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Many cultures — and at least one series of novels — have portrayed the world as being borne on the back of a giant turtle. And sea turtles, in particular the loggerhead, actually do carry an entire world on their backs — a world made up of microscopic creatures!

Many marine organisms bring other creatures along for the ride as they make their way through the ocean. Slow animals like sea turtles and whales are an ideal spot for smaller critters like barnacles to settle on. They're constantly swimming to new sources of food for the little guys to pull from the water.

To get a clearer picture of these little passengers, a 2020 study took a close look at the organisms that spend their time on loggerhead shells. The researchers characterized tiny creatures collectively referred to as meiofauna: organisms smaller than one millimeter. And they found that loggerheads may carry an average of 34,000 such critters on their shells.

One loggerhead was found with almost 150,000 animals, including nematodes, crustacean larvae, and shrimp! Meiofauna are found all over the ocean, tucked between sediment grains on the seafloor. They're an important part of the marine ecosystem, often the first to move in when new spaces open up.

However, most do not have a free-swimming larval stage, so it's been a head scratcher as to how they can live virtually everywhere. Researchers believe that sea turtles, especially loggerheads, play a large role in their dispersal. It's not because these tiny critters prefer a sea turtle's shell to other floating objects.

Rather, it's the unexpected consequence of the loggerhead's typical activities. Loggerheads migrate thousands of kilometers each year, sometimes crossing entire ocean basins as they move from nesting to foraging sites. They spend lots of time digging in the seafloor in search of tasty shellfish, and in the process, they stir up the sediment — which is full of meiofauna.

A sea turtle's shell has a lot of surface area, and if these opportunistic critters happen to drift down to rest on it, they're perfectly happy to set up shop. We're not sure yet how much time these hitchhikers spend shell-side. For some, the ride may last a lifetime.

For others, it's probably more like a long marine Uber ride: hopping on when a turtle digs into the sediment for a meal and back off at another foraging stop. That would help disperse these critters around the globe, though we'd need more research to show that's really happening. How long they stick around may also depend on where the animal settles on the shell.

The 2020 study found a greater diversity of animals living towards the back of the shell than the front or middle. This could be because animals towards the back are less likely to be knocked off the shell when the turtle is digging. They're also less likely to dry out when the turtle surfaces to breathe.

The turtles in this study were tagged and their shells wiped clean of hitchhikers. Since they return to the same nesting sites every season, the researchers hope to be able to observe how shells are taken over over a fixed time period. Sea turtles' traveling habits can make them difficult and expensive to track.

But studying their meiofauna might make it easier. Learning more about these hitchhikers could help researchers further define turtle migration routes and inform future conservation efforts for these endangered creatures. Helping turtles will surely help the little guys whose entire world is a turtle shell, too— so it works out for everyone.

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