This species has pinnate opposite leading and lagging branches that alternate along the axes to give distinctive thalli that typically range from 5-20 cm in height (Image A). Thalli are fairly rigid and generally maintain their shape when removed from water. Cortical cells cover the entire thallus, except for the apical 1-2 cells (Image B). The lagging branch in each pair of branches typically acquires a claw-like appearance (Image C).
To the inexperienced collector confusion with Plumaria plumosa (Hudson) Kuntze is a possibility, but that species has delicate and flaccid thalli that collapse when removed from water, as well as sparse cortication in ultimate branches and extended monosiphonous tips. A critical distinction is the regular alternation of leading and lagging branches in Ptilota spp. versus the alternating series of 2-3 in Plumaria plumosa (Hudson) Kuntze. More problematic is the European Ptilota gunneri P.C.Silva, Maggs & L.M.Irvine from the Canadian Arctic and more northerly waters of the NW Atlantic. In the last mentioned species the lagging branch in each pair typically acquires a spine-like rather than claw-like appearance, and cortication is delayed in some ultimate branches resulting in monosiphonous regions (this was not observed in our European specimens of this species). DNA data are necessary where an absolute identification is required.
We have genetically verified records from southern NB to northern NU and from the low intertidal to depths of 20m, although the biogeographical and ecological range is likely much greater for this species (Mathieson & Dawes 2017). Despite extensive sampling we have been unable to confirm that this genetic species extends into the North Pacific, Bearing Straight, or Beaufort Sea, where it is reported (see Bruce & Saunders 2016), although reports are found in Gabrielson & Lindstrom (2018, p. 118) for northern BC and southern AK. For further notes on the two species in our flora see Bruce & Saunders (2016).
Image A. Subtidal (6 m) specimen from Labrador, NL (GWS039430).
Image B. Apex of a leading axes displaying cortication developing below the terminal few cells (New River Beach, NB; GWS039126).
Image C. Claw-like shape of the lagging branch in a pair of opposite branches (GWS039126).