A turtle’s shell teems with thousands of microscopic animals, and the unique features of these hitchhikers could help scientists understand turtles’ travels and diets, according to a study published July 2 in Frontiers in Ecology & Evolution. By combining data on these so-called epibionts with stable isotope analysis, the researchers were able to identify specific organisms that may be useful in discriminating between sea turtle populations, helping to set conservation priorities that would otherwise depend on costly satellite tracking.
“I’m always excited when people use a novel technique to study sea turtles, because even though we’ve been studying them for decades, there’s still so much that we don’t know,” Erin Seney, a marine ecologist at the University of Central Florida who was not involved in the study, tells The Scientist. “It was great to see this unique approach of looking not just at what was living on the turtles, but the interconnectedness between the epibionts of the turtles and their stable isotope signature, which can be indicative of both what they’re eating and where they’re eating.”
While working on a separate project for his master’s degree in 2016, Ian Silver-Gorges, now a PhD candidate at Florida State University and the lead author on the new study, noticed something unique about the loggerhead turtles (Caretta caretta) hauling out on nesting beaches in the Gulf of Mexico. “I got to the first turtle I saw, and I said, ‘Oh my gosh, this is such a dirty turtle,’” he says, describing the kaleidoscope of epibionts—barnacles, algae, worms, and other creatures—cloaking the turtles’ shells. “For whatever reason, they really are just incredibly covered,” says Silver-Gorges, leading him to wonder how these communities grow and how they might be affected by a turtle’s wanderings.
Silver-Gorges now studies what epibionts can reveal about the ecology of loggerheads, which forage over tens of kilometers, making them difficult and expensive to track. The cheapest satellite tags cost roughly $1,000, and to make conservation recommendations for populations, scientists need a lot of data. Epibionts, if they can be harnessed in a meaningful way, could complement satellite tracking and other tools, such as stable isotope analysis from a small tissue sample, that reveals where and what types of food turtles have been eating. If turtles were foraging in different areas or focusing on different prey items, for example, might their epibionts be different too?
The team’s first paper, published last year, focused on meiofauna, a group of microscopic organisms that are larger than bacteria but still largely invisible to the naked eye. A single turtle, the authors found, could harbor as many as 150,000 meiofauna individuals, and this little mobile ecosystem is extremely diverse, including 23 of the 35 living animal phyla. Looking at just one taxonomic group, the nematodes, the team identified more than 110 genera across 24 turtles.
In their new study, Silver-Gorges and his colleagues combined that prior meiofauna information with new data on another fraction of epibionts, called macrofauna, and stable isotope analyses from the same turtles. Previously, a colleague had paired satellite tracking data and stable isotope readings from loggerheads captured in the Gulf of Mexico, creating a rough map of where the turtles go and what their isotope signatures look like after feeding in those areas. “Based on those data, essentially what you can do is assign the turtles that you sample, just based on stable isotopes, to broad foraging regions,” Silver-Gorges tells The Scientist, adding that the team could also then examine how the epibiont communities vary across those same regions.
To gather both the meiofauna and macrofauna data, the team sampled 23 female loggerheads coming ashore to nest on St. George Island off the coast of the Florida panhandle in 2018. The work, Silver-Gorges says, can be grueling—on a recent trip for another project, he walked roughly 180 miles over two weeks looking for turtles, often not finding them until late at night.
The researchers avoided disturbing the turtles until after they’d laid their eggs. Once that happened, they sprang into action, using putty knives and hammers to chip away larger epibionts such as barnacles. With the shell mostly clean, the team then scraped it to remove any of the larger macrofauna and used a sponge to collect the remaining meiofauna (the researchers note that while the turtles can feel this process, it doesn’t harm them). They also took a small tissue sample from the animal’s shoulder for stable isotope analysis before releasing each turtle.
Back in the lab, taxonomists sorted through the epibionts by hand, most often assigning meiofauna to a phylum and macrofauna to a family. Some of the researchers, who are experts in nematodes, focused in particular on these tiny worms, which are the most well-studied group of meiofauna (the model organism Caenorhabditis elegans being one example). They were able to identify many nematodes’ genus or even species.
