trophosome

Trophosome is a specialized organ found in certain siboglinid tube worms, particularly vestimentiferans such as Riftia pachyptila, that inhabit deep-sea hydrothermal vents and cold seeps. The organ houses endosymbiotic chemoautotrophic bacteria within large aggregations of bacteriocytes. In adult tubeworms the gut is reduced or absent, making the trophosome the principal site of nutrition for the animal.

The resident bacteria are primarily sulfur-oxidizing chemoautotrophs. They use energy from the oxidation of reduced sulfur compounds, along with carbon dioxide, to synthesize organic matter that nourishes the worm. The host supplies the bacteria with a steady flow of gases and substrates from the vent environment, facilitated by a specialized circulatory system and hemoglobins that help deliver oxygen and sulfide to the trophosome. In return, the worm provides a stable habitat and access to inorganic substrates.

Trophosome tissue is highly vascularized to maximize exchange of gases, nutrients, and produced organic matter between the host and its symbionts. The bacteria-containing organ thus enables a chemoautotrophic nutritional strategy that supports worms thriving in environments where sunlight-driven photosynthesis is impossible. The association is mutualistic: the tubeworm gains nutrition without a digestive tract, while the bacteria receive a protected niche and access to inorganic substrates. The trophosome and its symbiosis are key adaptations that underlie the ecological success of vestimentiferan tubeworms in vent and seep ecosystems.