Blades are simple, rarely once (twice) di(tri)chotomously branched, and are attached by a small discoid holdfast (Images A-C). Individual blades are borne on a short and very narrow (<1 mm) stipe, and are linear to lanceolate in outline being 1-2(3) cm tall and typically only 2-3 mm wide (Image A). Tips and margins routinely produce stolon-like outgrowths (Image B), which can function as stipes for new blades forming compound thalli, or grow along the substratum functioning as stolons that can also yield new blades (Image C).
In long section near the tip, blades are only 45-50 µm wide; medullary cells are axially elongate 15-28 µm wide and 25-75 µm tall and typically of 1-2 layers with thin cell walls; inner cortical cells typically form a single layer of axially flattened cells 12.5-23 µm wide and 8-12.5 µm tall; with an incomplete (inner cortical cells reach the surface in places) outer cortex typically of a single layer, the cells also axially flattened 5-10 µm wide and 3-5 µm tall (Image D). In transverse section mid thallus the cells are typically rounder in outline in the medulla and outer cortex, and blades reach 90-100 µm in width (Image E).
In the NW Atlantic this species can be confused with Erythrodermis traillii (Holmes ex Batters) Guiry & Garbary. In that species the blades are more ovate to irregular in outline; in longitudinal section the medullary cells tend to be less axially elongate with highly pronounced cell walls, and the outer cortical cells, isodiametric in shape, form two layers producing a complete covering.
This genetic group is likely introduced and has closer ties to Mediterranean populations of this species than those found in colder European waters. The possibility of cryptic species warrants investigation. We have only a few genetically verified records, seven from Woods Hole, MA, and one from RI indicating a wider distribution south of Cape Cod. Specimens were subtidal (5-8 m) on cobble and rock.
Image A. Vegetative habit of blades collected subtidally (8 m) at Woods Hole, MA (GWS030104). Scale = cm ruler.
Image B. Collection in which the blades have produced stolon-like outgrowths from the blade margins and tip (subtidal (6 m), Woods Hole, MA; GWS001878). Scale = cm ruler.
Image C. Collection for which the stolon-like outgrowths are functioning as a stolon (*) or as stipes to produce compound thalli (arrows) (subtidal (8 m), Woods Hole, MA; GWS030103). Scale = cm ruler.
Image D. Longitudinal section (aniline blue) close to the vegetative tip (GWS001878).
Image E. Transverse section (aniline blue) about mid thallus (GWS001878).