Thalli forming dark brownish to pinkish red bushy turfs, 0.5-3 cm tall (Images A & B). This species is heterotrichous with all filaments ecorticate and uniseriate. Prostrate filaments, cells 42-56 µm wide by 102-196 µm long with thick and laminated walls, have unilateral or opposite (pinnate) branching and adhere to the host by unicellular rhizoids (Image C) formed from the distal and the proximal end of the bearing cell, respectively. The rhizoids are characterized by distinct attachment pads (Image D). Erect filaments, main axial cells weakly clavate 45-70 µm wide by 160-320 µm long also with thick and laminated walls, arise from the prostrate system at ~90 degree angles from the distal end of the bearing cell (Image E) and also bear unilateral (Image F) to regularly opposite (pinnate) branches (Image G). Tetrasporangia, 54-62 µm wide by 65-82 µm long, have a tetrahedral division pattern and are sessile or on a stalk cell (Image H), which commonly form in small clusters (Image I).
Tufts commonly as epiphytes on robust subtidal algae, although this species can detach and contribute to dense floating mats along the beaches of southern MA and RI. We have twelve genetically verified records, all from southern MA and RI except for two from coastal NS. There are two genetic groups in our region with the recently uncovered species in the Maritime provinces matching well Spermothamnion cf. roseolum (C.Agardh) Pringsheim. Further taxonomic study is needed. There is a slight chance of confusion with the sporophyte stage of Bonnemaisonia hamifera Hariot, but that species has narrower erect axes, with irregular branching, with branches typically at 90 degrees, and an abundance of glands cells.
Image A. Specimen collected from the subtidal (6 m) on algae at Birchy Head, St. Margarets Bay, NS (GWS032079).
Image B. Pressed specimen from the subtidal (3 m) on rock at Covey Head, Mahone Bay, NS (GWS032009).
Image C. Prostrate system with unicellular attachment rhizoids (arrows) (drift as an epiphyte on a coarse red alga, Plantation Pt. Plymouth, MA; GWS000373; rehydrated from press).
Image D. Close up of a unicellular rhizoid with attachment pad (GWS000373; rehydrated from press).
Image E. Basal cell (arrow) of an erect axis arising from the prostrate system (GWS000373; rehydrated from press).
Image F. Unilateral branching in the erect filament system (GWS000373; rehydrated from press).
Image G. Typically regular opposite branching in the main axis of an erect filament (GWS000373; rehydrated from press).
Image H. Tetrahedral tetrasporangium subtended by a single cell stalk (GWS032079; rehydrated from press).
Image I. Tetrasporangia in a cluster (GWS032079; rehydrated from press).