Microscopy analysis of the yellow granules in a longitudinal section of P. hessleri’s branchial apparatus stem. Credit: Wang H, et al., 2025, PLOS Biology, CC-BY 4.0
In the depths of the Pacific Ocean a bright yellow worm is the only animal to survive and thrive on the hottest parts of deep-sea hydrothermal vents.
Arsenic – a highly toxic and carcinogenic metalloid – and hydrogen sulphide boil up from the Earth’s interior and accumulate within the worm’s tissues.
According to new analysis, arsenic can make up more than 1% of its total body weight – much higher than most other arsenic ‘hyperaccumulators’.
Image of the alvinellid worm, Paralvinella hessleri. A P. hessleri specimen with buccal tentacles extroverted, lateral view. Credit: Wang H, et al., 2025, PLOS Biology, CC-BY 4.0
The new research shows the species, Paralvinella hessleri, has made such an inhospitable region home by “fighting poison with poison”.
Arsenic accumulates in the animal’s outer layer of epithelial (skin) cells where it reacts with sulphide which has diffused in from the hydrothermal vent fluid.
This forms granules of orpiment (As₂S₃), a slightly less hazardous mineral. The bright yellow pigment is responsible for the extremophile’s striking appearance.
Microscopy analysis of the yellow granules. Cross section of branchial apparatus tip. Credit: Wang H, et al., 2025, PLOS Biology, CC-BY 4.0
“We were puzzled for a long time by the nature of the yellow intracellular granules, which had a vibrant colour and nearly perfect spherical shape,” write the authors of the paper published in the journal PLOS Biology. “It took us a combination of microscopy, spectroscopy, and Raman analysis to identify them as orpiment minerals – a surprising finding.”
Co-author Dr Hao Wang from the Chinese Academy of Sciences says: “Orpiment – the same toxic, golden mineral produced by this worm – was once prized by medieval and Renaissance painters.
Close-up image of P. hessleri worms close to the hydrothermal venting. Credit: Wang H, et al., 2025, PLOS Biology, CC-BY 4.0
“The bright yellow Paralvinella hessleri worms were unlike anything I had ever seen, standing out vividly against the white biofilm and dark hydrothermal vent landscape. It was hard to believe that any animal could survive, let alone thrive, in such an extreme and toxic environment.
“It’s a curious convergence of biology and art history, unfolding in the depths of the ocean.”