Rhodomela virgata Kjellman (Ar/A)

This species is highly variable in morphology ranging from densely branched plants with thin axes bringing to mind species like Polysiphonia sensu lato (Image A) to plants more reminiscent of the classic view of Rhodomela lycopodioides (Linnaeus) C.Agardh in the Canadian Arctic (Image B). Thalli are uniaxial with an obvious, rounded apical cell, and although construction is polysiphonous, this is obscured by a cortical covering in whole mount. Cross sections in mature thallus regions reveal a distinct central rosette pattern of this species, which consists of a central axial filament, 45-65 µm in diam., surrounded by 6(-8) slightly larger, 30-50 µm wide and 54-72 µm long (perpendicular to the axis), pericentral cells (Image C). The central rosette is surrounded by cortical cells similar in dimensions to the periaxial cells, 25-54 µm wide and 30-64 µm long. However, the central rosette stands out from the surrounding cortical tissue because the latter cells are characterized by dense cytoplasmic content relative to the central rosette cells (Image D). Long section reveals a similar pattern, but the periaxial cells are much taller (parallel to the axis) then the cortical cells, which get progressively shorter toward the outer cortex (Image E).

Genetically verified records extend from Nome, AK through the Arctic to the northern Maritime provinces (deep water, north coast PE). I am not yet convinced that this is the correct name for this species. Collected from mid intertidal pools to subtidal (22 m) on various hard substrata and coarse algae. This species overlaps in range with Rhodomela lycopodioides (Linnaeus) C.AgardhRhodomela sibirica A.Zinova & & Vinogradova and Rhodomela sp._1virgata. It differs from the last mentioned in that the axial cell is much smaller when viewed in cross section of older thallus regions (45-65 versus 75-100 µm), and is typically collected deeper than that species. Rhodomela sibirica A.Zinova & & Vinogradova has fewer branching orders (3-5), while Rhodomela lycopodioides (Linnaeus) C.Agardh lacks a clear central rosette in most cross sections.

Image A. This specimen was thought to be a Polysiphonia sp. when collected in the field (subtidal (8 m) on cobble, Durban Harbour, NU; GWS039244).

Image B. A typically ‘lycopodiodes’ sensu Europe morphology for a specimen from the subtidal (3 m) on rock, from Nome, AK (GWS042207).

Image C. Cross section of a vegetative axis near the thallus base reveals the central axial cell (A), periaxial cells (P) and densely cytoplasmic inner cortical cells (C) (subtidal (6 m) on rock, Churchill, MB; GWS005352).

Image D. Long section of a vegetative axis near the thallus base reveals the central axial cell (A), periaxial cells (P) and inner cortical cells, the latter shorter and more densely cytoplasmic than the former (C) (GWS005352).