“I got to the first turtle I saw, and I said, ‘Oh my gosh, this is such a dirty turtle.’”—Ian Silver-Gorges, Florida State University
The team used two different models to combine the stable isotope signatures and epibiont communities from each turtle and assign them to a likely foraging area. Both models produced similar, although not identical, results, showing that as a group, the turtles had foraged throughout the Gulf and into the subtropical Atlantic near Cuba. Almost half of the turtles appeared to have traveled to the southern Gulf of Mexico off the coast of Central America, while one-third returned to the nesting beach from the waters off the eastern Gulf near Florida and a few from the northern Gulf that includes St. George Island. Only one turtle had ventured south to the waters between Florida and Cuba, and none of the sampled turtles had crossed over to the Atlantic coast of Florida, a region known as the South Atlantic Bight.
Communities of meiofauna, but not macrofauna, were significantly different between these regions. Between the north, south, and eastern Gulf, bivalve molluscs, polychaete and turbellarian worms, and a handful of rarer phyla accounted for the majority of the differences, and the abundances of 27 nematode genera in particular were useful for discriminating between regions. In the paper, the team says that one reason meiofauna assemblages may be more unique to certain regions is because they don’t generally disperse very far. By contrast, more than 60 percent of the macrofauna the team identified had an open-water larval stage, meaning they might be able to colonize turtles far and wide.
The stable isotope data also correlated with the epibiont data—that is, turtles with more dissimilar nitrogen isotope values also had more dissimilar epibiont assemblages. Differences in the abundances of some nematode genera in particular correlated positively with both nitrogen ratios, which indicate how high in the food chain an animal feeds, and carbon ratios, which can be used to infer where an animal may have been feeding. A higher ratio of nitrogen-15 to nitrogen-14, for example, might indicate that a turtle is eating crabs or fish, while a particular ratio of carbon-13 to carbon-12 might tip off the researchers that a turtle is foraging in a particular habitat, such as seagrass. The specificity of the nematodes suggests, Silver-Gorges says, that either nematodes are a better indicator group than some other species, or that the ability to categorize them to a lower taxonomic level fine-tuned the conclusions the team was able to draw from the worms.
“Overall, I think it’s a wonderful study,” says Nathan Robinson, an independent marine biologist who was not involved in the study but who has done similar studies with diatoms, a type of marine microalgae, on turtles. “The concept of using epibionts to track sea turtles is something we’ve been, as a community, talking about for a long time, and [this] study is an important step to being able to apply it directly to conservation.”
Both Robinson and Seney point to important limitations of the study—for one, while the team sampled a good percentage of the turtles that nest on St. George Island, it’s a small proportion of the total number of loggerheads in the world. “I think they’ve done a really great study proving that this is a valuable approach . . . but because of the sample size, I think you would want to be hesitant to draw any big conclusions about loggerheads in general,” Seney tells The Scientist. “But they laid a great groundwork for future studies, and particularly for future work at the same nesting beach.”
In addition, because meiofauna are such a niche group, “not everyone in the world is or even wants to be a meiofaunal expert,” Robinson says. One solution is using molecular tools to sequence the DNA of meiofauna rather than identifying them by hand or sending them to experts. Currently, meiofauna DNA reference databases are somewhat poor, but as more scientists tackle this issue and sequencing tools become cheaper, they may become more useful.
Currently, Silver-Gorges and his colleagues are considering comparing molecular sequencing results to their taxonomic classifications, and they’re also combing the data more thoroughly to identify sentinel organisms to target in future sampling. Horseshoe crab larvae, for example, were found on almost every turtle assigned to the eastern and southern Gulf, but on none that were assigned to the northern Gulf. In addition, Silver-Gorges and his colleagues would like to link meiofauna to turtles with a known diet. “You might have, in one population, individuals that love tunicates that hang on the surface or some that dig for crabs,” Silver-Gorges tells The Scientist. While the differences in the epibiont communities could be driven in part by these dietary preferences, “we can’t yet go ahead and say that that’s exactly what’s happening.”
Editor’s note about the turtle image included in this article: These photographs were taken during research activities permitted by the Florida Fish and Wildlife Conservation Commission under permit number MTP-239 under conditions not detrimental to this animal. Please do not attempt to recreate the contents of this image without appropriate training and authorization. This photograph was taken under red or infrared light and converted to black-and-white in post-processing. Portions of this project were funded in whole or in part by a grant awarded from the Sea Turtle Grants Program. [Language courtesy of Florida Fish and Wildlife Conservation Commission]