Trilobites: This Snail Goes Through Metamorphosis. Then It Never Has to Eat Again.

Trilobites

The transformation of a deep sea mollusk is comparable to an average person growing as much as 60 feet tall with a giant sac of bacteria filling its guts.

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Four views of an adult Gigantopelta chessoia, which live deep below the ocean's surface near hydrothermal vents.CreditChong Chen

In the ocean off the coast of Antarctica, a snail lives around scorching hydrothermal vents. It’s 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.

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Not all animals eat. Some shallow water corals, for example, have algae living inside their tissues that take in sunlight and convert it to energy that provides the corals with nutrients. In the deep sea, there is no sun, but vents provide chemicals that bacteria break down. This is the basis of the deep sea food chain. Gigantopelta chessoia, instead of algae, have bacteria living in some of their cells that convert hydrogen sulfide and oxygen the snails absorb from the vents into energy.

Hydrothermal vents in waters off the coast of Antarctica, which emit chemicals that the bacteria in the snails transform into energy.CreditNERC ChEsSo Consortium

Because their guts and their radula, or snail teeth, were kind of small, and they seemed to be fine relying on bacteria, Dr. Chen and his colleagues originally thought these snails didn’t feed.

“But when we looked at the small guys, they had a very different anatomy,” said Dr. Chen. “Their internal organs were much more like a normal snail.”

And there was no bacteria inside them.

This was weird.

The only other snail in its family that relied on chemical-converting bacteria was an armadillo-esque mollusk called the scaly-foot gastropod. And in that species, the small ones looked just like the big ones. The two had evolved with the same ultimate adaptation, but through very different routes.

The team wanted to see if this transition from grazing to relying on bacteria was gradual — like how humans shift from breast milk to solid food as they grow — or sudden — like how a caterpillar may switch from eating plant matter to sipping nectar when it becomes a butterfly.

To find out, they gathered snails from their homes, 9,000 feet below the surface, and preserved their bodies so they could scan them and reconstruct the internal organs on a computer. By evaluating the relative size of these organs, they determined that the change was sudden.

Just as the snail’s body length reached 5 to 8 millimeters, the esophageal gland expanded dramatically and was teeming with bacteria. Indeed, this was a new type of metamorphosis, only visible from the inside.

However this change occurs, the snails gain an advantage by producing their own energy. They can grow bigger and make babies instead of searching for food.

Knowing about this change will also help researchers make more accurate calculations about the flow of energy in deep sea ecosystems. And in the future, looking at anatomy could prove useful in other ecosystems too.

“We think this crypto-metamorphosis could be common in other animals,” Dr. Chen said. “If we look closely enough, maybe it’s even present in systems like forests or coral reefs.”