The Iroquois people of North America believed that the earth rested on the back of a giant turtle swimming around in a vast ocean. It sounds quaint and fanciful but, during my visit to the Croatian Biological Congress, I learned that marine turtles do, in fact, carry worlds around on their backs, albeit on a somewhat smaller scale than envisaged by the Iroquois. The image at the top of the post shows a turtle at Pula Aquarium’s Marine Turtle Rescue Centre, with several barnacles clearly visible but Sunčia Bosak of the University of Zagreb gave a fascinating talk on yet smaller organisms living on turtles. She focussed on the diatoms but also talked about bacteria and a range of other organisms.

I’ve written many times about epiphytes on this blog – organisms that live on the back of other plants (most recently, “Springtime surprises …”). In the same way, smaller organisms can hitch a ride on the back of animals and studies have shown that these are not just common but, in many cases, the hitchhikers have strong preferences for exactly this type of habitat. Luc Denys, from Belgium, has found unique genera that live only on the backs of whales and there is now also evidence that some species and genera found on the backs of marine turtles are also restricted to this unusual habitat.

In a slight deviation from the Iroquois creation myth, diatoms were found not just on the shell (carapace) of the turtle, but also on the skin. Furthermore, different diatoms were found on skin and shell, and yet more differences were revealed when turtles from different parts of the world were compared. The diatoms that seem to live exclusively on turtles were more abundant on skin whereas the assemblages on shells were dominated by a range of generalist species.

The next twist to this story comes with a very recent paper (published in October 2022) that shows that not only do turtles have distinct assemblages of diatoms on their skin and shell, but that these diatoms, in turn, each have distinct assemblages of bacteria. I’ve talked before about how loose associations between the protozoan and Ophyridium and diatoms can benefit both (see “Intimate strangers …”). In brief, the hitchhikers extend the “metabolic toolkit of the host, giving it greater resilience to the uncertainties of life in the oceans. Because they, in turn, host a range of bacteria, the turtle-squatting diatoms take this to the next level, facilitating a rich external “microbiome” to complement the gut microbiome whose importance in humans is now recognised.

This is all part of an emerging field that is, ostensibly, about how species interact with each other in ways other than traditional view of “nature red in tooth and claw”. There are many studies now showing co-operation amongst organisms that challenge received wisdom not just in science. “Survival of the fittest” contributed not just to an understanding of nature in the late 19th and early 20th century, it also knitted in with ideas such as Nietzsche’s Übermensch to underpin political views that dominated much of the 20th century. How might a more co-operative view of nature synergise with politics? How does this challenge the harsh right-wing political views that dominates modern politics? This type of speculation may seem to be a long way from scraping diatoms off the back of a turtles in the Adriatic, but it is one of a number of studies all pointing to a more co-operative view of species interactions. Maybe there is a lesson we should all learn from that.


Denys, L. 1997. Morphology and taxonomy of epizoic diatoms (Epiphalaina and Tursiocola) on a sperm whale (Physeter macrocephalus) stranded on the coast of Belgium. Diatom Research 12: 1-18.

Filek, K., Lebbe, L., Willems, A., Chaerle, P., Vyverman, W., Žižek, M., and Bosak, S. 2022. More than just hitchhikers: a survey of bacterial communities associated with diatoms originating from sea turtles. FEMS Microbiology Ecology 98: fiac104.

Holmes, R.W. 1985. The morphology of diatoms epizoic on cetaceans and their transfer from Cocconeis to two new genera, Bennettella and Epipellis. British Phycological Journal 20: 43-57.

Kanjer, L., Filek, K., Mucko, M., Majewska, R., Gracan, R., Trotta, A., Panagopoulou, A., Corrente, M., DiBello, A. and Bosak, S. 2022. Surface microbiota of Mediterranean loggerhead sea turtles unravelled by 16S and 18S amplicon sequencing. Frontiers in Ecology and Evolution doi: 10.3389/fevo.2022.907368

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