Focus on a dense Symsagittifera roscoffensis (Graff, 1891) population around the Station Biologique de Roscoff (with gravid individu-

als exhibiting a double string of white ovocytes appearing along the body). On the upper left part, a (beautiful) plate (Delage, 1886) ded-

icated to the histological description of the main structures and tissues including the localization of the “enigmatic green bodies” (the

photosymbiotic algae, Tetraselmis convolutae).

Photograph by Wilfried Thomas & Xavier Bailly

Zoologists early questioned about the identity of a colouring matter present in several animal with the chlorophyll of plants

(Schultze, 1852). Indeed, the occurrence of photosynthetic organism inside metazoan tissues was rather astonishing given

the classical classification of Plant versus Animal kingdoms (today obsolete). However, the algal nature and origin of the

green bodies found in some marine and freshwater animals was deciphered from the studies dealing on the marine acoel

Symsagittifera roscoffensis (Graff, 1891) by Keeble & Gamble (1907). This flatworm lives in large populations, abundant

around the Station Biologique de Roscoff and is endemic from the Atlantic coasts. S. roscoffensis (formerly Convoluta

roscoffensis) is a gregarious intertidal photosymbiotic acoel around 4 mm in length that is found during daylight on the

upper part of beaches in calm residual flows on ebb tides. Like most acoels (a = non, coela = cavity), the main features of

S. roscoffensis are that it is hermaphroditic, soft-bodied (no rigid structures), has a sack-like gut with a solitary ventral

opening commonly called the mouth, lacks a mesoderm-lined body cavity (coelom), has multi-ciliated epidermal cells for

locomotion. The apparent simplicity of anatomo-morphological organisation of S. roscoffensis (Delage, 1886) early point-

ed up an enigmatic phylogenetic position. After intense controversial debates and huge amount of phylogenomics surveys,

acoels could be the earliest extant bilaterians, preceding the Protostomes/Deuterostomes split. Up to millions of individ-

uals congregate and form mats in more or less discrete patches or large populations on sand, notably where exposed to

sunlight that favours photosynthetic activity of the endosymbiotic algae, the prasinophyte Tetraselmis convolutae. Tide

return is the signal for retreat into the sediment to escape dispersal by waves. During the breeding period from September

to June S. roscoffensis may lay up to 20-30 eggs per sexually mature individual, all confined in a translucent mucilaginous

egg capsule. The colourless juveniles that develop directly without intermediate larval stages must individually obtain the

appropriate free-living green unicellular algae (horizontal transmission) in order to reach the adult stage. Captive algal

cells are thought to divide and their photosynthetic activity contributes to host growth by provision of photosynthates.

Both cultures of the acoel and the free living algae are currently controlled in the Station Biologique de Roscoff for pho-

tosymbiosis exploration and developmental studies.

Delage Y. 1886. Etudes histologiques sur les planaires rhabdocoeles acoeles. Archives de Zoologie Expérimentale et Générale, Serie II,

4:109-160.

Graff L. 1891. Die Organisation der Turbellaria acoela; mit einem Anhange über den Bau und die Bedeutung der Chlorophyllzellen

von Convoluta Roscoffensis von Gottlieb Haberlandt. Leipzig: Engelmann.

Keebles F. & Gamble F.W. 1907. The origin and nature of the green cells of Convoluta roscoffensis. Quaterly Journal of Microscopical

Science, 51: 167-217.

Schultze M. 1852. Note sur l’identité d’une matière colorante existant chez plusieurs animaux et identiques à la chlorophylle des végé-

taux. Comptes-rendus des Séances de l’Académie des Sciences, Tome XXXIV: 683-685.

Xavier Bailly, PhD

Station Biologique de Roscoff

bailly@sb-roscoff.fr