Paracatenula

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. Five species of Paracatenula have been described - P. erato, P. kalliope, P. polyhymnia, P. urania and P. galateia, named after muses and nymphs of the Greek mythology. Several more species have been morphologically and molecularly identified, but are not formally described. Paracatenula are globally distributed in warm temperate to tropical regions and have been collected from Belize (Caribbean Sea), Egypt (Red Sea), Australia (Pacific Ocean) and Italy (Mediterranean Sea). They occur in the oxic-anoxic interface of subtidal sands and have been found in water depths up to 40 m. Paracatenula can reach a length of up to 15 mm and a width of 0.4 mm. Several larger species of Paracatenula, such as P. galateia are flattened like a leaf, while all smaller species are round. All Paracatenula species examined so far were found to harbor bacterial symbionts in their trophosome. The frontal part of the worms - the rostrum - is bacteria-free, houses the brain, and is transparent, while the trophosome region appears white due to light refracting inclusions in the bacterial symbionts. Some species of Paracatenula such as P. galateia possess a statocyst with a single statolith. Sexual reproduction has never been observed in Paracatenula. Instead, the worms reproduce by asexual fission or fragmentation, a process called paratomy, where a fragment can regrow a head in few days. The bacteriocytes of dividing worms are split during the fission process and the population of symbiotic bacteria is distributed to the two daughter individuals. Paracatenula host their symbionts within bacteriocytes in the trophosome. These bacteria, named Cand. Riegeria, belong to the lineage alphaproteobacteria forming a monophyletic group within the order Rhodospirillales. The co-speciation between host and bacteria suggests a strict vertical transmission of the bacteria in which the endosymbionts are directly transferred from parents to their offspring. The symbiosis is assumed to be beneficial for both partners. The lack of both a gut lumen and a mouth indicate that the host derives most of its nutrition from its symbionts, which have the potential for carbon dioxide fixation and sulfur oxidation. In return, the host provides its symbionts with a stable supply of electron donors such as sulfide and oxygen in a dynamic and heterogeneous environment. Furthermore, symbionts living intracellulary in the worms are protected from predation as well as competition for nutrients by other bacteria.