This species has a highly distinctive morphology across its range. Straw coloured (typically near apices) to dark blackish red (typically near the base), plants range from 5-35 cm in height and are composed of regularly alternating branches, which show central thickening (midribs) in the lower regions of the thalli (Image A). If you have any doubt, another useful feature to look for are the distinct apical cells at the apices of the marginal branchlets (easiest to observe in branch axils; Image B).
Our collections range from Nome, AK, throughout the Arctic extending down to the Peggys Cove area of NS. Always subtidal, we have collections at depths from 5-30 m with the more southerly populations always deep (e.g. our collections around NS and PE are typically deeper than 15 m). Although there are other species of this genus in Canada (BC), this is the only species in our Arctic and Atlantic waters. There is little chance of confusing it with other taxa.
Image A. Typical specimen displaying the regularly alternating branches and thickening in the the centre of the blades (arrows) toward the plant base (subtidal (8 m) on rock, Sledge I., Nome, AK; GWS042379).
Image B. Distinctive apical cell terminating marginal branchlet in this uniaxial species (subtidal (15 m) on rock, off Lingan Power Plant, Cape Breton, NS; GWS002333).