Deep beneath the northern South China Sea, where the mud holds almost no oxygen and hydrogen sulfide hangs thick, a worm-like mollusc has been found to carry a structure no animal was known to possess. Researchers led by Hong Kong University of Science and Technology have named it the dracosphera, a layered bead that shelters sulfur-eating bacteria within the animal's own cells. The finding, published in Nature Communications, suggests that life in the dark has found yet another way to bind two organisms into one working whole, and it may hint at how the cellular machinery we inherited from an
Deep-Sea Mollusc Found to House Bacteria in a Novel Multilayered Organelle, the 'Dracosphera'
Neither an ordinary cell, nor a typical digestive vacuole
Mark
So what is actually new here? Symbiotic bacteria in deep-sea animals isn't new.
Mimi
No, mussels, tubeworms and clams all host them inside cells. What's new is a dedicated organelle: a sphere of 10 to 15 micrometres with more than 20 membrane layers, packed with sulfur-oxidizing bacteria.
Mark
And it isn't just a normal cell?
Mimi
Right. It lacks host nuclei and lysosomes, so the team says it's neither an ordinary cell nor a typical digestive vacuole.
Luke
I'd note the language, though. The paper offers the first clues that such an organelle exists. That's careful wording, not a closed case.
Mark
What does it do for the animal?
Mimi
Probably two things. It may help manage oxygen in mud that has almost none, and it may support nutrient exchange, since the host appears to digest some of its bacteria.
Luke
The oxygen part is a hypothesis. The researchers say the membranes might slow diffusion, and they say plainly it still needs confirming. The nutrient idea is inferred from enzyme signals and cellular structures nearby.
Mark
What about the claim that it lives at cold seeps worldwide?
Luke
That's extrapolation. They found traces off Kyushu and at Site F in the South China Sea, and there are records of large relatives in the Gulf of Mexico. Global distribution and a role in carbon, sulfur and methane cycles are hypothesized, not shown.
Mimi
Still, the anatomy is striking: no hindgut, no anus, a body cavity mostly given over to bacteria-filled glands.
Mark
So the next step is testing the oxygen idea?
Luke
That's the obvious one, and nothing in the reporting says how or when it will happen.
The Pulse
- A mollusc living in oxygen-starved seafloor mud depends entirely on bacteria it cannot do without.
- An unknown multi-layered structure, the dracosphera, was found packed with hundreds to thousands of symbiotic bacteria inside single host cells.
- Researchers are testing whether its membranes throttle oxygen flow so the scarce supply reaches the host and its partners, though the mechanism is not yet confirmed.
- Traces of the animal at Japanese and South China Sea cold seeps point to a wider presence that could influence deep-sea methane, sulfur and carbon cycles.
Deep beneath the northern South China Sea, where the mud holds almost no oxygen and hydrogen sulfide hangs thick, a worm-like mollusc has been found to carry a structure no animal was known to possess. Researchers led by Hong Kong University of Science and Technology have named it the dracosphera, a layered bead that shelters sulfur-eating bacteria within the animal's own cells. The finding, published in Nature Communications, suggests that life in the dark has found yet another way to bind two organisms into one working whole, and it may hint at how the cellular machinery we inherited from ancient partnerships first came to be.
Beneath the floor of the northern South China Sea, in mud so poor in oxygen that it is thick with hydrogen sulfide, lives a mollusc that can stretch past 150 millimeters. Chaetoderma shenloong, nicknamed the Divine Dragon Glistenworm, was named one of the Top-Ten Marine Species of 2024. Now a team led by Prof. Qian Pei-Yuan of the Hong Kong University of Science and Technology has described a structure inside it that no animal was known to have.
The researchers used anatomy, micro-computed tomography and three-dimensional reconstruction to study how the animal survives. Its digestive system has departed sharply from the usual mollusc plan, with most of the gut gone and a large digestive gland system filling the body. Those glands hold vast numbers of sulfur-oxidizing bacteria that supply nearly all of its energy. Within them sits the dracosphera, a spherical body wrapped in more than 20 membrane layers, each cell carrying over 1,600 symbionts on average. Lacking host nuclei and lysosomes, it is neither an ordinary cell nor a simple digestive vacuole, and the team believes it exists solely to house and maintain its bacterial partners.
The arrangement creates a hard bind. The bacteria are adapted to function on very little oxygen, while the mollusc must burrow deep into the mud to draw in the hydrogen sulfide they feed on. The team suspects the layered membranes slow oxygen diffusion so that the scarce supply favors host cells, but they stress this idea still needs proof. Signs of digestion around the symbionts also suggest the host may draw nutrients from some of its bacteria.
