Rubrointrusa membranacea (Magnus) S.L.Clayden & G.W.Saunders (BC/Ar/A)

This species imparts a reddish colour to the host invertebrates (Image A). Basal filaments endozoic, of cylindrical to barrel-shaped to irregular cells, forming freely-branched filaments, or partially aggregated to form more-or-less continuous expanses within host (Image B). Although it has been difficult to ascertain with certainty, there appear to be some cellular fusions as is characteristic of the Meiodiscaceae (Images C & D). In culture erect cells, 7-8.5 µm by 12-16 µm with multiple discoid to broadly ribbon-shaped plastids lacking pyrenoids, occur in short filaments (Image E), which can be terminated by tetrasporangia, 16-18 µm wide by 26-29 µm tall (Image F). In field specimens erect filaments are rarely 1-2 celled, but are more typically restricted to tetrasporangial initials composed of a stalk cell and tetrasporocyte initial (Image G), or the endozoic filament cell itself is the stalk cell of the tetrasporangial initial (Image H). These can be locally abundant (Image I). Tetrasporangia are cruciate, but are smaller than those observed in culture, 14-17 µm wide by 18-21 µm tall (Image J).

We have genetically verified records from Nome, AK and the Atlantic provinces of Canada, although this commonly overlooked species is likely widely distributed in the NW Atlantic and likely extends throughout the Arctic. This species lives in a variety of invertebrate hosts with collections from the mid intertidal (typically in pools) to subtidal (15 m). We have no genetically verified records from BC to date, although this species is reported for that flora (Gabrielson & Lindstrom 2018).

EGWS000711 copyImage A. Macro of this species imparting a reddish colour to its invertebrate host (low intertidal in invertebrate on Ascophyllum nodosum (Linnaeus) Le Jolis, Wallace Cove Lighthouse, NB; EGWS000711).

EGWS000711-0016Image B. Close up of the endozoic basal filaments with their irregular branching and cell sizes and shapes (EGWS000711).

Image C. Putative cellular fusion (asterisk) in an endozoic filament (subtidal (15 m) in invertebrate, Scatarie Bank, off Cape Breton, NS; GWS042833, rehydrated from press).

Image D. Putative cellular fusion (asterisk) in an endozoic filament (GWS042833, rehydrated from press).

GWSC030-0031Image E. Erect filament displaying the regular vegetative cells with multiple parietal discoid to ribbon-like plastids (image from culture; isolated from the low intertidal, Long Eddy Point, Grand Manan, NB; GWSC030).

GWSC030-0030Image F. Erect filament bearing a terminal decussate cruciate tetrasporangium (image from culture; GWSC030).

Image G. Tetrasporangial initial in surface view with the stalk cell (arrow) borne directly on an endozoic filament cell (GWS042833, rehydrated from press).

Image H. Tetrasporangial initial in section with the tetrasporocyte initial (arrow) borne on a stalk cell within an endozoic filament (GWS042833, rehydrated from press, aniline stained).

Image I. Locally abundant tetrasporangial initials viewed in section (GWS042833, rehydrated from press, aniline stained).

Image J. Mature tetrasporangium with the cruciate tetrasporocyte subtended by a stalk cell (GWS042833, rehydrated from press).