A genus of flatworms has evolved such an effective symbiosis with bacteria that it no longer has a mouth or anus. Instead, all of Paracetenula's energy and nutrients come via the Candidatus bacteria, which weigh as much as the worm that carries them. The partnership may seem strange to us, but it clearly works for both sides – they've been together for a minimum of 500 million years.

We think of animals as being active chasers of food, but representatives of our kingdom are not above having others do the work for them. Corals rely on the energy produced by photosynthesizing algae for much of their food. Even the human digestive system works because bacteria extract some nutrients from what we consume, making it easier for us to absorb what we need. Giant tubeworms living around hydrothermal vents live entirely off symbiotic bacteria.

However, when Oliver Jäckle studied Paracetenula for his PhD thesis at the Max Plank Institute for Marine Microbiology he found the worms, which make organic compounds out of carbon dioxide and hydrogen sulphide, were doing something unlike anything we have seen before.

"In all chemosynthetic symbioses known to date, the host digests the bacteria to access their nutrients," Jäckle said in a statement. "Other chemosynthetic symbionts additionally use so-called transporter proteins that deliver nutrition to their hosts. In the Paracatenula symbiosis, we did not find either in large quantities. Everything pointed to a different mechanism."

Jäckle has explained the process in Proceedings of the National Academy of Sciences. The bacteria produce nutrition-rich droplets, which senior author Dr Harald Gruber-Vodicka compares to fruit. “The bacteria continuously bear fruit, which the worm reaps,” Gruber-Vodicka said. “In other symbioses, it's more like harvesting a cornfield, their bacteria are completely mowed down, the worm digests most of the bacterial cells.”

For other animals, most of what goes in must come out, but Paracetenula appears to play by different rules. It never developed a replacement waste disposal mechanism after abandoning its anus. “Everything the bacteria provide is apparently used by the worm, one way or the other,” Gruber-Vodicka said. 

Meanwhile, Candidatus made its own cuts. Its genome is far smaller than its closest non-symbiotic relatives, having deleted all the genes necessary to survive on its own, instead focusing on the development of a system of nutrient processing and energy storage the paper calls “unparalleled”. For the flatworms, this replaces the energy reserves other animals have in specialized cells.

There are multiple species of Paracatenula, and each has a particular species of Candidatus on which it relies.

The discovery required bringing together genomic analysis, electron microscopy and deep knowledge of the most comparable symbiotic relationships. Jäckle also had to work out how to grow Paracatenula in the lab sustainably.