The Worm That Eats Its Own Mouth

In February 1977, geologists aboard the submersible Alvin dropped to 2,500 meters near the Galápagos Rift looking for hot water. They found it, and around the vents, fields of red-tipped white worms in chitinous tubes — Riftia pachyptila, up to two meters long, with no mouth, no gut, and no anus.

Monika Bright and Andrea Nussbaumer wanted to know how the next generation got its bacteria. The textbook guess was vertical: parent passes microbes to egg. The Vienna group worked out, in a 2006 Nature paper, that it isn't. Riftia larvae are free-swimming and aposymbiotic — born clean. They drift, find a vent, settle, and the bacterium Candidatus Endoriftia persephone infects them through the skin.

Here is the part the textbooks tend to skim. The larva has a normal gut: mouth, foregut, midgut, hindgut, anus. Once symbionts are inside, the host doesn't keep them in some new pouch — it dismantles the digestive tract it just grew. The midgut is remodeled into the trophosome, the dense organ that will hold a billion bacteria per gram in the adult. The mouth and anus close. Whatever was a working digestive system at the planktonic stage becomes a sealed bioreactor.

The adult breathes for both partners. Its red plume is full of an unusual hemoglobin that carries oxygen and hydrogen sulfide at the same time, on separate sites, without the sulfide poisoning the oxygen binding. The blood delivers both to the trophosome; the bacteria oxidize sulfide for energy and fix carbon into food the worm absorbs.

The worm spends months building a digestive system, then spends the rest of its life undoing it.