Paracatenula are marine flatworms that occur in shallow water sediments. These flatworms are fascinating as they lack a mouth and a digestive system and instead live in an ancient symbiosis with bacteria. Their bacterial symbionts can make up a large proportion of the total biomass are thought to be responsable for their host nutrition.
Besides a sheltered and stable environment the symbionts gain a 'shuttle service' through the sediment that is illustrated in this video of Paracatenula sp. standrea from the sediments of Elba, Italy.
Max Planck Institute for Marine Microbiology