In the ocean off the coast of Antarctica, a snail lives around scorching hydrothermal vents. Its name is Gigantopelta chessoia. From the outside, it looks like any other shelled slug. But on the inside, something strange is happening, scientists report in Proceedings of the Royal Society B, like no metamorphosis ever observed in any other animal on the planet.

“We’re calling it crypto-metamorphosis,” said Chong Chen, a deep sea biologist at Japan Agency for Marine-Earth Science and Technology who uncovered this hidden transition that is unlike the external body changes most other animals undergo during metamorphosis.

Once the snail reaches a certain body length, its digestive system stops growing. Its teeth, stomach and intestine make way for an expanding esophageal gland. The organ gets so big, it takes up most of the snail’s body, and basically becomes a new organ. Bacteria colonize it, and the snail, which grazed for food when it was smaller, no longer needs to eat. Instead it just sits there getting bigger, surviving on energy the bacteria produces inside the snail’s cells.

To make a human comparison, imagine growing from an average size adult to one 30 to 60 feet tall, with a giant sac of bacteria living inside you.