Roscoff’s Flatworm, one of the first widely studied photosynthetic relationships

Symbiotic relationships necessary for the survival of certain species

The symbiosis between Roscoff’s Flatworm and a unicellular green algae

Corals are far from being the only species to benefit from a symbiotic relationship to ensure their survival. Different types of symbiosis exist, (between plants and fungi, between plants and animals, between humans and bacteria), as well as different modes of symbiosis (mutualism, parasitism, commensalism and phoresis). Today we are talking about one of the first widely studied photosynthetic animal systems, that of the Roscoff’s Flatworm or Symsagittifera roscoffensis with a unicellular green alga, Tetraselmis convolutae.

Present on a majority of the French coastline, particularly on the Brittany coast, the Roscoff flatworm (4-5 mm adult) has become a study model used in research. Several questions remain to this day, such as the precise functioning of the symbiotic relationship between the worm and the alga as well as the development of photosymbiosis.

The study of its symbiosis allows the understanding of different phenomena, such as climate change for example. Researcher Xavier Bailly, co-responsible for the marine biological resource centre at the Roscoff biological station (origin of the worm’s name), cultivates these worms in a controlled and mastered manner with the aim of artificially inducing symbiosis from cultivated algae strains. He has developed a kit to understand, among other things, the animal-algae symbiosis as well as ocean acidification and its consequences.

Because of this algal symbiosis, Symsagittifera roscoffensis differs from other worms with a quite singular attitude during tides. Indeed, the normal behaviour of a worm is to take refuge in the sand during low tides so as not to be visible to any predators present. The Roscoff’s worm takes advantage of the descending sea level to come to the surface, and thus optimize the photosynthetic work of its alga Tetraselmis convolutae that it hosts. In addition, the worm will anticipate the high tide by taking refuge in the sand so as not to be carried away by the current.

Roscoff flatworms need this symbiotic relationship to live. Once born, the worms have only a few days to associate with the algae and take advantage of the nutrients they provide. This association takes place horizontally, meaning that the juveniles will ingest the algae, which will then be phagocytised by the worm cells. As they grow, the worm’s digestive tract will regress before becoming non-functional, which is why symbiosis is obligatory once adult since it becomes entirely dependent on the photosynthetic work of the algae. This relationship, when a unicellular organism lives inside the cells of its host, is called endosymbiosis, just like the zooxanthellae within corals.

Once phagocytised, the algae will be able to contribute to the nutrition of the worm. Indeed, their photosynthetic activity will provide fatty acids and amino acids essential for the development of the worm, as well as glucose and fructose necessary for its survival.

Like any association, the benefits are reciprocal between the algae and the worm. In exchange for an important contribution in nutrients, the worm serves as a host to the algae, which is then protected from predators.

 

Symbiosis, one of the factors explaining the evolution of species

Symbioses are found in the majority of living things, but their importance varies from species to species. This system of mutual dependence between living organisms is even a key factor in the evolution of species.

This is the work of the famous American biologist Lynn Margulis (1938-2011), known for presenting her theory on endosymbiosis in 1966, according to which life is not only the product of competition between species, but has developed thanks to such close associations between certain species that it has led to their fusion into a single complex organism (1970, The Origin of Eukaryotic Cells).

Its theory was long ignored by the scientific community before it became the new explanatory scheme for evolution.