Researchers at The Hong Kong University of Science and Technology (HKUST) have discovered a previously unknown specialized organelle structure in a deep-sea mollusc living at cold seeps in the South China Sea. Dedicated to maintaining bacterial symbiosis, this newly identified survival mechanism provides new insights into how deep-sea organisms adapt to nearly oxygen-free environments and opens new directions for research on chemosymbiosis, organelle evolution, and deep-sea ecosystems.
The study was led by Prof. QIAN Pei-Yuan, Chair Professor in the Department of Ocean Science at HKUST , in collaboration with Prof. SUN Jin of the Ocean University of China, Prof. QIU Jian-Wen of Hong Kong Baptist University (HKBU), and research teams from multiple institutions worldwide. The findings were published in the international journal Nature Communications , titled “ A multilamellar organelle for chemosymbiosis in an aplacophoran mollusc adapted to anoxic cold seep sediment ”.
The Divine Dragon Glistenworm from the Deep-Sea
The study focused on Chaetoderma shenloong (also known as the Divine Dragon Glistenworm), which was named one of the “Top-Ten Marine Species of 2024”. This deep-sea mollusc inhabits cold seep environments and belongs to the aplacophoran class Caudofoveata. It can grow to more than 150 mm in length, far exceeding the size of most related species.
Although cold seep ecosystems receive no sunlight, they are rich in methane and hydrogen sulfide. Many organisms in these extreme environments rely not on photosynthesis, but on partnerships with chemosynthetic bacteria for survival. Previous studies have shown that large chemosymbiotic animals, such as deep-sea mussels, tubeworms, and clams, provide living space for symbiotic bacteria within their cells. However, there has been no clear evidence that animals could evolve specialized organelles dedicated to maintaining bacterial symbiosis. This study provides the first clues that such a biological innovation may exist.
The team also discovered that Chaetoderma shenloong primarily inhabits sediment layers tens of centimeters beneath the seafloor surface. These mud layers contain almost no oxygen and are rich in hydrogen sulfide, representing an extreme environment for most animals, which suggests that Chaetoderma shenloong has evolved a unique adaptation mechanism.
Discovery of the Novel Specialised Organelle: Dracosphera
To understand how Chaetoderma shenloong survives in this extreme environment, the research team utilized anatomical analysis, micro-computed tomography, and three-dimensional image reconstruction technology to examine its body structure in depth. The results showed that its digestive system differs significantly from general molluscs: its radula is heavily reduced, and the midgut, hindgut, and anus have almost completely disappeared. Instead, a highly developed digestive gland system occupies most of its body cavity. The study further revealed that these digestive glands harbor large numbers of symbiotic bacteria, which act as the primary source of nutrients and energy for Chaetoderma shenloong .
The team subsequently discovered a novel cellular structure housing these symbiotic bacteria. This spherical structure, approximately 10 to 15 micrometres in diameter, is wrapped in more than 20 membrane layers, and accommodates hundreds to thousands of sulfur-oxidizing symbiotic bacteria. Because its shape resembles a bead wrapped in layers, the team named it the " dracosphera " (dragon ball) in Latin.
Further research found that large numbers of sulfur-oxidizing bacteria aggregate within the dracosphera, with a single host cell accommodating over 1,600 symbionts on average. Notably, the dracosphera lacks common cellular structures like host nuclei and lysosomes, meaning it is neither an ordinary cell, nor a typical digestive vacuole. Researchers believe the dracosphera is a specialized structure dedicated to accommodating and maintaining symbiotic bacteria, reflecting a highly intimate cooperative relationship established between Chaetoderma shenloong and its symbionts.
