Plants form bushy tufts 8-16 cm tall with the regularly dichotomous branching fronds arising from a stoloniferous holdfast (Image A). Axes are terete, except at the points of branching where they become somewhat compressed, and are thicker in apical regions when reproductive (Image A). Axes are dark reddish brown to black, although erect tips can become greenish to yellowish red in colour while stolons are light and can become pink. Branch angles at the dichotomies are typically narrow (<45 degrees). In longitudinal section the medulla is composed of axially elongate, as well as transverse, filaments, which support a thick cortex of ovoid cells that get smaller toward the surface (Image B). Occasionally, the connection between the cells of the axially elongate medullary filament cells have a ball and socket appearance (Image C; see Bárbara et al. 2013).
This species can be confused with Polyides rotunda (Hudson) Greville, but that species lacks a stoloniferous base, has few transverse medullary filaments (most being axially elongate) and typically has wider branch angles (45-90 degrees). It also has a broader range in the NW Atlantic ranging from CN to the Island of NL.
We have records from around PE, Northumberland Shore, NB, Cape Breton, NS, and Birchy Hd. in St. Margarets Bay of this introduced species. Typically collected from rock in the subtidal (3-10 m) and a common drift element in its range.
Image A. Dichotomously branched fronds arising from the stoloniferous holdfasts as is characteristic of this species (subtidal (10 m) on rock, White Pt., Cape Breton, NS; GWS007817).
Image B. Longitudinal section displaying the central filamentous medulla of axially elongate as well as transverse filaments, which bears a thick cortex of ovoid cells that get smaller toward the surface (GWS007817).
Image C. Some of the axially elongate medullary filament cells join with a ball and socket appearance (arrow; GWS007817).