Percursaria percursa (C.Agardh) Rosenvinge (BC/Ar/A)

This species is most commonly encountered in estuarine and brackish water habitats where it forms unattached green mats entangled with other green algae, which are themselves tangled among stalks of the various marsh vegetation (Image A). The unbranched filaments start out uniseriate, 17-20 µm wide, with disc-shaped to square cells that are 14-16 µm wide and only 7-14 µm tall, each containing a parietal ring to cup shaped chloroplast containing multiple pyrenoids (Image B). Filaments eventually become biseriate and up to 30 µm wide (Image C).

We currently lack genetic data for this species and the account here is based on a single collection. More data are needed on the morphological variation of this species, as well as its true biogeographical and and ecological range as accounts in the literature provide a mix of inconsistent information. This is not surprising for a species that is so easily overlooked, at least in my experience. Furthermore, this species might be confused for Ulothrix spp. to the budding seaweed enthusiast when the filaments are uniseriate (Image B). Nonetheless, when the filaments transition to biseriate development (Image C) (up to four rows by some accounts) this species is easily identified from the tangled mix of green species with which it grows (Image A).

Image A. Mat dominated by Rhizoclonium riparium (Roth) Harvey among which rare filaments of Percursaria percusa were found (uppermost intertidal tangled with salt marsh vegetation, St. Andrews (Blockhouse), NB; GWS044465A).

Image B. Filaments are uniseriate over part of their length with the disc-shaped to square cells containing a parietal chloroplast with multiple pyrenoids (GWS044465A).

Image C. Filaments become biseriate revealing the habit typically associated with this species (GWS044465A).