This species forms crusts (Image A) to globose cushions (Image B) along the margins of seagrass. Vegetative construction consists of a monostromatic basal layer each cell of which produces an erect filament (Image C) with cellular fusions connecting adjacent cells of both filament types (Image D). A sorus initiates centrally expanding toward the margins and contains both free curved sterile filaments (paraphyses; Images C & D) among which asexual cruciately divided tetrasporangia develop (Image E). Tetrasporangia consist of two cells, the subtending regenerative stalk cell and the terminal tetrasporocyte, these 16-26 µm wide by 24-41 µm long (Image F).
This species is relatively easy to identify as few red crusts grow along the margins of seagrass, which when combined with cellular fusions, curved sterile filaments and regenerative tetrasporangia render it unique. This was the very first species I worked on for my MSc and resulted in my first publication (Saunders & Bird 1989)… time flies. It is still one of my favourites and along with its allies has been critical to a number of studies in our lab over the past many decades.
Image A. Crustose thalli along the margin of seagrass from a mid intertidal pool on seagrass, Wallace Cove Lighthouse (sheltered side), NB (EGWS001122).
Image B. Globose thalli along the margin of seagrass from the subtidal (4 m) at Beach south of Kiju Point, Gwaii Haanas, BC (GWS030892).
Image C. Transverse section through a reproductive thallus showing general vegetative and reproductive features (EGWS001122).
Image D. Cellular fusion (arrow) between two erect filament cells (EGWS001122).
Image E. Cruciate tetrasporangium among curved sterile filaments (EGWS001122).
Image F. Regenerative stalk cells (arrows) subtending developing tetrasporocytes (EGWS001122).