Paracatenula

Paracatenula is a genus of millimeter sized free-living marine gutless catenulid flatworms.

Paracatenula spp. are found worldwide in warm temperate to tropical subtidal sediments. They are part of the interstitial meiofauna of sandy sediments. Adult Paracatenula lack a mouth and a gut and are associated with intracellular symbiotic alphaproteobacteria of the genus Candidatus Riegeria. The symbionts are housed in bacteriocytes in a specialized organ, the trophosome (Greek 'food'). Ca. Riegeria can make up half of the worms' biomass. The beneficial symbiosis with the carbon dioxide fixing and sulfur-oxidizing endosymbionts allows the marine flatworm to live in nutrient poor environments. The symbionts not only provide the nutrition but also maintain the primary energy reserves in the symbiosis.