Off California, scientists found a sea spider that appears to farm methane-eating bacteria on its own body

Along methane seeps scattered off the California coast, researchers have documented sea spiders that appear to grow their own food supply directly on their limbs, cultivating colonies of methane-eating bacteria in a living film across their bodies and then scraping it off to eat. The behavior, described by a team working with remotely operated vehicles thousands of feet below the surface, adds sea spiders to a short list of animals known to actively farm microbes for nutrition rather than simply hosting them internally for shelter.

The methane seeps where the spiders were collected

The animals were gathered from cold seep sites along the Pacific coast of North America, including locations off Del Mar and Palos Verdes in Southern California, with related specimens turning up as far north as the Sanak seep off Alaska. These seeps release methane gas from beneath the seafloor, creating pockets of chemical energy in an otherwise dark, food-scarce environment nearly 3,350 feet down, far below where sunlight reaches and far from the surface food chains most animals ultimately depend on for energy.

How the bacteria-farming actually works

Researchers led by Occidental College marine biologist Shana Goffredi identified three previously unknown species in the sea spider genus Sericosura, part of the family Ammotheidae, each carrying communities of methane- and methanol-consuming bacteria embedded in a sticky, extracellular layer covering their legs and bodies. Rather than absorbing nutrients produced by the bacteria through their shells, the spiders appear to periodically scrape the bacterial film from their own limbs and consume it directly, a foraging strategy closer to tending a crop than hosting an internal symbiont the way many other deep-sea animals do.

The lab experiment that confirmed the spiders were actually eating the bacteria

To prove the spiders were genuinely feeding on their bacterial coating rather than simply carrying it along for the ride, the research team ran live incubations using methane and methanol labeled with a heavy carbon isotope, then tracked where that labeled carbon ended up using a technique called nanoscale secondary ion mass spectrometry. Within five days, the labeled carbon had shown up inside the spiders’ own tissue, according to the peer-reviewed study describing the findings, direct evidence that carbon originating in the bacteria was being absorbed and used by the animal itself rather than simply passing through unused.

Why this counts as a new kind of symbiosis

Plenty of deep-sea animals rely on bacteria for energy, but most, such as tube worms at hydrothermal vents, house their microbial partners internally in specialized organs built specifically for that purpose. The Sericosura spiders instead keep a mix of methanotrophic bacteria from the family Methylomonadaceae alongside methylotrophic bacteria from Methylophagaceae and Methylophilaceae arranged externally across the body’s surface, a pattern researchers described to Phys.org as closer to cultivation than to passive hosting. That distinction is what marks the discovery as a previously undocumented form of methane-based symbiosis rather than a variation on the internal-hosting model already known from vent ecosystems.

What the discovery adds to the study of methane seeps

Methane seep ecosystems have drawn growing scientific interest partly because the bacteria living in them consume methane, a potent greenhouse gas, before it escapes into the water column and eventually the atmosphere. Documenting an animal that farms those same bacteria for food gives researchers a new data point on how seep ecosystems cycle methane and support species that would otherwise have little access to energy in a lightless, food-limited environment far removed from surface-driven food webs, and it raises the possibility that similar farming behavior has simply gone undetected in other seep-dwelling invertebrates until now.

Where the research goes from here

The team’s findings, drawn from expeditions using remotely operated vehicles capable of working at seep depths, set up further questions about how widespread bacteria-farming might be among sea spiders and related seep-dwelling invertebrates that have historically been harder to study than their better-known hydrothermal vent counterparts. Because methane seeps exist in scattered pockets across ocean floors worldwide, researchers who worked on the Sericosura specimens say similar farming behavior could plausibly turn up in related species elsewhere, pending further collection expeditions and genetic comparison work across other seep sites.

Sea spiders as a group have long been considered biological oddities even before this discovery, since most of their organs extend into their legs rather than staying confined to a central body, a body plan that already sets them apart from nearly every other marine arthropod. Adding an external, cultivated food source to that list of unusual traits gives researchers another reason to treat the group as a useful model for studying how animals adapt to extreme, energy-poor environments, particularly as more remotely operated vehicle surveys reach deeper and more remote methane seep sites that earlier generations of ocean research simply lacked the technology to visit.

This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.

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