This post is by MSU postdoc John Phillips
Some of you may be familiar with the term ‘troglodyte’, which is a somewhat old-timey derogatory term for an unintelligent person. The Greek root troglo- means “cave” so a troglodyte is a cave person. While we use(d?) this as an insult, caves are actually fascinating places to study, explore, and even earn a Ph.D!
Caves serve as fascinating evolutionary laboratories and are home to a variety of species, many of which have converged on adaptations allowing them to thrive underground. From invertebrates to fish to salamanders, cave-obligate species have repeatedly lost vision/eyes, deactivated pigments, slowed metabolic rates, and evolved behaviors to survive in a nutrient poor environment where most organic material gets washed in from the surface! Many cave food webs are based on bat guano, thus highlighting the importance of bats to the persistence of many cave species. Additionally, cave species are often overlooked when it comes to conservation efforts. This can be a HUGE problem because cave species are imperiled when you combine habitat-specialization, high rates of endemism and low rates of dispersal with a suite of anthropogenic threats (think groundwater pollution or climate change).
Much of my research involves the study of biogeography–I like discovering when and how species got to where they are. Cave systems seemed like an excellent ecosystem which has been relatively ignored in genetic studies. Furthermore, I was living only an hour from the beautiful Ozark Plateau, which is known as a biodiversity hotspot with many endemic species (including several cave species) but was lacking for studies testing biogeographic hypotheses which can be crucial for conservation efforts. The Ozarks are made from limestone karst, which is easily fragmentable rock and often dissolves in ways that produce amazing caves and subsequently their fauna. There are over 10,000 caves in the Ozark Plateau, many of which have not been well-studied to understand their biodiversity.
In one of my studies, I looked at the Grotto Salamander (Eurycea spelaea) that is unique among salamanders. While there are only 12 described cave-obligate species of salamanders in the world almost are paedomorphic, which means they retain characteristics of larvae throughout their life (predominantly gills and a fully aquatic lifestyle, see picture of the Tennessee Cave Salamander (Gyrinophilus palleucus)). Typically, these salamanders will never leave the cave. However, the Grotto Salamander larvae (pictured) can inhabit surface streams and possess fully functional eyes. After several years as larvae they metamorphose into adults, losing their gills, pigments, and eyes, whereupon they leave the water and are free to climb about the cave walls.
As a group, salamanders employ various life-history strategies, but none as unique as this. All grotto salamanders obligately metamorphose, indicating this an evolved strategy as opposed to something environmentally driven. Because of this unique life-history shift, my colleagues (Ron Bonett: University of Tulsa, Sarah Emel: UMass – Amherst, and Danté Fenolio: San Antonio Zoo) were interested in testing colonization patterns of Grotto Salamanders across the Ozark Plateau. Grotto Salamanders occupy a much larger range than other cave salamanders (See #1 on the map below). Could this be due to the surface-dwelling larvae following drainage patterns? Or do the terrestrial adults disperse underground more readily that their fully aquatic relatives? SPOILER ALERT: It is actually hard to distinguish between the two causes, but using the DNA of these salamanders we find that the geologic history of the Ozarks and major changes in drainage basins of the regions combine to explain a majority of genetic variation.
How much genetic variation? Well we have discovered three highly divergent lineages of grotto salamanders dating back 10–15 million years. While these three groups have not changed noticeably in their morphological features (so far as we can tell yet), they are considerably more different genetically than many other species of salamanders are to one another. Therefore, my colleagues and I have “re-elevated” each lineage to species status based on strongly supported genetic differences and geographical separation (see lower map). This phenomenon–where multiple species are unknowingly classified as a single species–is known as “cryptic speciation”. This has turned out to be quite common in cave species. Partially due to their lack of study. Hopefully our efforts here will help conservation agencies (in Oklahoma, Arkansas, Missouri, and Kansas) better manage these lineages across their range.
For more info on my Grotto Salamander work, feel free to read: Phillips, J.G., Fenolio, D.B., Emel, S.L., Bonett, R.M., 2017. Hydrologic and geologic history of the Ozark Plateau drive phylogenomic patterns in a cave-obligate salamander. J. Biogeogr. 44, 2463–2474.
This work was done as part of my Ph.D. at the University of Tulsa in Oklahoma. Currently I am a postdoc at MSU with BEACON, EEBB, and the Department of Integrative Biology where I study evolution in Stickleback fish! Hopefully I will have another blog post down the line as my work here progresses.