The Old Narrative
When I travel to the Rosenstiel School of Marine and Atmospheric Science, I traverse the Rickenbacker Bridge which overlooks Biscayne Bay. Miami, like much of South Florida, is a haven for large game fish like marlins, mackerel, and tuna. Or, at least, it once was. Conservationists and fishermen alike have collaborated to protect the charismatic billfish from such a fate, shaping economic, environmental and maritime policy in the process.
However, the biology is often ignored or unknown, especially when it comes to billfish evolution. I believe one must know the past before one tinkers with the future because history has a knack for repeating itself. While knowledge of billfish evolution is sparse, more than enough exists to piece together a story that can positively affect the conservation movement. Our journey, however, begins with DNA and ends with the fossil record.
It has long been believed that billfish were related to mackerel and tuna (Eliason 2010). Scombroids, as these fish are collectively known, thrive in marine waters around the world. Renowned for their size as well as their taste, the tuna and mackerel drive the global seafood market. They are also known for their numbers. Or, at least, they historically were.
Fossils from North America, Europe, and Russia suggest that the earliest Scombroids evolved during the late Cretaceous Period about 70 million years ago. At this time, the Earth was lush and tropical and sea levels were much higher than today. The seas teemed with enormous fish, giant squids, and massive marine reptiles.
However, a mass extinction wiped out most of the colossal creatures that dominated the Cretaceous seas. The ancestors of marlin, mackerel, and tuna survived the volcanic eruptions that permanently altered the Mesozoic climate, the continuous pumping of greenhouse gases from the Deccan Traps that caused a warming effect much like the one happening today. Neither the dinosaurs nor the marine reptiles survived. The ammonites perished, as did the fishy behemoths like Xiphactinus. More than 60% of the species on Earth went completely extinct.
Ernest Hemingway depicted the struggle between man and marlin in “The Old Man and the Sea”. Another less documented struggle is that of survival and the ever-changing elements. The Scombroids are endotherms like mammals, capable of generating heat to maintain a stable body temperature. This adaptation gives them the ability to endure thermal changes to the surrounding environment, and probably contributed to the Scombroids’ success after the Cretaceous.
The New Narrative and Its Evidence
Numerous morphological (fossil and skeletal comparisons) and molecular studies have been conducted to determine the relationships between scombroid family members, producing conflicting results. Alex Little and his colleagues from Queen’s University in Canada sought to resolve the classification of scombroid fishes using DNA sequencing. The team collected muscle samples from numerous species of billfishes (e.g. swordfish, striped marlin, blue marlin) and tuna (e.g. bigeye tuna, yellowfin tuna, bullet tuna) near the Hawaiian Islands in the Pacific Ocean. Next, they sequenced the DNA from nine mitochondrial loci and three nuclear loci from each muscle sample. Finally, they used statistical analysis to compare their results with sequences from additional species of fishes available on GenBank (publically available DNA sequences) in order to generate the best genetic relationships for scombroid fishes (Eliason 2010, Little 2010).
Remarkably, the team found that tuna and billfish are only distantly related. This finding suggests that regional endothermy and continuous swimming has arisen independently in these two groups of fishes. What’s more, the authors found that billfishes are closely related to flatfishes (Pleuronectiformes) and jacks (Carangidae) (Eliason 2010, Little 2010).
This is an astonishing result considering the differences in
lifestyle, physiology and morphology observed among these fishes. While billfishes are extremely athletic, and have regional endothermy and elongated bills, flatfish primarily live on the sea floor and are usually asymmetrical with their two eyes located on the same side of their head (Eliason 2010).
The first billfish, however, did not appear until the period that followed, the Paleocene. The earliest fossils of billfish were found in Turkmenistan about ten years ago by Russian paleontologists. They belong to one species, Hemingwaya sarissa, appropriately named after the author that made their marlin brethren famous. Spanning a foot from the tip of the bill to the tip of the tail, these small fish resembled a cross between needlefish and sailfish. They were long, slender, and streamlined. Unlike modern billfish that are known to stun predators and prey alike with their swords, the small Hemingwayids hovered close to the surface, did not swim actively, and hunted their food in short bursts of speed. Their home was the Tethys Ocean, a tropical expanse of water that submerged the Mediterranean and most of Western Africa.
It is believed that Hemingwaya and other ancient billfish had the same environmental preferences as modern species. Fossil evidence bolsters this argument, and also suggests that billfish originated in the Tethys. Only the swordfish evolved in the North Atlantic. Most billfish fossils from subsequent periods were discovered in Europe, Russia, and the United States.
