Paracatenula is a genus of millimeter sized free-living marine gutless catenulid flatworms. Paracatenula is 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 Cand. Riegeria.The symbionts are housed in bacteriocytes in a specialized organ, the trophosome (Greek. τροφος trophos ‘food’). The bacteria can make up half of the worms' biomass. The benefitial symbiosis with carbon dioxide fixing and sulfur-oxidizing endosymbionts allows the marine flatworm to live in nutrient poor environments.