In the ice-covered central Arctic Ocean, far from any coastline, food on the ground is hard to find. When scientists take core samples of the seafloor here, which can lie more than 2.5 miles below the surface, they typically pull up muck that supports few, if any, organisms visible to the naked eye. Yet in 2011, one such sample appeared to contain, in the words of the student who first saw it, “a polar bear!”
What looked like white fur, recalls marine biologist Antje Boetius, of Germany’s Alfred Wegener Institute, was an almost equally surprising piece of sea sponge. “In this area, you’d have maybe one sponge every square kilometer or so. What a coincidence, we thought, to hit a sponge.”
Yet when the scientists returned to the same spot in 2016 with lights and cameras, they found the area—at the top of an extinct submarine volcano, known as a seamount—almost entirely covered in sponges. Some were more than three feet across.
The discovery left the researchers with a burning question: What in the world were these sponges eating? In an area seemingly devoid of food, “it was absolutely not clear how they could grow to that density,” Boetius says.
The sea sponges, it turned out, were feasting on the fossilized remains of what used to be a vibrant tube worm colony, thriving on the methane the once-active volcano released, according to a new paper published in the journal Nature Communications. And helping them turn this seemingly non-food into nourishment, they found, are symbiotic bacteria.
It’s the first time that scientists have found an animal that eats fossils. “The finding that sponges use food sources that other organisms cannot is very cool,” says marine ecologist Jasper de Goeij of the University of Amsterdam, who was not involved in the study. “And it corroborates earlier findings that the symbiosis with bacteria allows huge flexibility in acquiring food.”
On active underwater volcanoes, living tube worms settle atop the empty tubes of dead ones, generation after generation, creating the appearance of “hairy hills,” says marine biologist Antje Boetius, of Germany’s Alfred Wegener Institute. When the volcano’s activity subsides and the methane the worms turn into food stops flowing, the worms die. Their tubes remain, however, and fossilize into chitin and proteins.
This symbiotic relationship is what allows sponges to survive here, says sponge expert and lead author Teresa Morganti, of the Max Planck Institute for Marine Microbiology in Bremen, Germany.
Previous studies had already shown that a history of volcanic activity may continue to affect the local ecosystem even after the volcano goes extinct, says marine ecologist Emmelie Åström, of the Arctic University of Norway, who was not involved in the study. Still, she adds, “I am surprised by this dense sponge garden so far north, which shows we don’t know all that exists in deep oceans.”
How did sponges, which don’t seem to move around very much, or at all, manage to reach this all-you-can-eat fossil tube worm buffet on an Arctic seamount? Marine biologist and co-author Autun Purser, of Germany’s Alfred Wegener Institute, suspects they arrived as larvae.
“There are gardens of similar sponges in more southerly Norwegian waters,” he says. “So possibly, larvae came across from there.” Some lucky ones, adrift on the current, must have gotten stuck on the summit, where they found an unexpected profusion of food.
As the sponges spent more time feasting on fossil worm tubes, the symbiotic bacteria that helped them digest it likely proliferated. Adult sponges pass this highly adapted microbiome on to the next generation when they reproduce, by budding genetically identical baby sponges from their bodies. (Sponges can also reproduce sexually, but it leads to larvae that can be swept away by the current—a risky strategy when you live in inhospitable surroundings but the only way to colonize new areas.)
The team also found convincing evidence that adult sponges can move, leaving a trail of silica-based skeletal elements called spicules. They mostly move uphill, Morganti discovered, where it may be easier to catch local currents carrying bits of fossilized worm tubes. Moving uphill may also make space for the next generation, allowing smaller sponges to mature in spots more sheltered from currents.
These sponges can harbor small animals like shrimps, they learned, which probably feed on their leftovers—and the occasional mouthful of sponge. Sea stars, too, eat dying sponges.
But how long can this unusual ecosystem survive, living off the remains of an extinct community? “These sponges have a very low metabolism,” Morganti says, “so I don’t see how they could possibly finish their food here.”
A more likely threat to this colony of sea sponges may come from climate change, which is reducing Arctic ice cover and may encourage the growth of algae. That might kick the food chain into higher gear and result in more food raining down onto the seafloor. That in itself wouldn’t harm the sponges, Purser says. But it might create opportunities for other animals—perhaps a faster-growing sponge species that can’t survive in this area today—to outcompete them.
“From my experience in these northern locations,” he says, “when things start to change, the ecosystem becomes unbalanced in such a way that we don’t really know what animals are most likely to thrive.”