The discovery may reach beyond this one animal. Its traces have turned up at cold seeps off Kyushu and at Site F in the South China Sea, and the team suspects it lives at seeps worldwide and helps drive deep-sea cycles of methane, sulfur and carbon. Whether that holds is still to be tested, but the dracosphera already offers a fresh view of how organelles may form from long-term partnerships between hosts and microbes.
Tens of centimeters beneath the floor of the northern South China Sea, in mud that holds almost no oxygen and is thick with hydrogen sulfide, lives a mollusc that can grow past 150 millimeters long. Chaetoderma shenloong, nicknamed the Divine Dragon Glistenworm, belongs to the aplacophoran class Caudofoveata and is far larger than most of its relatives. It was named one of the "Top-Ten Marine Species of 2024". Now a team led by the Hong Kong University of Science and Technology has described a structure inside it that had not been seen in any animal before.
The study appeared in Nature Communications under the title "A multilamellar organelle for chemosymbiosis in an aplacophoran mollusc adapted to anoxic cold seep sediment." It was led by Prof. Qian Pei-Yuan, Chair Professor in HKUST's Department of Ocean Science, working with Prof. Sun Jin of the Ocean University of China and Prof. Qiu Jian-Wen of Hong Kong Baptist University, along with teams from several institutions worldwide.
Cold seeps receive no sunlight but are rich in methane and hydrogen sulfide, so many animals there depend on chemosynthetic bacteria rather than photosynthesis. Deep-sea mussels, tubeworms and clams are known to shelter such bacteria inside their cells. What had not been shown was an animal evolving a dedicated organelle just to keep symbionts. This study offers the first clues that one exists.
To see how the worm-like mollusc copes, the researchers used anatomical analysis, micro-computed tomography and three-dimensional reconstruction. Its digestive system departs sharply from the usual mollusc plan: the radula is heavily reduced, and the midgut, hindgut and anus have nearly vanished. In their place, a well-developed digestive gland system fills most of the body cavity. Those glands hold large numbers of symbiotic bacteria, the animal's main source of nutrients and energy.
Inside the glands the team found the new structure. It is spherical, about 10 to 15 micrometres across, wrapped in more than 20 membrane layers, and holds hundreds to thousands of sulfur-oxidizing bacteria. A single host cell contains more than 1,600 symbionts on average. Because it looks like a bead wrapped in layers, the researchers named it the dracosphera, Latin for "dragon ball." It lacks host nuclei and lysosomes, so it is neither an ordinary cell nor a typical digestive vacuole. The team regards it as a specialized structure for housing and maintaining the bacteria.
Genome analysis of both host and symbionts added to the picture. The bacteria have evolved to keep up normal metabolism on very little oxygen. The mollusc, meanwhile, must live deep in the mud to take up hydrogen sulfide for them, which puts both partners in a severe oxygen shortage. The team hypothesizes that the layered membranes slow oxygen diffusion, letting the scarce supply go preferentially to host cells while the symbiosis keeps running. They stress that this mechanism still needs confirmation.
The dracosphera may also handle nutrient exchange. Digestion-related cellular structures and enzyme activity signals surround the symbiont region, suggesting the host obtains nutrients it cannot make by breaking down some of the bacteria.
The researchers see implications for evolutionary biology. Mitochondria and chloroplasts are thought to have arisen from long-term partnerships between ancient microorganisms and their hosts, and they believe the dracosphera shows a new animal-bacteria arrangement that could shed light on how organelles form.
The animal may also be widespread. The team found traces of it at the mud volcano cold seep off Kyushu, Japan, and at the Site F cold seep in the northern South China Sea. With records of large related species in the Gulf of Mexico, they hypothesize it occurs at cold seeps globally and may take part in deep-sea methane, sulfur and carbon cycles. Whether it does remains to be tested.
Co-first authors are Dr. Li Yunlong and Dr. Chen Chong of JAMSTEC. Funding came from sources including the National Key Research and Development Program of China, the Laoshan Laboratory, the Natural Science Foundation of Shandong Province, the Guangzhou Laboratory and the HKSAR Government.
Notable Quotes
The layered membranes may slow oxygen diffusion so scarce oxygen goes preferentially to host cells, but this mechanism remains to be confirmed.— HKUST-led research team (paraphrased)
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