Plants are unbranched, attached (have a rhizoidal holdfast), dark green filaments up to 60+ cm high and 0.6-1.2 mm wide (Image A). Remarkably, this species is uniseriate, i.e., consists of a single file of cells, which are discernible without magnification (Featured Image above). The dark green colour, cells visible to the naked eye, and unbranched filaments ensure no confusion with identification within its range in our waters. However, preliminary molecular data indicate that unattached morphologies of this species are assigned another name. Taxonomic work is needed.
We have collections from just north of Cape Cod, MA, extending through northern New England and Atlantic Canada, to Churchill, MB. This species is common in mid to low intertidal pools and extends into the subtidal (8 m). It typically grows on rock, although we have a few collections that were growing on species of red algae.
Image A. Specimen from a mid intertidal pool displaying the unbranched filaments composed of large cells that are visible without magnification (Lepreau, NB; GWS039515).