Thalli are typically feathery in habit and range from 3-8 cm tall (Image A). Main axes are thin and wiry, multiseriate, and irregularly branched (Image A), becoming densely corticated in lower reaches of the axes. Axes are typically pinnate in younger regions with two well-developed rows of opposite branchlets (Image B), this feature becoming denuded on older thalli (Image A). The branchlets are unbranched and multiersiate, lack cortication, and each segment can produce a pericyst (Image C). Intact branch tips reveal a distinct apical cell, which undergoes a transverse division to produce a subapical cell. Subapical cells undergo longitudinal divisions to produce the multiseriate appearance, as well as the distinct segments that characterize this species (Image C). Most longitudinal cells in each segment subsequently undergo secondary transverse divisions (Image D). Plastids are parietal and discoid, reportedly lacking pyrenoids.
This species has two distinct mitotypes and chlorotypes. One type is essentially North Atlantic with genetically verified records from Norway, as well as throughout Atlantic Canada extending to Baffin Island, NU, and Churchill, MB, in the Arctic. The second type, confined to the Arctic, extends from the Boulder Patch, AK, to Baffin Island, NU, and Churchill, MB. However, two specimens from Churchill, MB, and one from Baffin Island, NU, have the Arctic mitotype but the Atlantic chlorotype. The ITS between these two populations differs by a single ambiguity, ‘Y’ (C and T) versus a ‘T’. These data suggest that these two populations are intermixing in the Arctic and should be considered a single species pending further study.
We have collected this species from Nome, AK (failed to yield sequence data), the Beaufort Sea (Boulder Patch, AK), throughout the Canadian Arctic extending into colder waters of Atlantic Canada. Many of our collections are from the drift, with attached specimens subtidal (3-20 m) and growing on rock or other algae. An unusual specimen was taken from a ‘seaweed ball’ washed up on Conrad’s Beach, Lawrencetown, NS (Image E).
Image A. Feathery thalli from subtidal (3 m) on rock in the Bay of Fundy (East of Cape Spencer, Saint John Harbour monitoring site, NB; GWS042857).
Image B. Closeup of a main axis and paired branchlets near a tip (GWS042857).
Image C. Branchlets are uncorticated and each segment can produce a pericyst (arrows) (GWS042857).
Image D. Segment cells can undergo secondary transverse divisions (arrows), which are easily viewed on the branchlets (GWS042857).
Image E. Some ~10 species of seaweed, including Chaetopteris plumosa, were identified from this seaweed ball, which was largely composed of Desmarestia aculeata (Linnaeus) J.V.Lamouroux knitted together with Ahnfeltia plicata (Hudson) Fries (seaweed ‘tumbleweed’ drift on beach, Conrad’s Beach, Lawrencetown, NS; EGWS000830).