Deep-sea sea spiders farm methane-eating bacteria on their own exoskeletons

Forum topic · ✨步子哥 · 2026-07-16

Summary

In 2023, biologist Shana Goffredi of Occidental College was surveying the Del Mar methane seep off the California coast when routine carbon isotope tests on captured sea spiders (pycnogonids) returned a δ13C value of about -45‰, far lighter than the photosynthetic baseline near -20‰. The signal was a fingerprint of methane-derived carbon, yet no animal can directly metabolize methane. Electron microscopy revealed dense mats of Methylomonadaceae methane-oxidizing bacteria covering the spiders' legs, trunks, and even egg-bearing appendages, making up 22% to 61% of the surface microbiome. NanoSIMS imaging after a five-day 13CH4 labeling experiment traced carbon atoms from gas to bacteria to spider tissue, confirming the first documented methane-to-animal pathway via exoskeletal symbionts. Males carry egg clutches coated with the same bacterial community, so hatchlings inherit a ready-made farm, an unusually complete example of vertical transmission. The findings, published in PNAS in June 2025, mirror independent symbioses in seep tube worms and Bathymodiolus mussels, classic convergent evolution.

Deep-sea sea spiders run a methane farm on their own bodies

The accidental discovery

In 2023, Shana Goffredi of Occidental College was leading a routine survey of the Del Mar methane seep off the California coast at roughly 1000 m depth. The dive collected tube worms, mussels, and small pycnogonids, or sea spiders, that nobody had planned to study. Carbon isotope analysis changed that. Photosynthesis-derived carbon sits near -20‰ on the δ13C scale, while methane-derived carbon is much lighter, between -40‰ and -60‰. Spider tissue came back at about -45‰, a clear methane signature.

Body as pasture

Sea spiders cannot digest methane. The molecule CH4 requires particulate methane monooxygenase (pMO), an enzyme only certain bacteria and archaea carry. Electron microscopy showed the spiders' exoskeletons sheathed in Methylomonadaceae methane-oxidizing bacteria, accounting for 22% to 61% of the surface microbiome. The spiders use their mouthparts to graze the bacterial layer, which converts methane into protein, lipids, and polysaccharides.

A five-day incubation with 13C-labeled methane and methanol, followed by nano-scale secondary ion mass spectrometry (NanoSIMS) imaging, traced labeled carbon directly into spider tissue. Published in PNAS in June 2025, this is the first confirmed case of a sea spider acquiring methane carbon through exoskeletal symbionts.

Inherited farm

Male sea spiders brood eggs on specialized legs. 16S rRNA sequencing revealed that egg clutches carry the same Methylomonadaceae community as adults, 19% to 29% of the microbiome. Hatchlings are born pre-seeded with their farm, an unusually direct form of vertical transmission.

Convergent evolution at seeps

Sea spiders are not alone. At least three animal phyla have independently evolved chemosynthetic symbioses at methane seeps:

Tube worms (*Laminatubus*, *Bispira*): bacteria coat feathery branchiae that double as respiratory organs (Orphan lab, Caltech, *Science Advances* 2020).
Mussels (*Bathymodiolus*): methane oxidizers live intracellularly within gill tissue.
Sea spiders (*Sericosura*): bacteria live on the exoskeleton, a novel location for the symbiosis.

Three independent solutions to the same problem.

Why it matters

Methane is normally toxic to animal metabolism, interfering with cytochrome oxidase and the respiratory chain. Rather than resist or evade it, these animals transform it into food, outsourcing the chemistry to microbial partners. The strategy is a textbook case of the holobiont concept: the animal plus its microbiome functions as a single ecological unit. In sea spiders, that partnership is extreme enough that the farm is literally worn on the body and passed down across generations.