A community of sponges with a morbid secret lives on an underwater mountain in the Arctic Ocean. With little to eat in the nutrient-depleted water, the sponges survive by digesting the remains of long-dead animals that once lived on the seamount peaks where the sponges now reside. And they've been preying on the bodies of their extinct neighbors for centuries.
Scientists discovered these ghoulish organisms in the Langseth Ridge, a former volcanic seamount in the Central Arctic, at depths ranging from 1,640 to 1,969 feet (500 to 600 meters) with temperatures hovering just above freezing. Researchers discovered thousands of sponges covering an area of 5.8 square miles (15 square kilometers) at such freezing depths.
"The main question was, 'How could such a community survive in this area?'" lead study author Teresa Morganti, a postdoctoral researcher at the Max Planck Institute for Marine Microbiology in Bremen, Germany, told Live Science. "We hypothesized that they could use a local food source - in this case, the remnants of this ancient seep community."
Seafloor ecosystems often congregate around hydrothermal vents (also known as seeps) in sections of the deep ocean where nutrients are scarce, but volcanic activity in this part of the seamount halted thousands of years ago. There are no powerful ocean currents that could deliver food from above or below to the sponges.
The sponges, however, discovered a rich and plentiful food supply: dissolved substances from a long-dead tubeworm and bivalve graveyard, which the sponges digest with the help of symbiotic bacteria.
There were not only thousands of sponges grouped on the seamount peaks, but many of them had grown rather enormous, reaching up to 3 feet (1 m) in diameter. Many of the sponges were actively reproducing, with "substantial budding," the scientists said in a paper published in the journal Nature Communications on Feb. 8.
The researchers discovered a dense biomass beneath the sponges, mostly made up of tubes left behind by marine worms that died out when the seamount's volcanic activity stopped about 2,000 to 3,000 years ago, and sponge tracks over the fossil mats revealed where the sponges hunted for food before settling down atop the preserved remains.
Many of the sponges were at least 300 years old and were home to a variety of bacteria. According to the study authors, bacteria in the phylum Chloroflexi likely played a major role in decomposing the fossilized tubeworms and producing dissolved organic materials that kept the sponges well-fed.
But, if the sponges have been here for centuries - and there are now hundreds of them - is their food supply in jeopardy? According to Morganti, the sponges' metabolic rate is unusually slow.
"Because these are big individuals living in the central Arctic where the temperature is low, their metabolism is very low in general," she explained. "They aren't consuming this food source very fast, so I think they have plenty of food there."
