishes and foxfaces, and the scats and spadefishes. All of these families look a bit like one another in their general appearance, but recent genetic study has argued that this is most likely an artificial grouping that poorly reflects their true evolutionary origins. The fossil record is practically littered with enigmatic taxa that fit into this picture, showing that the ancestral stock from which many of these families (independently) emerged possessed a tall, laterally compressed body. Paleontologists point towards a few morphologically “primitive” fossils in particular that seem to bear a special relation to Antigonia—Acanthonemus, Zorzinichthys, and Caprosimilis.
But the group that the caproids seem to actually belong nearest to (at least, based on the latest and greatest genetic research) might come as a bit of a surprise—the lophiiforms & tetraodontiforms. For those not initiated in this arcane piscine nomenclature, the former includes all of the various anglerfish families, while the latter includes a hodgepodge formed of filefishes, triggerfishes, and pufferfishes. One would be hard-pressed to look at Antigonia and see the resemblance to some of these groups—a boarfish certainly looks little like an Antennarius frogfish or a Diodon porcupinefish—but there are other examples here that reveal closer ties. For instance, the flattened shape of a filefish is not far off from Antigonia, and the unique manner in which tetraodontiforms swim, propelling themselves via oscillations of the dorsal and anal fins, also occurs predominantly in this family’s namesake, Capros aper.
The Caproidae as it is presently defined includes just two genera—Capros and Antigonia—though just how closely related these two really are to one another remains a bit of a mystery. At times they’ve been treated in separate families, and the data supporting their shared ancestry is fairly weak (i.e. they may simply be isolated twigs at the base of this family tree). Capros aper is the only species in its genus and occurs commonly in the Mediterranean and nearby Atlantic waters. Unlike Antigonia, which are mostly observed as either solitary individuals or in loose groups, Capros forms large shoals that move about primarily with a tetraodontiform wiggle to their fins. On the other hand, Antigonia swim more like a surgeonfish or wrasse, propelling themselves with thrusts of the pectoral fins.
So where does this leave us in answering our original question… what is Antigonia? I’ve taken you on this phylogenetic journey for a reason. Aside from being a rare and unusual aquarium fish and an important part of rariphotic ecosystems, perhaps the most fascinating thing about the boarfishes is the evolutionary link they form between the more traditional perch-like fishes and the specialized morphologies and behaviors of the tetraodontiforms. Not every piece of this puzzle is fully understood just yet, but it’s not hard to envision a scenario wherein a “primitive” fish like Zorzinichthys gives rise to something like Antigonia… which eventually leads to basal tetraodontiforms (like Hollardia, seen in the video below) and, ultimately, pufferfishes.