A deep-sea mollusc builds a 'dragon ball' for more than 1,600 symbionts

A team led by HKUST studied Chaetoderma shenloong in South China Sea cold-seep sediment and found a spherical structure called the dracosphera inside its digestive glands. Roughly 10-15 micrometres across and wrapped in more than 20 membranes, it houses hundreds to thousands of sulphur-oxidising bacteria, averaging over 1,600 symbionts in one host cell. The animal's midgut, hindgut and anus are nearly absent, so it relies heavily on the symbionts for energy. The team hypothesises that the membranes regulate scarce oxygen and nutrient exchange, but that mechanism is not directly confirmed; this is a clue to organelle evolution, not a confirmed new organelle type.