Paracatenula

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

Adult Paracatenula lack a mouth and a gut and are associated with intracellular symbiotic alphaproteobacteria of the genus Candidatus Riegeria.

erato, P. kalliope, P. polyhymnia, P. urania and P. galateia, named after muses and nymphs of the Greek mythology.

All Paracatenula species examined so far were found to harbor bacterial symbionts in specialized symbiont-housing cells that form the nutritive organ - the trophosome.

[Ref 6] Although Paracatenula produce sperm and eggs that can be very informative to differentiate between species, sexual reproduction has not been observed.

[Ref 5] Instead, the worms reproduce by asexual fission or fragmentation, a process called paratomy.

Paracatenula worms have high regenerative capabilities and can regenerate a lost head including the brain within 10–14 days[Ref 8][Ref 9] The bacteriocytes of dividing worms are split during the fission process and the population of symbiotic bacteria is distributed to the two daughter individuals.

Riegeria, belong to the lineage of Alphaproteobacteria forming a monophyletic group within the order Rhodospirillales[Ref 3] and the family Rhodospirillaceae.