A deep-sea mollusc reveals a 'dragon ball' symbiotic organelle

Researchers found a structure specialized for housing chemosynthetic bacteria in the aplacophoran mollusc Chaetoderma shenloong, which lives in anoxic deep-sea cold-seep sediment. The roughly 12-micrometre multilayered spheres fill an enlarged, reticulated digestive gland and farm sulfur-oxidizing bacteria; the host has lost most of its conventional gut. The team named the structure dracosphera, inspired by Dragon Ball. Genomic and spatial-transcriptomic evidence indicates that the bacteria are microaerobic while the host lives tens of centimetres below the seabed in oxygen-free sediment. Multiple membrane layers may restrict oxygen diffusion and create a controlled microenvironment for the symbionts. The paper describes a previously unknown animal organelle dedicated to chemosynthesis, suggesting how larger animals can integrate microbial metabolism in dark, hypoxic habitats. Whether it meets every definition of an organelle, how it develops and is inherited, and how the host controls bacterial abundance need study in more species and living specimens.