Fimbrifolium dichotomum (Lepechin) Hansen (Ar/A)

Thalli are typically found as clumps, 6-12 cm high, of bright to brownish red fronds that are characterized by sparse to dense marginal (at times surface as well) proliferations (at times difficult to discern, especially in northern and deeper collections, but which will nonetheless be uncovered with careful observation) (Image A). Some fronds display weak venation when held up to the light. This species can be highly variable in terms of frond width with most individuals in our collections falling in the 0.5 to 0.8 (1.5) cm range (Image A), but with some of our northerly most specimens becoming as narrow as 0.1 cm at their widest (Image B). Microscopic observation of whole mounted fronds will reveal the small apical cell, especially evident at the tips of the proliferations (a number of branch tips should be checked to ascertain if a species is multiaxial or uniaxial as broken tips can lead to erroneous decisions) (Image C), as well as the incomplete outer cortex which produces rosette-like patterns around the exposed inner cortical cells (Image D). In transverse section blades are 170-200 µm wide with the medulla 3-6 cell layers thick, the cells 50-75 by 50-150 µm in dimensions, transitioning to a single layered inner cortex and incomplete outer cortex (Image E).

We have verified (genetic and morphological, this species highly distinctive in the NW Atlantic flora) records from the drift and attached subtidal (30 m) from Baffin Island, Torngats and the island of NL, coastal NS, as well as the lower Bay of Fundy on both the NB and NS sides, and a single record from Jasper Beach, ME, as well as from Kamchatka. This species can be highly variable in morphology, some of which is recorded here (Images A & B) and in the BOLD taxonomy browser. Nonetheless, the combination of distinct apical cells at the branch tips, pseudoparenchymatous medulla, incomplete outer cortex giving the surface rosettes and marginal proliferations render this species unique in the NW Atlantic flora.

GWS039640.JPGImage A. Typical specimen for coastal NS, subtidal (18 m) in St. Margarets Bay (GWS039640).

GWS039235.JPGImage B. Characteristically narrow fronds of a subtidal (8 m) specimen from northerly waters; Durban Harbour, Baffin I., NU (GWS039235).

GWS007028-0001.jpgImage C. Distinct apical cell at the tip of a marginal proliferation viewed in whole mount (subtidal (25 m) at Bonne Bay, NL; GWS007028).

GWS007028-0002.jpgImage D. Incomplete outer cortex results in a distinctive rosette pattern around the exposed inner cortical cells (observed in whole mount; GWS007028).

GWS007028-0003.jpgImage E. Transverse section (aniline stained) reveals the pseudoparenchymatous medulla of large cells transitioning to the thin inner and incomplete outer cortex (GWS007028).