Thalli are microscopic, typically 0.1-1 mm tall, with sparse branching to one or, less frequently, two orders (Image A). Branches start with a slightly oblique division of an intercalary cell (Image B), which then continues to divide to form a lateral filament (Image C). Filaments are gelatinous in appearance, 14-26 µm wide, the cells are squat to elongate, 8-11 µm wide by 7-16 µm long, with rounded to slightly angular corners (Image D). Chloroplast are stellate with a central pyrenoid (Images D & E). The basal attachment cell is little modified from other vegetative cells (Image E).
Stylonema alsidii is reportedly widespread in our flora growing on various hard substrata or as an epiphyte on diverse algae from the low intertidal to subtidal (15 m). In my experience it is an epiphyte, but this is definitely a species that is easily overlooked. Despite the previous it can be relatively abundant in some areas. Typically a summer to early autumn annual. There is a slight chance of confusion with Erythrotrichia carnea (Dillwyn) J.Agardh, but that species is more robust, has larger vegetative cells, is typically unbranched, and has a modified basal cell.
Image A. Habit of an individual growing on drift Ceramium sp. 1G0SL, Beach to east of Parlee Park Beach, NB (EGWS001132).
Image B. Initiation of a branch by an oblique division of an intercalary cell (drift on Leptosiphonia sp., Beach to east of Parlee Park Beach, NB; EGWS001129).
Image C. More developed lateral branch (EGWS001132).
Image D. Close up of squat to slightly elongate vegetative cells each with a stellate chloroplast and central pyrenoid (EGWS001129).
Image E. Close up of an unmodified basal attachment cell (EGWS001129).