Acoels are almost exclusively marine worms. Members of the Sagittiferidae and Convolutidae contain symbiotic unicellular algae and therefore prefer to remain in shallower waters exposed to sunlight, to optimise conditions for the photosynthesising endosymbionts.

Symsagittifera roscoffensis is entirely marine and is distributed around the Atlantic coast of Europe in shallow waters of sheltered sandy bays. The highest densities are found in southern England and northern France, north of Spain and Portugal.

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