The Symbiont Alga of the FlatwormConvoluta PsammophilaBekl. Observed at the Electron Microscope

Abstract

Convoluta psammophila is an acoel turbellarian found for the first time on the Black Sea coasts, and only recently at Marina di Pisa. It forms a symbiosis with an unicellular green alga. The symbiont alga has a thin plasmatic membrane and a chlamydomonad-type cup-shaped chloroplast. The chloroplast has a number of perforations with cytoplasmic strands going through them. The round shaped pyrenoid lies inside the chloroplast in the lower half of the « cup »; inside it there are fingers of cytoplasm. The algal cell contains various cytoplasmic inclusions such as mitochondria, dictyosomes, bodies of a probable lipidie nature, etc. Numerous vacuoles can be found containing electron-dense granules; they seem to correspond to the polyphosphate containing vacuoles described in other algae. Four basal bodies, arranged in two pairs, lie very close to the nucleus. On the whole the alga appears very similar to the symbiont of C. roscoffensis, generally considered as a species of the genus Carteria. Both algae however can be assigned to this genus only temporarily. As far as the symbiont of C. psammophila is concerned a more precise classification will be possible only after its free-living form has been isolated.