Blades light, more typically dark, green; at times blackish green near the margins (Image A). Blades highly irregular in outline and size some approaching 30 cm, perforations may be present (Image A). Typically turning black at the margins and turning paper blackish on pressing (Image B). Attached by a small holdfast with a distinct stipe (Image C). In surface view cells lower on the thallus are irregular in outline, rounded to angular, and compact, 12-33 µm diam., and contain 2-5+ pyrenoids (Image D). This species is monostromatic in section, 35-50 µm wide, with the chloroplast parietal (Image E). Vegetative cells transform into reproductive structures (gametangia or sporangia) and release flagellate zoids (Image F).
This species can be difficult to distinguish from species of Ulva in the field, however, the latter are distromatic rather than monostromatic in section, which can be verified in the lab. In the field, individuals with the dark blackish green hue at the margins are best assigned to Ulvaria obscura. There is a second genetic species that we have collected in BC, which needs taxonomic study and could result in confusion in the NE Pacific.
We have genetically verified records from throughout BC, the Canadian Arctic, and Atlantic Canada extending to just north of Cape Cod, MA. Individuals range from the mid intertidal pools to subtidal (11 m) growing on rock, as well as a wide variety of invertebrates or other algae.
Image A. Fresh specimen from a low intertidal pool on rock displaying the irregular blade with occasional perforations (Lepreau exposed biodiversity site, NB: GWS036965)
Image B. Pressed specimen from a low intertidal pool on rock, which turned the herbarium paper black after pressing (Maces Bay, NB; ULV20).
Image C. View of the basal holdfast and short stipe (low intertidal pool on Corallina officinalis Linnaeus, Musquash Head Lighthouse, Saint John Harbour monitoring site, NB; GWS044468).
Image D. Surface view of cells lower on the blade displaying their shape and 2-5+ pyrenoids per cell (GWS044468).
Image E. Section of blade displaying the monostromatic habit of the blade and parietal chloroplasts (low intertidal on rock, Lepreau exposed biodiversity site, NB: EGWS000702).
Image F. Cells that have noticeably transformed to reproduction; many empty, a few full, and one in which a few zoids were still trying to escape (GWS044468).