This bright red filamentous species forms elongate tufts or clumps to 12 cm in height (Image A). It has delicate and feathery fronds with opposite and pinnate branching (Image B). The branch tips are pointed, gland cells are absent, and the basal cells of the determinate branches are considerably smaller than the adjacent cells (Image C). Carposporophytes are produced near the plant tips and consist of 3-5 gonimolobes in varying stages of development (Image D).
We have genetically verified records from the low intertidal into the subtidal (10 m) growing on rocks and a variety of invertebrates and other algae. Collections in the NW Atlantic range from northern MA through to the southern Maritime Provinces. In the Pacific we have genetically verified records from southern Vancouver Island, BC, as well as the nearby San Juan Islands, WA, and from northern and southern Haida Gwaii, BC. In the Pacific comparison to Antithamnionella pacifica (Harvey) E.M.Wollaston and Antithamnionella spirographidis (Schiffner) E.M.Wollaston must be considered (Bruce & Saunders in prep.). Careful observation will prevent confusion with species such as Antithamnion cruciatum (C.Agardh) Nägeli, Antithamnion hubbsii E.Y.Dawson, Pterothamnion plumula (J.Ellis) Nägeli, or Scagelia pylaisaei (Montagne) M.J.Wynne.
Image A. Bright red tufts of a specimen collected subtidal (7 m) on algae at Paddys Head, NS (GWS011855).
Image B. A frond showing the opposite pinnate branching habit (Lepreau, Bay of Fundy, NB; EGWS000386).
Image C. Close-up of a frond emphasizing the tapered apical cell tips, distinctly smaller basal cells of the determinate branches, and the absence of gland cells (EGWS000386).
Image D. Developing carposporophyte with gonimolobes in different stages of development (GWS009767).