An animal without a stomach
In the sandy, shallow sea floor off the coast of Italy lives a group of flatworms that have completely abandoned a digestive system. The genus Paracatenula includes several species of worms, often just a few millimeters long, that possess no mouth, gut, or anus. These animals survive entirely on an ancient partnership with bacteria living inside their bodies. This relationship is estimated to be over 500 million years old, making it the oldest known symbiosis between an animal and chemosynthetic bacteria.
The worm belongs to the Catenulida, a group of flatworms found in both marine and freshwater environments. Instead of a digestive tract, the body of a Paracatenula worm is packed with specialized cells called bacteriocytes. These cells house immense populations of symbiotic bacteria. This bacteria-filled tissue, called a trophosome, can make up between 33% and 50% of the worm's total body volume, the highest proportion of any known animal-bacterial symbiosis. The worm's own brain is located in front of this large bacterial mass.
A bacterial power plant
The bacteria inside Paracatenula are its power plant and its kitchen. The specific symbionts are classified as Candidatus Riegeria. These bacteria are chemosynthetic, meaning they generate energy from chemical reactions rather than from sunlight. They harness energy from sulfur compounds in the sediment to fix carbon dioxide into organic molecules, producing all the nutrition the worm needs to live.
The way the worm receives its food is unusual among symbiotic relationships. In many similar partnerships, the host animal simply digests its resident bacteria to absorb their nutrients. Paracatenula, however, appears to "farm" its bacteria rather than consuming them directly. The Candidatus Riegeria bacteria release their nutrients to the worm in tiny, bubble-like packages called outer-membrane vesicles. This allows for a continuous harvest of food without destroying the bacterial population.
This stable, ancient partnership has allowed the bacteria's genomes to become highly specialized and reduced over hundreds of millions of years. The symbionts are passed down directly from parent to offspring, a process known as vertical transmission. The worms themselves reproduce asexually through a process of splitting, ensuring that each new worm receives a starter culture of its essential bacterial partners.