Methane-Eating Sea Spiders: A New Symbiotic Relationship in the Ocean Depths

In a fascinating new discovery, scientists have uncovered three previously unknown species of sea spiders that thrive on a unique diet of methane-fueled microbes in the depths of the ocean. The findings, published in the Proceedings of the National Academy of Sciences, suggest that these tiny creatures may play a crucial role in preventing methane from reaching Earth's atmosphere, potentially mitigating the effects of global warming.

The newly discovered sea spiders, part of the Sericosura genus, were found off the coasts of Southern California and Alaska. They lack the necessary appendages to capture prey, making them more akin to farmers harvesting methane-fueled bacteria from their own bodies. In this symbiotic relationship, bacteria take up real estate on the spiders' exoskeletons, and in return, the microbes convert carbon-rich methane and oxygen into sugars and fats that the spiders can eat.

"Just like you would eat eggs for breakfast, the sea spider grazes the surface of its body, and it munches all those bacteria for nutrition," said Shana Goffredi, a professor and chair of biology at Occidental College in Los Angeles and the study's principal investigator. This unique nutrition strategy has never been observed in sea spiders before.

The deep-sea ecosystem where these spiders make their home is dark and inaccessible to sunlight, so microbes have evolved to use chemicals instead of sunlight for energy. When marine life dies and sinks to the seafloor, methane gas is released during decomposition, escaping through the cracks of sediment as bubbles. The microbes that feed on the methane attach themselves to sea animals to remain among the bubbles.

By analyzing isotopes in the spiders' tissues, scientists determined that the bacteria were not just hitching a ride from an eight-legged friend—they were also being eaten. "This is really the beauty of the symbiosis between the two: The bacteria get that perfect Goldilocks zone with everything they need," said marine biologist Nicole Dubilier, a professor and director of the department of symbiosis at the Max Planck Institute for Marine Microbiology in Germany. "Even if 80% of the population are eaten (by the spiders), it’s worth it for the 20% to keep surviving and reproducing."

The newly discovered Sericosura spiders are translucent and only span about 0.4 inch (1 centimeter) in length, so they likely can't travel very far. Each of the three sea spider species collected for the study were from different areas off the coast of Southern California and Alaska—a clue that the populations are highly localized.

Since these Sericosura species are the first sea spiders caught feeding on the methane-fueled microbes, other deep-sea animals such as tube worms and sponges are known to share a similar diet. Goffredi believes that one day, similar microbes found on these spiders could be cultured to reduce water contaminants elsewhere.

Studying microbiome inheritance in animals could potentially help scientists understand more about how humans' gut bacteria is passed between mothers and newborn children, Dubilier said. Further exploration of the seafloor could also uncover additional, similar sea spider species.

"People tend to think of the deep sea as a kind of homogeneous ecosystem, but that’s actually untrue," Goffredi said. "There’s a lot of biodiversity by region and animals are very localized to specific habitats on the seafloor." She cautions that if we decide to use the seafloor for mining, for example, we must be very careful to avoid causing irreparable harm to very specific habitats that aren't found anywhere else.