Deep-Sea Worm Houses Bacteria in 'Dragon Ball' Organelles

Takeaway: this worm thrives in oxygen-poor, sulfide-rich mud by farming bacteria inside itself.

How did the team find the 'dragon ball'?

The study was led by Qian Pei-Yuan, chair professor in HKUST's Department of Ocean Science, with Sun Jin of the Ocean University of China, Qiu Jian-Wen of Hong Kong Baptist University and collaborators from multiple institutions worldwide.

Using anatomical analysis, micro-computed tomography and 3D image reconstruction, the team mapped the worm's body. Its digestive system barely resembles a typical mollusk's:

• The radula — the toothed feeding ribbon mollusks use to scrape food — is heavily reduced.
• The midgut, hindgut and anus have almost completely disappeared.
• A highly developed digestive gland system fills most of the body cavity.

Those digestive glands harbor large numbers of symbiotic bacteria, which serve as the worm's primary source of nutrients and energy.

Within the gland cells, the researchers found the dracosphera: a sphere wrapped in layered membranes, each holding hundreds to thousands of sulfur-oxidizing bacteria — over 1,600 per host cell on average. The structure contains no host nuclei and no lysosomes, the cell's digestive compartments. That rules it out as an ordinary cell or a typical digestive vacuole. The researchers interpret it as a purpose-built container for symbionts.

Takeaway: the worm essentially replaced its gut with bacteria farms enclosed in a new kind of organelle.

What does the structure actually do?

Genomic analysis of the worm and its bacteria showed the symbionts have evolved to keep their metabolism running on minimal oxygen. The worm, meanwhile, must stay buried in sulfide-rich mud to supply its tenants with chemicals.

The team hypothesizes that the dracosphera's multilayer membranes act as a natural barrier that slows oxygen diffusion, steering scarce oxygen toward host cells while the symbiosis keeps running. That mechanism remains unconfirmed pending further research.

The dracosphera may also mediate nutrient exchange. Digestion-related structures and enzyme activity surround the symbiont region, suggesting the host breaks down a portion of the bacteria to obtain nutrients it cannot make itself.

Takeaway: the membranes may ration oxygen, and the host may eat some of its tenants — a two-way deal.

Why does this matter beyond one worm?

Scientists hold that cell-in-cell symbiosis drove major transitions in life's history: mitochondria in animals and plants, and chloroplasts in plants, likely descended from ancient bacteria that took up long-term residence in host cells. The dracosphera offers a present-day model of how such partnerships begin and how new organelles might form.

The team found traces of Chaetoderma shenloong at a mud volcano cold seep off Kyushu, Japan, and at the Site F cold seep in the northern South China Sea. Records of large related species in the Gulf of Mexico support a hypothesis that this symbiont-dependent worm may inhabit cold seeps worldwide, participating in deep-sea methane, sulfur and carbon cycles.

These hypotheses about distribution and ecosystem roles await further sampling. For now, the confirmed result stands: a deep-sea worm built an organelle no one had seen before.

Publication: Yunlong Li et al., Nature Communications (2026). DOI: 10.1038/s41467-026-78965-0.