Paracatenula marine flatworm

Bacteria growing inside this Paracatenula flatworm provide their host with care packages of fats, sugars and other nutrients. Credit: Oliver Jäckle/Max Planck Institute for Marine Microbiology

Ecology

Gutless worms rely on handouts from internal microbes

Paracatenula worms store their energy in a peculiar ‘battery’: bacteria that dwell in the worms themselves.

If the worm Paracatenula had a mouth, one could say that the critter was born with a spoon in it. Despite lacking a gut, these sea-dwelling animals are always well fed, thanks to bacteria living inside the worms’ bodies that supply them with all the nutrients they need — in handy packages.

Paracatenula worms and a type of bacteria known as Candidatus Riegeria have been in a close relationship for at least 500 million years. Harald Gruber-Vodicka at the Max Planck Institute for Marine Microbiology in Bremen, Germany, and his colleagues studied a species of Paracatenula that lives off the coast of the Italian island of Elba. They found that over the past half a billion years, the worm’s bacterial partner has drastically reduced its genome size, keeping only the DNA needed for essential functions. Still, the microbe is able to provide the worm with all kinds of nutrients, including fats, sugars and amino acids.

In most similar partnerships, the host organism digests the bacteria to access the food that the microbe has made. But Paracatenula feeds on small, nutrient-packed droplets secreted by its partner bacteria.