Dracosphera Helps Chaetoderma shenloong Adapt to Hypoxic Environments
To understand the function of the dracosphera, the research team analyzed the genomes of Chaetoderma shenloong and its symbiotic bacteria, discovering that the symbiotic bacteria have evolved the ability to adapt to low-oxygen environments, maintaining normal metabolism even with minimal oxygen. At the same time, Chaetoderma shenloong must live deep within seafloor mud to absorb chemicals like hydrogen sulfide for the symbiotic bacteria to use. However, the extreme scarcity of oxygen in these mud layers poses a severe survival challenge for both the host and the symbionts.
The team hypothesizes that the special structure composed of multilayer membranes surrounding the dracosphera may help regulate the distribution of oxygen within the body. These multilamellar membranes might act as a natural barrier, slowing the diffusion of oxygen and allowing the limited oxygen to be preferentially supplied to host cells for use, while maintaining the normal operation of the symbiotic system. However, this mechanism remains to be confirmed by further research.
Researchers found that the dracosphera may also participate in nutrient exchange between the host and the symbiotic bacteria. Abundant digestion-related cellular structures and enzyme activity signals exist around its symbiotic region, indicating that the host may acquire essential nutrients it cannot produce itself by breaking down a portion of the symbiotic bacteria. This mutually beneficial relationship allows Chaetoderma shenloong to survive in resource-limited deep-sea environments.
Promoting Evolutionary Biology and Deep-Sea Ecological Research
Scientists have long believed that symbiotic relationships between cells play a crucial role in the process of life evolution. For instance, mitochondria present in animal and plant cells today, and chloroplasts within plant cells, are thought to have gradually evolved from long-term symbiosis between ancient microorganisms and their hosts. The research team believes that the dracosphera demonstrates a novel cooperative model between animals and symbiotic bacteria, providing important clues for studying how organelles form and how complex life evolves new functions through symbiotic relationships.
In addition to evolutionary biology, the research also helps deepen the scientific community's understanding of deep-sea cold seep ecosystems. The research team found traces of Chaetoderma shenloong at the mud volcano cold seep off Kyushu, Japan, and the Site F cold seep in the northern South China Sea. Combined with records of large closely related species in the Gulf of Mexico, it is hypothesized that Chaetoderma shenloong , which relies on symbiotic bacteria for survival, may be widely distributed across multiple deep-sea cold seep areas globally. These organisms may participate in the methane, sulfur, and carbon cycles in the deep sea, playing an important role in maintaining the operation of cold seep ecosystems. As humanity's understanding of deep-sea environments continues to deepen, these previously little-known organisms may bring more new discoveries to our understanding of Earth's life evolution and the operation of marine ecosystems.
The co-corresponding authors are Prof. Sun Jin (OUC), Prof. Qian Pei-Yuan (HKUST / Guangzhou Marine Laboratory), and Prof. Qiu Jian-Wen (HKBU). The co-first authors of this study are Dr. LI Yunlong (formerly of Ocean University of China) and Dr. CHEN Chong (Japan Agency for Marine-Earth Science and Technology, JAMSTEC). The work benefited from the contributions of Dr. LIU Xu (OUC), Mr. LI Menggong (OUC, PhD candidate), Dr. YAN Guoyong (Guangzhou Marine Laboratory), Prof. ZHOU Haiyun (Sun Yat-sen University), Miss WANG Hui (OUC), Dr. ZHENG Xiaoshan (Qingdao Institute of Bioenergy and Bioprocess Technology, Chinese Academy of Sciences), Dr. LIU Shanshan (BGI), Mr. TAO Yinghao (ZEISS), Miss ZHANG Ni (ZEISS), Miss CHU Jingyi (OUC), Dr. LI Yi-Xuan (HKBU), and Dr. Hiromi K. Watanabe (JAMSTEC). The work also received support from the National Key Research and Development Program of China, the Laoshan Laboratory, the Natural Science Foundation of Shandong Province, the Guangzhou Laboratory and research funds from the HKSAR Government, among other sources.
Nature Communications
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A multilamellar organelle for chemosymbiosis in an aplacophoran mollusc adapted to anoxic cold seep sediment
30-Oct-2026