Gametophytes for this species form small clumps of reddish-brown blades from relatively expansive crustose bases. Individual blades are 2-6 cm tall and 2-6 mm wide, with irregular branching in older parts but becoming more typically dichotomous toward the apices (Image A). In transverse section there are 4-7 large colourless medullary cells (largest ~225 µm) grading to the outer anticlinal cortex of 4-6 cell layers (Image B). Reproductive females display obvious surface swellings at the site of carposporophyte development (Image C). In section carposporophytes form in the medulla (Image D); the developing gonimolobes crammed in the spaces between the thick-walled medullary cells (Image E). The putative sporophyte (life history remains uncertain; see Maggs et al. 2013) is crustose (Image F).
We have genetically verified records for this species from throughout Canadian waters, although there is a genetic break between the Atlantic and Pacific populations (Maggs et al. 2013). Typically rare, found on rock or cobble from low intertidal (typically pools) to subtidal (17 m). This species is relatively distinct in the NW Atlantic, although hasty identification can result in confusion with the gametophytes of Mastocarpus stellatus (Stackhouse in Withering) Guiry, which are nonetheless distinct in usually producing surface papillae and in having a filamentous rather than pseudoparenchymatous (Image B) medulla in transverse section.
Image A. Subtidal (6 m) gametophytic specimen from Wallace Cove, NB (GWS039478).
Image B. Transverse section of a gametophyte mid thallus (subtidal (4 m) Rockland, ME; GWS024025).
Image C. Reproductive female with obvious surface swellings at the site of gonimoblast development (subtidal (6 m) on rock, Black Point (north), Saint John Harbour, NB; GWS048728).
Image D. Transverse section reveals carposporophyte development in the medulla (GWS048728).
Image E. Close up of a carposporophyte in transverse section displaying gonimoblasts developing among the thick-walled medullary cells (GWS048728).
Image F. Tetrasporophytic crust, subtidal (3 m) on rock (Covey Head, Mahone Bay, NS; GWS032011).