When you are looking at a sea turtle, you are usually not just looking at a single organism. Sea turtles never swim alone. Many creatures live directly on or even inside a sea turtle. As for all animals, this so-called symbiotic relationship does not have to be negative for the turtle.
Mutualism
In a mutualistic relationship both organisms benefit from living closely together in a symbiotic relationship. We can for example observe this on cleaning stations. Different fish nibble on old skin and small creatures such as parasites living on the sea turtle. The fish gets dinner, the turtle a free surface clean!
Commensalism
In this type of symbiotic relationship, one organism benefits, the other is neither harmed nor helped. The most obvious hitchhikers catching a ride on a turtles shell are so called remoras. These fish use bigger sea creatures such as sharks, mantas or turtles as a taxi – even if occasionally a rather slow one.
They rest on the turtles shell, hide under them and just occasionally snatch a bit of discarded food from the turtle. We can sometimes see the turtles getting impatient with a remora, trying to shove it away with it’s flippers.
Last year, we also spotted a rather unexpected hitchhiker on one of our named green turtles called GöWi – a sea cucumber! It most likely walked onto the turtles shell when the turtle had a nap earlier. It had to cling on quite tightly when the little turtle sped up and down the reef!
The same is true for this marine snail sitting on another juvenile green turtle we know very well called Beag.
Parasitism
Parasites are living in or on another organism, the so-called host. The parasite benefits, while the host is harmed in the process.
Most obvious organisms living on the outside of the turtle, so-called ectoparasites, are barnacles. These are not parasites per se, but become parasitic and harmful in excessive numbers. There are different types of barnacles known from sea turtles, the glueing and embedding types.
The typical barnacle uses a special substance it secretes, the ‘cement’, to glue itself onto a surface of the host. Glueing barnacles belonging to the genus Chelonibia are found on turtles, as their name already indicates (Chelonia = turtle).
©Stephanie Köhnk
Embedding barnacles infiltrate the skin or shell of the host turtle, thus causing more damage to the tissue. These barnacle wounds can also be an opening for bacteria to enter the turtles body, thus causing real illness.
Excessive barnacle cover can be a sign of general bad health of a turtle. Usually sea turtles are debilitated first, and then become covered in an extensive amount of other organisms, such as barnacles and algae.
Luckily turtles are very resilient and can sometimes recover from such infestations.
Through our photo ID program, we have been able to document the development of a hawksbill in Noonu Atoll, which recovered from very high barnacle cover and seems to be doing very well again!
©Karin Nistler, Prodivers
What the Hitchhikers Can Tell Us
As turtles are endangered of becoming extinct, it is very important for us to know how healthy our present sea turtle populations are. Healthier populations are more likely to withstand further stress such as man-made pollution or climate change. Our photo ID program helps us monitor the presence of parasites or severely weakened turtles in certain areas.
Furthermore, the creatures living on a sea turtle might actually be able to tell us more about the origins of the turtle itself. Just like their host, the parasite also has a specific genetic code. In the future, we might be able to infer the source population of the olive ridley turtles we find entangled in ghost nets in the Maldives, without having to invasively take tissue samples from the turtle, but just by looking at the organisms living on the turtle.
