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Ecology

The Hitchhiker’s Guide to the Galax-Sea

Van de Vijver, B., Robert, K., Majewska, R., Frankovich, T. A., Panagopoulou, A., & Bosak, S. (2020). Geographical variation in the diatom communities associated with loggerhead sea turtles (Caretta caretta). PloS one, 15(7), e0236513.

The Hitchhikers and their Host

A loggerhead sea turtle. Image credit: Creative Commons

Sea turtles are one of the most well-known ocean reptiles. They a graceful swimmers that can be found all over the world, and are inextricably linked to many healthy ocean ecosystems. There are several species of sea turtles including green sea turtles, loggerhead sea turtles, leatherback sea turtles, and olive ridley sea turtles, just to name a few.

Just like humans have microscopic species living on and within our body, called our microbiome, all species of sea turtles can grow and carry a unique set of organisms on their shells and skin. These hitchhikers, or epibionts, adhere to a sea turtles’ shell similarly to the way they would a rock or coral on the ocean floor. A specific type of phytoplankton, called a diatom, is one of the single-celled organisms most commonly found on sea turtle shells. When the phytoplankton are really dense, the can form a film-like substance that can cover the skin and shell of the sea turtle.

A few species of diatoms. Image credit: Creative Commons

There are around 5,000 known species of diatoms, and their geographical range can vary from just a couple of miles across to worldwide. Turtles are a highly migratory species, which led Bart Van de Vijver and his fellow researchers to ask the question: Can you tell where a sea turtle has been from what it’s carrying on its back?

I Scratch Your Back, You Scratch Mine

Van de Vijver and his team decided they wanted to focus their studies on loggerhead sea turtles in four areas of the world – Croatia, Greece, South Africa, and Florida (USA). In each of these locations, five turtles were randomly selected to get their shells sampled for diatoms. Turtles from Greece and South Africa had their shells scrubbed using a single-use toothbrush, turtles from Croatia had their shells scraped with a sharp tool (don’t worry – this didn’t hurt the turtles!), and the turtles from Florida had their shells swabbed with Q-tips.

These samples were then brought to the lab and analyzed to figure out what species of diatoms were on the turtles’ shells. The diatoms were photographed using an Electron Scanning Microscope, which is a special kind of microscope that can provide detailed images of the surface of cells. Scientists were then able to tell what type of diatoms were present on each turtle’s shell.

What Did They Find?

The loggerhead turtles that these scientist studied had over 400 different species of diatoms on their shells and skin. They found a unique array of species, including species that are endemic to, or can only be located on, the backs of turtles. It also seemed that the way that they sampled the turtles had a major effect on the diversity of life that they saw. Florida sea turtles had the least amount of biodiversity. Most of the species were not particularly “sticky”; they were the species that would have been easily knocked off the shell with the cotton swab. On the other hand, the Croatian turtles had higher biodiversity, which the scientists attributed to the knifelike tool they used to scrape samples off the shells.

A sea turtle grazing on sea grass. Image credit: Creative Commons

Even though the biodiversity of diatoms in the Florida turtles was lower than that of the others, Van de Vijver and his team still found significant differences between the makeups of species on the turtles at all locations. These turtles are grazers, mostly eating sea grass off the ocean floor. The scientists hypothesized that when these turtles turned up the sand by pulling out the grass, many diatoms that were in the sand, and specific to the region they were in, would stick to the shells of the turtles as they continued to move along. This answered their original question; they were able to figure out where a sea turtle lived based on their hitchhikers.

Sea Turtles: an Endangered Species

Why should we care about these turtles and their hitchhikers? The reasons are twofold. As a reminder, diatoms’ habitat ranges can be really limited. These turtles can act as a vector to bring species of diatoms to new areas. Since diatoms can reproduce asexually, a single cell in a new location could expand a diatom’s habitat range. Secondly, turtles are an endangered species, so scientists are constantly trying to improve way to ID and track individuals. Looking at the makeup of diatoms for individual turtles could help scientists determine their home turf, and even possibly help identify a specific turtle. Looks like these hitchhikers aren’t complete freeloaders after all!

 

I am currently a Master’s candidate in Environmental and Ocean Sciences at the University of San Diego, and I study the stickiness of phytoplankton using 3D images. By tracking collisions of phytoplankton, I can see how sticky they are by observing how often they stay together when they collide instead of bouncing off of each other.  When I am not working on my thesis you can find me on the beach, reading a book, or working on a painting!

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