This species forms green sacs that soon tear open to form monostromatic blades 2-14 cm in dimensions (Image A). Rehydrated material reveals surface cells that are angular and irregular to cuboidal or rectangular in outline, 6-24 µm in dimensions, and closely compacted together (Image B). In this species there is a greater tendency to develop rectangular cells that run parallel to the thallus axis lower to mid thallus (Image C), but this can be subtle or absent toward the apices in some individuals (Image B). In section the blades are clearly monostromatic, 20-35 µm wide, with the cells roughly cuboidal to slightly rectangular in outline (Image D). Fresh material reveals 1(2) pyrenoids per cell typically with 2(3) in cells closer to the base.
Based on low number of collections (n = 9) this appears to be a rare genetic group in the NW Atlantic (n = 5). We have few verified records for BC (n = 3), all from Haida Gwaii, which likely reflects a sampling artifact (see Monostroma cf. fractum C.-C.Jao and Monostroma cf. grevillei (Thuret) Withering). This species extends from upper intertidal pools to the lowest intertidal (based on our few records), growing on rock, other hard substrata, as well as other algae. It can be difficult to distinguish from Monostroma cf. grevillei (Thuret) Withering with which it overlaps in range in the Arctic and NW Atlantic (BC?), but that species tends to have fewer rectangular cells and a persistent sac habit before or in lieu of transitioning to the blade morphology (i.e. even large individuals can retain the sac morphology in some habitats). In BC where Monostroma cf. arcticum overlaps with Monostroma cf. fractum C.-C.Jao more study is needed. Based on our single collection to date, that latter genetic group is characterized by more regularly rectangular cells. All species of Monostroma can be confused with Gayralia oxysperma (Kützing) K.L.Vinogradova ex Scagel & al., but in surface view that species has its cells in groups of 2-4 with relatively larger gaps between these groups (i.e. they are not compact in surface view as is the case for Monostroma spp.; Images B & C).
The “cf.” used for all Monostroma spp. here is an indicator that taxonomy for the genus needs considerable work. There are unequivocally three genetic groups in our flora to which I tentatively apply three names (based on sequence data in BOLD a fourth genetic group collected in WA is distant to the three referred to here, and may represent another species of this genus). Accepting these preliminary name assignments for now, the Atlantic and Arctic Monostroma cf. grevillei (Thuret) Withering (matching sequence data from Germany, Central Norway and the UK; all relatively near the type locality in France) is a close sibling species to the Pacific Monostroma cf. fractum C.-C.Jao (for which we have only 1(2 in the works?) collection from Haida Gwaii. Based on the few collections in hand (no current matches in GenBank), Monostroma cf. arcticum Wittrock, with a type locality in the Arctic waters of Norway, is distributed in northern New England and the Maritime Provinces, Haida Gwaii, and Kamchatka, possibly consistent with a circumpolar distribution. Obviously we need considerably more hits on all three genetic groups to sort out the taxonomy and full distribution of the respective species.
Image A. Pressed specimens from low intertidal on rock at our Lepreau exposed biodiversity station, NB (GWS005974).
Image B. Surface view of cells on the upper portion of a blade (low intertidal pool on rock, Letete, NB; GWS005934; rehydrated from press).
Image C. Surface view of cells on the lower mid portion of the blade (mid upper intertidal pool on rock, Starboard, ME; GWS003637; rehydrated from press).
Image D. Section of the blade (mid upper intertidal pool on rock, Cape Neddick, ME; GWS003538; rehydrated from press).