This species forms dull green turfs or limited carpets in the uppermost intertidal and is easily dismissed (Image A). Our single collection consists of tangled filaments, which when spread in our collecting tray suggested perhaps a species of Urospora (e.g. Urospora neglecta (Kornmann) Lokhorst & Trask), but a look under the microscope dispelled that notion. In whole mount most of the thallus consists of narrow uniseriate filaments, 12-16 µm wide, the cells of which are disc-shaped to square, 11-15 µm wide by 4-12 µm tall, and at times rectangular where single descending rhizoids are produced (Image B). In addition to rhizoids being produced singly, they were commonly formed in pairs (Image C) and series (Image D). In localized regions filaments became biseriate, 20-23 µm wide, with the cells disc-shaped to squat in outline (Image E). More rarely filaments became pluriseriate, presumably as a result of gametangial production although details were difficult to discern (Image F). More commonly, filaments locally produced a false biseriate type of development and relatively large knots (Image G). In fresh material each cell has a lobed to stellate chloroplast with a single pyrenoid (Image H).
Molecular data confirm that this is a new record for our flora. This species is actually more common in terrestrial environments (Rindi et al. 2004), and in hindsight this collection may have been just above the high water mark. Another species, Prasiola crispa (Lightfoot) Kützing, is recorded for our flora (Mathieson & Dawes 2017), and is reportedly easily confused with Rosenvingiella radicans (Brodie et al. 2007). We have not yet encountered Prasiola crispa (Lightfoot) Kützing in our surveys to date and vouchers on which published records are based should be carefully assessed as some may be assignable to Rosenvingiella radicans. This species is easily distinguished from its congener Rosenvingiella polyrhiza (Rosenvinge) P.C.Silva.
Image A. Our only collection from the uppermost intertidal on rock at Musquash Head Lighthouse, Saint John Harbour monitoring site (GWS044470).
Image B. Uniseriate development, as is typical of this species, the filaments thin with disc shaped to square cells, at times rectangular where unicellular descending rhizoids were formed (GWS044470).
Image C. Rhizoids were commonly produced in pairs (GWS044470).
Image D. Rhizoids were occasionally produced in series (GWS044470).
Image E. Localized regions of filaments can become biseriate (GWS044470).
Image F. Localized regions of filaments can become pluriseriate, presumably owing to gametangial production, but details were difficult to discern (GWS044470; covid has me working at home with rather limited microscopy).
Image G. Paired filaments and knots were relatively common (GWS044470).
Image H. Each cell contains a single lobed to stellate chloroplast with a single pyrenoid (best seen adjacent and below the arrow; GWS044470).