Rhodomela sibirica A.Zinova & Vinogradova (Ar)

Individuals of this species range from 8-20 cm in height with terete axes, and branching only to 3-4 orders (Image A), although short spine-like outgrowths occur on some individuals (likely the remnants of seasonally shed branches; Image B). The tip has an obvious dome-shaped apical cell (Image C) and the polysiphonous construction is obscured in whole mount by surface cortication (Image D). In cross section the central rosette is clear, the central axial cell, 55-62 µm in diam., surrounded by six larger, 30-100 µm wide by 50-104 µm long (perpendicular to the main axis) periaxial cells (Image E). The periaxial cells are surrounded by 3-4 inner cortical layers of similar sized cells, which abruptly transitions to a small-celled, 12-24 µm in diam., 1-2 layered outer cortex (Image F). In long section a similar pattern is observed except that the cortical cells are not as tall (parallel to the axis) as their underlying periaxial cells and rapidly decrease in height (Image G) to the thin outer 1-2 layered cortex (Image H).

Genetically verified records are subtidal (2-20 m) on hard substrata or rarely other algae, with collections from Nome, AK, to Baffin Island (we did not encounter this species in our Labrador collections). This is distinct from all other species of Rhodomela in our flora as it only attains 3-4 levels of branching order. Unlike Rhodomela lycopodioides (Linnaeus) C.Agardh, the central rosette is obvious despite the periaxial and surrounding cortical cells being similar in size in cross section. The large-celled inner cortex combined with an abrupt transition to a thin, 1-2 layered, outer cortex is different from Rhodomela virgata Kjellman  and Rhodomela sp. 1virgata.

Image A. Typical specimen of this sparsely branched species from the subtidal (8 m) near Nome, AK (GWS042334).

Image B. Small spine-like projections along younger axis (rehydrated; subtidal (12 m) on rock, Churchill, MB; GWS005418).

Image C. A view of the dome-shaped apical cell on a young vegetative axis (rehydrated; subtidal (12 m) on rock, Churchill, MB; GWS005411).

Image D. Cortication obscures the polysiphonous construction of this species when viewed in whole mount (rehydrated; GWS005411).

Image E. Cross section revealing the central rosette consisting of a central axial cell (A), surrounding periaxial cells (P), and 3-4 layers of inner cortical cells (C) (rehydrated; drift, Churchill, MB; GWS005526).

Image F. Close up of cross section revealing the 1-2 layers of small outer cortical cells (rehydrated; GWS005526).

Image G. Long section revealing the central rosette consisting of a central axial cell (A), surrounding periaxial cells (P), and 3-4 layers of inner cortical cells (C) (rehydrated; GWS005526).

Image H. Close up of long section revealing the 1-2 layers of small outer cortical cells (A, P, C as per previous figures) (rehydrated; GWS005526).