From the Paleocene through the Miocene (which ended 27 million years ago), the world experienced remarkable billfish diversity. The Paleorhynchids were the most diverse, with 22 species known to date. Members of this family include the 10 species of Aglyptorhynchus, the 3 species of Homorhynchus, and the 9 species of Palaeorhynchus. All of these genera emerged in the early Eocene approximately 55 million years ago, and are represented in rocks deposited at various depths, types of water, and temperatures. They were extremely successful, existing in the fossil record through the Miocene 30 million years later. Their existence also coincides with two contemporary but now-extinct billfish families—the Blochiidae and Xiphiorhynchinae.
Unlike modern billfish, their forebears possessed two bills of equal length—one on the upper jaw and one on the lower jaw. The purpose of the double-bill is currently unknown. The extant billfish families—the Istiophoridae (marlin and sailfish) and the Xiphiinae (swordfish)—are also unique because they wield single swords. Though not as ancient as the Hemingwayids or the Paleorhynchids, modern marlins and swordfish also have rich evolutionary histories.
The swordfish are and always have been represented by a single species. Though fossils of Xiphias gladius (the swordfish) are hard to come by, the oldest have been found in Italian rocks dating 15 million years. The Istiophorids appeared at the same time and like the swordfish they have barely changed since they first evolved. The Blue Marlin (Makaira nigricans) are the most common billfish fossils, found in Miocene deposits in Southern California as well as Italy. Swordfish, like their marlin cousins, currently inhabit all of the world’s oceans.
Even though science has shed significant light on billfish evolution, there is still a 10-million-year gap between the advent of modern species and the extinction of their forebears. How do we bridge that gap (see below)? We can start by digging through the archives.
Dr. Harry Fierstine, the foremost authority on billfish evolution, recommends greater scrutiny of museum collections from the early and middle Miocene to complete the fossil record.
Secondly, he also recommends a detailed cladistical analysis of both extinct and extant billfishes. By using both morphological and molecular data, meaningful phylogenetic relationships can be determined between Scombroids and swordfish (Fierstine 2006). In other words, we should re-examine the way we classify modern billfish species.
Because recent DNA evidence strongly suggests that billfish diverged from flatfish (not tuna, mackerel, or other scombrids) (Little 2010), we should also re-examine the way we classify extinct billfish species. A missing link between flatfish and billfish could be found, because the earliest flatfish also emerged during the Paleocene. A 50-million-year-old transitional fossil called Heteronectes showed us that ancient flatfish resembled perch, with eyes on both sides of their heads (Friedman 2008). This morphological evidence indicates that the environmental and predatory pressures that streamlined the tuna also shaped the billfish (Little 2010). Fierstine’s fossils are geologic illustrations of the story told by the DNA.
The evolutionary relationships found in Little’s study demonstrate that shared selective pressures can lead to similar adaptations in distantly related species. Equally, it also shows how disparate selective pressures can lead to divergent adaptations in closely related species (Eliason 2010). Thus, the story of the billfish is that of convergent evolution with tuna—how two distantly related fish can evolve similar solutions to the same niche. They are apex predators like the dolphin and the shark, endotherms tailored for the chase.
I hope this paper inspires dialogue about the often-overlooked evolutionary story of the billfish. I also hope that this dialogue will be incorporated in larger discussions about the conservation of these charismatic sport fish.
Eliason, Erika (2010). Billfishes are closely related to flatfish. Journal of Experimental Biology. http://jeb.biologists.org/content/213/17/vi.full.pdf
Fierstine, H.L. Fossil History of Billfishes (Xiphioidei), Bulletin of Marine Science (2006) http://digitalcommons.calpoly.edu/cgi/viewcontent.cgi?article=1021&context=bio_fac
Friedman, M. (2008). The evolutionary origin of flatfish asymmetry. Nature. July 2008; 454(7201):209-12 http://www.ncbi.nlm.nih.gov/pubmed/18615083
Little, A. G., Lougheed, S. C. and Moyes, C. D. (2010). Evolutionary affinity of billfishes (Xiphiidae and Istiophoridae) and flatfishes (Plueronectiformes): Independent and trans-subordinal origins of endothermy in teleost fishes. Mol. Phylogenet. Evol. 56, 897-904 http://www.sciencedirect.com/science/article/pii/S1055790310001880