Sea turtles are the gentle giants of the ocean, synonymous with mental images of wise old creatures slowly migrating alone across the world’s seas – but turns out they do not travel solo. Similar to the Great A’Tuin from the Discworld novels written by Terry Pratchett, loggerhead turtles carry an entire world on their back.

Life can be found in the strangest places. It is not a novel concept that large animals support microscopic life. Even humans carry a vast range of microbes with us, including viruses, bacteria, lice, and ticks. This is no different for marine animals, especially animals like whales, turtles, and large crustaceans, as they have large, solid surfaces, and move slowly, allowing microscopic communities to flourish.

Carrying The Worlds On Their Backs

It has been known for years that sea turtles harbor entire metropolises on their backs, including visible organisms such as barnacles, algae and crustaceans, but a new study provides us with a more in depth look at the micro-worlds turtles carry on their shells. A paper published in May 2020 in the journal Diversity indicates that loggerhead sea turtles (Caretta caretta) carry surprisingly diverse and abundant populations of tiny creatures on their shells, with an average of 34 000 individual meiofauna – microscopic organisms smaller than one millimeter – inhabiting this area. 

Meiofauna are bigger than bacteria, but are still too small to see with the human eye, and it is estimated that 100 meiofauna organisms can comfortably fit on a pinhead. The study found that one individual loggerhead carried more than 146 000 individual microscopic animals on its shell, including nematodes, crustacean larvae and shrimp. These findings are significant, as it raises important questions about loggerhead conservation as well as marine meiofauna.

This study was led by researchers from Florida State University as well as a research team from Brazil, and suggests that turtles are far more important at an ecological level than previously believed, and the results also contribute to the motivation for turtle conservation.

Researching Loggerhead Turtle Shells

During 2018, the researchers sampled the shells of 24 loggerhead turtles on Florida’s St. George’s Island, looking for organisms roughly between 1 mm and 0.032 mm in size. The turtle shells were sampled by first removing barnacle growths, and then carefully scraping and sponging the very top layer of their carapace (upper section of the shell) after the turtles came ashore to lay their eggs. The turtle shells were divided into sections – forward, middle and posterior – to document the different meiofauna communities living on these sections. Based on the analysis of the shell sections, the posterior shells hosted a larger diversity of species. After the samples were collected, the unharmed turtles were released and returned to the water. Samples were then taken to be analyzed at Florida State University.

Results of the Study

The researchers were delighted and pleasantly surprised to find that the samples taken from the 24 loggerhead turtles contained a vast abundance of meiofauna diversity, far greater than previous research suggested. The biggest surprise was the abundance of microscopic nematodes – known as roundworms – which were previously discounted from living on turtle carapaces. This study determined that more than 7000 nematodes, comprising 111 different species, were found on the loggerhead shells.

This result more than doubles the previously thought 100 species of meiofauna thought to be living on shells, as nematodes were not previously included in the analysis of meiofauna communities on loggerhead shells. The total number of meiofauna found on the turtles will likely increase in the future as there are more meiofauna species on loggerhead shells that are yet to be characterized. With regard to the new results, the lead author of the study Jeroen Engels said “To find nematodes on loggerhead turtle carapaces is no surprise, but when we compared their numbers and diversity to those from other hard surfaces or even on marine plant life, we realized their carapaces are abounded with this microscopic life, the extent to which had hardly been documented in the past.”

The researchers expected to find that the shells were dominated by a certain single species that is well-adapted to that environment. However, they now know that shells contain many microhabitats and niches, allowing for fully functioning and diverse communities. The researchers also found that each individual loggerhead turtle carried its own unique community of meiofauna, comprising different species that were found on some turtles and not others. 

This finding has left researchers with the question – where were these turtles colonized, and are they responsible for the success of meiofauna in the ocean? Despite smaller organisms not being able to travel great distances, microscopic organisms often have a large geographic range, and this study indicates that loggerhead turtles and other turtle species may be contributing to this large range, acting as a turtle highway, allowing microorganisms to spread.

Although it is not exactly certain how these meiofauna communities came to live on the back of turtles, the feeding habits of turtles may hint at an answer. Some loggerheads dig through the sediment on the seabed in search of clams and crabs, kicking up sand and the meiofauna within. The meiofauna then land on the back of the turtle and catch a ride throughout the world’s oceans.

What Does This Mean For The Turtles?

This latest study highlights the ecological importance of turtles in the greater marine environment. The results of the study – with turtles literally carrying entire microscopic worlds on their backs – also acts as a motivation for turtle conservation, as the loss of turtle species will negatively impact microscopic organism communities throughout our oceans. It also indirectly assists with sea turtle conservation initiatives, as a closer investigation of the communities on their shells might indicate to researchers where the sea turtles spend their time, much like satellite tags can. These claims still need to be researched, but if it proves to be true, proactive conservation measures can be put in place at the areas these turtles frequent, and the forage and breeding grounds of sea turtles can be linked. Mapping of meiofauna assemblages will help to identify critical sea turtle habitat, as well as indicate the health of marine sediments throughout the oceans.