Symsagittifera roscoffensis (flatworm)

Described for the first time at the Roscoff Biological Station in 1891, this small flatworm (2 to 5 mm max.) is a platyhelminth from the class Turbellaria, belonging to the Acoelomorpha group, represented exclusively by marine worms.  It is abundant along the European Atlantic coast. It has a gregarious life style and millions of individuals live in relatively dark green mats in shallow waters. Symsagittifera roscoffensis is a photosymbiotic organism, living in association with a unicellular alga, Tetraselmis convolutae, a prasinophyte, and each worm harbours up to 25,000 algal cells.

EMBRC expertise


  • Held in aquariums;
  • Can be raised in the laboratory, which makes it possible to follow all the developmental stages — from the first cleavage to the non-symbiotic juvenile stage — and artificially induce symbiosis after contact with algal strains, which can also be cultivated in the lab.

Available tools

  • The reproductive season stretches from September to June, and the worms are particularly abundant during the summer.
  • It can be easily raised in the laboratory and development is rapid, 4 to 5 days. 
  • The embryos are transparent and are thus suitable for cell imaging techniques, in situ in toto hybridisation, and immunohistochemistry.

Scientific advantages of this model organism

  • Symsagittifera roscoffensis is hermaphroditic and an obligate outcrosser, with rapid development in 3 to 5 days. Juveniles ingest live algae shortly after hatching, leading to symbiosis, which becomes obligate and giving the flatworm its green coloration in 10 to 15 days. This small worm has been a model organism for studying photosymbiosis in metazoans since the 19th century.
  • After a decline over several decades, the interest in this organism has increased since it was demonstrated that acoelomorphs occupy a special phylogenetic position at the base of the bilaterian branch. They thus represent the oldest lineage of metazoans with bilateral symmetry, preceding the emergence of protostomes and deuterostomes. Symsagittifera roscoffensis is thus a model of choice for evo-devo studies, in particular for the study of the transition from radial to bilateral symmetry.
  • Given the high regeneration capacity of its tissues due to the presence of totipotent stem cells, it is a model of choice for the study of regeneration and stem cell biology. In particular, it is useful for the analysis of regeneration mechanisms that allow muscle and brain to reconstitute themselves.


  • EST libraries
  • Complete mitochondrial genome