Not far from the coasts of California and Alaska, three newly discovered species of sea spiders thrive where methane escapes from the ocean floor.
Far from being hostile to this greenhouse gas, these tiny creatures measuring barely half an inch (1 cm) indirectly consume it through a unique biological partnership. At these depths, life doesn't depend on the Sun but on gases released by sediments. Researchers discovered that sea spiders of the Sericosura genus host bacteria on their shells that transform methane into nutrients.
The males carry the spider eggs, also ensuring the transmission of nutritious bacteria. Credit: Shana Goffredi/PNAS
Thanks to this intimate pact, the spiders don't need to capture prey. They simply graze on the bacteria that colonize their own exoskeleton. A behavior never observed before in these animals, according to the team led by Shana Goffredi, a biologist at Occidental College in Los Angeles.
Unlike their predatory cousins, these spiders have neither hooks nor specialized appendages to trap other invertebrates. Their survival tools are on their very bodies: a miniature farm, constantly regenerated by methane bubbles emerging from the seafloor. Isotopic analysis of tissues confirms that carbon from methane is indeed integrated into their diet.
The reproduction of these sea spiders reveals another aspect of their intimate association with bacteria. In these species, females release their eggs through tiny openings at the "knees." Males immediately collect them and carefully wrap them around their legs, forming protective biological bracelets.
During incubation, the eggs remain in close contact with the male's exoskeleton, already colonized by methanotrophic bacteria. This direct exposure allows the young spiders, still inside their eggs, to be introduced very early to their future food sources.
Once hatched, the larvae inherit not only genes but also useful microbes, like a living legacy. These pioneer bacteria, already adapted to the methanotrophic lifestyle, immediately settle on the young exoskeletons, providing them with nourishment from their first moments.
This vertical microbial transfer, from father to offspring, had never been observed in sea spiders. It could provide a new model for studying microbiome inheritance in other animals, including humans.
This symbiosis, already known in tube worms or sponges, takes on a novel form in an arthropod here. Nicole Dubilier, a microbiologist at the Max Planck Institute, highlights the precision of this partnership: even though most bacteria are consumed, their reproduction continues thanks to this stable balance.
Researchers believe these species play an important role in capturing underwater methane, potentially preventing its release into the atmosphere. This reinforces the idea that ocean depths harbor natural climate regulation mechanisms.
The discovery highlights the extreme localization of deep-sea ecosystems. Each species appears attached to a very specific area.