By Liz Boatman
Each year, Smithsonian Secretary Wayne G. Clough recognizes up to 10 scholars throughout the institution “for exemplary scholarship and outstanding contribution towards the increase and diffusion of knowledge.” Among this year’s recipients is Dave Johnson, Curator of Fishes in the Department of Vertebrate Zoology at the National Museum of Natural History.
Dave was recognized in the category of article or book chapter for his 2011 publication, entitled “A ‘living fossil’ eel (Anguilliformes: Protanguillidae, fam. nov.) from an undersea cave in Palau” (full paper here). The article presented not only the discovery of a new species of eel (video)—representing an entirely new family and genus—but also an in-depth, carefully crafted argument demonstrating the primitive nature of this species.
Dave specializes in the study of comparative anatomy, primarily the skeletal features of teleost fishes, with an emphasis on ontogeny. In other words, Dave studies the cartilage and bone structures of young fishes, and how those structures grow, recede, or change in shape throughout development.
To do so, Dave must first “clear and stain” whole fish specimens preserved in alcohol, to yield articulated skeletons that can be dissected and studied. He uses the enzyme trypsin to digest the soft tissues and then stains the cartilage and bone with blue and red biological dyes. Using stereomicroscopy, Dave finally records stunning high-resolution images of the fish’s skeletal structures. Although it’s careful work—and a larger specimen can take weeks to process—the results of this approach can clearly be seen in one of the striking composite plates that Dave included in his publication on the new eel, Protanguilla palau.
Such work is central to understanding how different taxa of fishes are evolutionarily related and may or may not agree with hypotheses based on genetic analyses. The incongruence often seen between these two different data sets is particularly interesting and can drive additional research.
Dave’s morphological study of Protanguilla is important not only for demonstrating that this animal is a true eel but also for finding that it has several primitive skeletal characters only found otherwise in non-eel related fishes (e.g., the presence of several skull bones, including an upper jaw bone, and gill rakers). In collaboration with his colleague Vic Springer, Senior Scientist Emeritus, Dave is now completing a comparative study of the gill-arch muscles of Protanguilla and other eels, that will provide additional evidence supporting its placement as the most primitive extant eel species.
Ultimately, the team concluded that Protanguilla not only diverged from other eel species a very long time ago but also that it exhibits characters common to both recent eels and Cretaceous eels. Hence, the team dubbed Protanguilla a “living fossil.” This finding, says Dave, “is why this [work] was so exciting…It was just a really strange eel—a primitive eel.”
To date, Protanguilla is only known from a single cave on the fringing reef of Palau, and even in this cave, the species is difficult to locate. For this reason, the team has chosen not to collect additional specimens until more is known about the species’ life habits and potential for continued survival.
Interestingly, the cave is part of a ridge that formed only 60-70 million years ago. The molecular-clock analysis in the paper indicates that Protanguilla diverged from other eels over 200 million years ago. If true, Protoanguilla arrived in Palau relatively recently. Dave suspects that Protanguilla might exist in other caves in the region (possibly in much deeper water), because the larvae of most eels spend up to two months at sea before settling in a cave. In other words, Protanguilla palau’s offspring might also go through a stage that allows for wide dispersal.
Johnson, G. D., Ida, H., Sakaue, J., Sado, T., Asahida, T., & Miya, M. (2011). A ‘living fossil’ eel (Anguilliformes: Protanguillidae, fam. nov.) from an undersea cave in Palau. Proceedings of the Royal Society B: Biological Sciences, rspb20111289.