Thalli form pinkish red delicate tufts 1-5 cm tall (Image A). The tips are compact and densely branched (Image B), which is evident as apical red ‘dots’ when a specimen is viewed in a tray wet or as a press (Image A). Mid thallus each axial cell bears two opposite determinate branches, with these alternating at 90o to each adjacent cell to give an overall decussate (two planes) pattern (Image C). The basal cell of the branches is smaller and morphologically distinct from subsequent cells in each branch (Image D). Gland cells can be rare to common, they are formed adaxially on short 2-3 celled branchlets (Images E & F). Occasionally short depending rhizoid-like filaments are produced from the basal cells of the branches (Image G). Tetrasporangia are decussate in division pattern, typically oval, 34-42 µm wide by 78-88 µm long, and are produced on distinct one-celled stalks replacing branchlets of the determinate branches (Image H). Sporangia can appear as polysporangia, but this is likely an infection (fungal?) of the developing sporangium (Image I).
We have only three genetically verified records, two from south of Cape Cod and one from the Northumberland Strait, NB. Two are subtidal (3-5 m) on various hard substrata and one was a drift epiphyte on other algae. There are differences in the rbcL sequence of NW Atlantic specimens compared to data in GenBank for a specimen from closer to the type locality and our species may represent a cryptic sibling. Taxonomic study is needed. Careful observation will prevent confusion with species such as Antithamnion hubbsii E.Y.Dawson, Antithamnion cf. sparsum Tokida, Antithamnionella floccosa (O.F.Müller) Whittick, Pterothamnion plumula (J.Ellis) Nägeli, or Scagelia pylaisaei (Montagne) M.J.Wynne.
Image A. Fresh specimen with the compact tips apparent as red spots (drift, Pointe du Chêne Rd., Northumberland Strait, NB; GWS045977).
Image B. Close up of a compact tip (low intertidal on rock, Cap des Caissie, North of Shediac, NB; GWS007967; rehydrated from press; aniline blue stained).
Image C. Indeterminate axis with opposite pairs of determinate axes alternating in and opposing to the image plane in a decussate pattern (GWS045977).
Image D. Cells at the base of branches (arrows) are smaller than the subsequent vegetative cells (subtidal (6 m) on algae, Birchy Head, St. Margarets Bay, NS; GWS030111; rehydrated from press).
Image E. Lens shaped gland cell (G) adaxial on a short three-celled branchlet (drift on red algae, Kouchibouguac lagoon seagrass beds, NB; GWS006241; rehydrated from press).
Image F. Gland cells (arrows) in fresh material (GWS045977).
Image G. Descending rhizoid-like filament extending from the basal cells of a branch lower on the thallus (GWS045977).
Image H. Decussate tetrasporangium on a small single-cell stalk (arrow) (GWS030111; rehydrated from press).
Image I. Presumptive fungal infection of a developing sporangium (GWS045977).
Seaweed of Canada 