Hungry sea sponges feast on fossils atop an extinct underwater volcano
In the ice-covered central Arctic Ocean, far from any coast, food on the ground is hard to come by. When scientists take core samples from the seabed here, which can be more than 2.5 miles below the surface, they typically scrape up mud that supports few or no organisms visible to the naked eye. Yet in 2011, one such specimen appeared to contain, in the words of the student who first saw it, “a polar bear!”
What looked like white fur, marine biologist Antje Boetius of Germany’s Alfred Wegener Institute recalls, was an almost equally startling piece of sea sponge. “In that area, you would have maybe one sponge per square kilometer or so. What a coincidence, we thought, to hit a sponge.
Yet when scientists returned to the same spot in 2016 with lights and cameras, they found the area – atop an extinct undersea volcano known as a seamount – almost entirely covered in sponges. Some were over a meter in diameter.
The discovery left researchers with a burning question: what the hell were these sponges eating? In an area seemingly devoid of food, “it was absolutely not clear how they could achieve this density,” says Boetius.
According to a new article published in the journal Nature Communication. And by helping them to transform this non-food into food, they discovered that these are symbiotic bacteria.
This is the first time that scientists have discovered an animal that eats fossils. “The finding that sponges use food sources that other organisms can’t is very cool,” says marine ecologist Jasper de Goeij from the University of Amsterdam, who was not involved in the study. . “And this supports previous findings that symbiosis with bacteria allows for great flexibility in food acquisition.”
On active submarine volcanoes, living tubeworms settle atop the empty tubes of the dead, generation after generation, creating the appearance of “hairy hills”, says marine biologist Antje Boetius, of the Alfred Wegener Institute. in Germany. When the activity of the volcano decreases and the methane that the worms turn into food stops flowing, the worms die. Their tubes however remain and fossilize into chitin and proteins. (See photos of more amazing creatures living in the icy Arctic Ocean.)
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 could continue to affect the local ecosystem even after the volcano has died out, says marine ecologist Emmelie Åström, from the Arctic University of Norway, who did not not participated in the study. Still, she adds, “I am surprised by this dense sponge garden so far north, which shows that we don’t know everything that exists in the deep oceans.”
How did the sponges, which don’t seem to move much, if at all, manage to reach this buffet of fossil tube worms at will on an Arctic seamount? Marine biologist and co-author Autun Purser, of Germany’s Alfred Wegener Institute, suspects they arrived as larvae.
“There are similar sponge gardens in Norwegian waters further south,” he says. “So maybe larvae came from there.” A lucky few, adrift on the current, must have gotten stuck at the top, where they found an unexpected abundance of food.
As sponges spent more time feasting on fossil worm tubes, the symbiotic bacteria that helped them digest them likely proliferated. Adult sponges pass on this highly adapted microbiome to the next generation when they reproduce, breeding genetically identical baby sponges out of their bodies. (Sponges can also reproduce sexually, but this leads to larvae that can be swept away by the current – a risky strategy when living in an inhospitable environment but the only way to colonize new areas.)
The team also found compelling evidence that adult sponges can move around, leaving a trail of silica-based skeletal features called spicules. They mostly move uphill, Morganti found, where it may be easier to catch local currents carrying pieces of fossilized worm tubes. Moving up can also make room for the next generation, allowing smaller sponges to mature in places more sheltered from currents.
These sponges can harbor small animals like shrimp, they learned, which likely feed on their remains and sometimes a bite of the sponge. Starfish also eat dying sponges. (These animals also live in extreme environments.)
But how long can this unusual ecosystem survive, living on the remains of an extinct community? “These sponges have a very low metabolism,” says Morganti, “so I don’t see how they could end up 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 promote algae growth. This could kick the food chain into high gear and rain more food onto the seabed. That in itself wouldn’t harm the sponges, Purser says. But it could create opportunities for other animals — perhaps a faster-growing species of sponge that can’t survive in this region today — to outcompete them.
“From my experience in these northern areas,” he says, “when things start to change, the ecosystem becomes out of balance in such a way that we don’t really know which animals are most likely to thrive.