by Hiba Alhomoud, Cornell University
Three students aboard the J. B. Heiser stagger to reel in an endless length of water-drenched rope. The stationary boat rocks back and forth on the surface of the North Atlantic, seven miles off the coast of Maine. These budding scientists struggle to haul in a plastic barrel that has been buried in the mud four hundred feet below the water surface—a baited trap to catch the deep sea dwelling worm of the ocean, the enigmatic hagfish.
The rest of the students eagerly await the moment of unveiling; today is the day they will first set sight, smell and touch on the slimy marine creature. Their excitement at the retrieval of the barrel is brusquely broken by its gaping emptiness—an emptiness that is much like the puzzling evolutionary history of the hagfish. Dr. William Bemis, director of the Shoals Marine Lab on Appledore Island, tries to lighten the load of their disappointment: “Fishing is different from catching,” he jokes.
When you’re missing jaws, fins, scales, and a skeleton, you can’t blame someone for mistaking you for a worm—which is precisely the mistake scientist Carl von Linne had first made when he classified hagfishes as worms in 1758. Hagfish are often not considered to be true fish—and for good reason. These two-foot-long, cylindrical tubes lack all the major features that any ordinary fish would need to flaunt in order to earn its name. But hagfish are cold-blooded marine animals with gills, so they can’t not be fish. It’s not difficult to see why these rule-breakers have been the cause of many a debate between scientists, debates that won’t be finding their peaceful closure anytime soon.
Hagfish are special because they lie at the evolutionary crossroads of the origin of all vertebrates. Whether these slime hags could be the ancestors of all organisms with a backbone is one of the most problematic issues in the evolution of vertebrates. After more than one hundred and fifty years of studying the sixty species of hagfishes, the answer to this classic question remains as dark as the deep ocean in which they reside.
Hagfish lack many of the physical features that all vertebrates have, the most essential being a backbone. This means that hagfish branched off from the path along which vertebrates later evolved. So early scientists refereeing the game of evolution—the ones who group related organisms together, otherwise known as phylogeneticists—had to exclude hagfish from playing on the same team as their vertebrate cousins. But this wasn’t entirely the problem—the problem was that scientists were stumped as to which team hagfish actually belonged in. Ever since then, the blacklisted slime rods have been hanging around on the sidelines, waiting for the experts to make up their minds.
Part of the dilemma lies in the fossil record—or, in this case, the lack of one. Because hagfish don’t have bones, which are necessary for fossil formation, only one fossil has been uncovered—a single find that came from coastal rock deposited roughly 330 million years ago. Scientists uncovered this fossil in the form of a little nodule, about the size of your hand. But the split between hagfishes and vertebrates is predicted to have occurred around 530 million years ago, which leaves 200 million years of empty pages in the fossil record. And that’s not much to work with.
Even the fossils we do have aren’t particularly helpful—as Bemis flips through his presentation slides to one depicting the hazy image of a hagfish fossil, he remarks, “I want to impress upon you how crummy this fossil is.” According to Bemis, dealing with such old evolutionary splits is difficult because so much change can accumulate over such long periods of time. And if you don’t have the fossils to catalog the progression of these changes, there’s not much you can do to predict 200 million years’ worth of evolution.
Until relatively recently, biologists have mainly relied on common bodily features to group related animals together. For hagfishes, this meant being lumped together with lampreys, another family of sea-dwelling creatures that don’t bare much resemblance to fish. Much like hagfish, lampreys are missing fins, scales, and bones. On the other hand, lampreys boast the rudimentary material for a backbone; in this sense, lampreys were evolution’s beta software for vertebrae. But if lampreys are the most primitive living vertebrates, where does that leave hagfish? This is what Bemis refers to as “a perennial question in zoology.” And as with any story, there are two sides to this one.
Scientists on one side declared that hagfish and lampreys are distinct enough to not be thrown into one box with the same label. These scientists approached the enigma in the traditional fashion of comparing shared, specialized characteristics between different kinds of organisms in order to forge relationships between them. But because this outcome clashed with the earlier hagfish enthusiasts who had decided to lump the two animals together, a new group of scientists had to come to the rescue.
This group of scientists was unleashed with the recent wave of modern genomic studies, fully strapped with new and improved technologies that would crack the code once and for all. Scientists in this field are known as molecular systematists—they study the molecules of organisms, specifically their genes, to learn about their evolutionary origins. Genomes, unlike fossils, can carry a more intact and continuous historical record. The genome is essentially a blueprint that contains the assembly instructions for a living creature. Evolution is constantly and continuously re-drafting this blueprint, producing new species of living organisms.
With the modern tools available today, biologists who study genes can track these subtle changes in effectively the same way that fossils are used for this purpose. The difference is that genes don’t get buried, roughed up, or lost; for the most part, they’re preserved, like molecular fingerprints, in every cell of any given organism. In other words, genes are easily accessible. Guillermo Orti, Associate Professor at the School of Biological Sciences in the University of Nebraska, attests to this: “There is little doubt that molecular data are and will be most commonly used for phylogenetics. Part of the reason is the ease of collection.”
So when scientists like Orti set out to demystify the mystery of the hagfish using genetic data, the aim was to find clarity. As is often the case with scientific inquiry, however, the water only got murkier—findings from genetic studies raised more questions than they were able to answer. These budding researchers’ findings reverted right back to the claim that lampreys and hagfish are much too alike to be separable. The answer to the question of whether the hagfish is our Grandfather Backbone was buried way deeper than six feet under. And the more they kept digging, it seemed, the further down scientists were pushing it.
Orti acknowledges that the molecular data used for the genetic approach is not crystal clear. When molecular systematists use this technique, they don’t work with the entire genome to deliver results; only a few genes are taken into account. This makes for a pretty small sample, which in turn makes errors more likely. Dr. Thomas Near, Assistant Professor of Ecology and Evolutionary Biology at Yale University, doesn’t disagree: “I am a molecular systematist, but I do not think that molecules are giving us a robust answer to this question.”
Anatomy was making its point: that the hagfish is the singular origin of all vertebrates. But genes were making another point: that hagfish are too closely related to lampreys—in terms of their genetic codes—to belong in their own special group. The aim was to find two approaches that would back each other up, but scientists soon realized that they had unearthed a war waiting to be waged—age-old anatomy had been holding down the fort for centuries, but genetics was armed with new and improved weaponry and was ready to step up. So what’s a conflicted biologist to do?
According to Orti, the answer may lie in combining the two approaches: “We anticipate that in just a few years, efforts along these two fronts will converge.” The potential of a comparative approach, in which different ways of answering the same question are combined in a cooperative manner instead of being pit against each other, is eagerly endorsed by many scientists. “I'm not sure what it will support and that is the excitement of the chase!” expresses Dr. J. B. Heiser, Senior Lecturer in the Department of Ecology and Evolutionary Biology at Cornell University. The goal is to sweep the entire genome to create a large data set. With higher numbers of deciphered genes, this “phylogenomic” approach can assemble a larger fleet that is more resistant to inevitable attacks by systematic errors.
Bemis sees the genetic efforts to date as skimming the cream off the question: “Quickly done, not very thoughtful approaches seem to prevail; the hard work of actually understanding hagfishes as a group has yet to be done.” Perhaps two centuries’ worth of research is just the tip of a titanic and relentless iceberg. One thing is for sure: with a bottomless well of curiosity, these resolved scientists will keep hacking their way down to the clandestine secret of the hagfish until it is finally ready to tell its tale.
[Photo by Willy Bemis]