With an n = 1 (possibly 2, awaiting molecular data), this is a preliminary account. This species forms green sacs up to at least 10 cm tall (Image A) that eventually tear open to form monostromatic blades. Rehydrated material reveals surface cells that are angular and irregular to cuboidal or rectangular in outline, 6-25 µm in dimensions, and closely compacted together (Image B). Toward the base the rectangular cells running parallel to the plant axis become more prevalent and can reach >80 µm in length (Image C). In section the blades are clearly monostromatic, 18-20 µm wide, with the cells slightly rectangular in profile (Image D). Rehydrated material reveals 1(2) pyrenoids per cell (Image B) increasing to 2-3(4) in cells closer to the base (Image C).
Based on the low number of collections in hand (n = 1, possibly 2) from Haida Gwaii, it is difficult to state much about this genetic group (see Monostroma cf. arcticum Wittrock and Monostroma cf. grevillei (Thuret) Withering). It is certainly a closely related sibling species to Monostroma cf. grevillei (Thuret) Withering, which we have not yet encountered in BC. The two specimens that we do have for Monostroma fractum are both from 10 m subtidal, one each growing on rock and an invert. In BC where Monostroma cf. arcticum Wittrock overlaps with Monostroma fractum more study is needed. Based on the limited observations to date, Monostroma fractum is characterized by more regularly rectangular cells that are significantly enhanced toward the base of the plant (Image C). 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 the Canadian 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. Specimen from subtidal (10 m) on cobble at Tanu Island, Gwaii Haanas, BC (GWS031002).
Image B. Surface view near the top of a thallus (subtidal (10 m) on inverts, Chaatl Island, Haida Gwaii, BC (GWS012569; rehydrated from silica vial).
Image C. Surface view near the base of a thallus (GWS031002; rehydrated from silica vial).
Image D. Section from closer to the top of a thallus (GWS031002; this specimen is not distromatic, but failed to separate fully on rehydration from the silica vial).