Plants form bushy tufts 6-15 cm tall with the regularly dichotomous branched fronds arising from a discoid holdfast (Image A). Axes are terete, dark reddish brown to black, although erect tips can become greenish to yellowish in colour. Branch angles at the dichotomies are typically wide (45-90 degrees). In longitudinal section the medulla is composed predominantly of axially elongate filaments, which support a thick cortex of ovoid cells that get smaller toward the surface (Image B). Postfertilization development is evident by the production of relatively large surface nemathecia (Image C) in which multiple carposporophytes are produced (Image D).
This species can be confused with Furcellaria lumbricalis (Hudson) Lamouroux, but that species has a stoloniferous base, has many transverse medullary filaments among the axially elongate filaments and typically has narrower branch angles (<45 degrees). It also has a narrower range in the NW Atlantic ranging from PE, Northumberland Shore, NB, and Cape Breton, NS.
We have genetically verified records from CN to the Island of NL, typically on rock from low intertidal to shallow subtidal, but with some collections from depth (30 m).
Image A. Typical habit (dichotomously branched with terete axes) for a low intertidal specimen from Cape St. Marys, NS (GWS003517).
Image B. Longitudinal section (unstained) revealing a central medulla composed predominantly of axially elongate filaments supporting a thick cortex of ovoid cells that reduce in size toward the surface (subtidal (5 m), Cape Neddick, ME; GWS002676).
Image C. Wart-like nemathecia (arrows) containing the carposporophytes (subtidal (2 m) Split Rock, NB; GWS045086).
Image D. Section of female nemathecium revealing developing carposporophytes (GWS045086).