This species ranges in morphology from sparsely to densely branched individuals for which the secondary axes commonly overtop the primary axis (Image A). Only rarely are third or fourth order branches observed. However, in our experience individuals from the Arctic and extending into Atlantic Canada can very much have a percurrent primary axis with shorter secondary axes, which can confound identification with most identification guides (Image B). When viewed in whole mount the tips lack an obvious apical cell (Image C) while the surface cells near the apices are round to angular, 9.5-14 µm in diameter, with slight spacing (Image D). Squash mount reveals elongate medullary cells, 2-3 layers of compact subcortex and an outer layer of clavate assimilatory filaments (Image E). In transverse section of younger axes there is a mix of smaller and larger cells centrally, the cells gradually decreasing in size to the 2-3 layered subcortex, which bears the assimilatory filaments (Image F). In older axes, the cells centrally are small, surrounded by larger cells, which then grade to smaller cells out to the subcortex and coating of assimilatory filaments (Image G). At higher magnification of younger axes the assimilatory filaments are 3-4 cells long and only slightly clavate (Image H). In mature axes the assimilatory filaments can become modified appearing as 5-7 celled paraphyses being more elongate clavate in overall appearance (Image I). Continued observation will reveal the nested unilocular sporangia, 12-20 µm wide by 41-52 µm tall, which are oblong to elliptical in outline and borne on the lowest cells of the assimilatory filaments (Image J).
This species is widely distributed in the NW Atlantic and extends into the Arctic flora. We have genetically verified records from upper intertidal pools to 20 m subtidal on a variety of hard substrata and other algae. Our BC collections actually form a distinct genetic group and taxonomic work is needed. Possibly confused with Acrothrix gracilis Kylin, that species is lighter in colour, uniaxial, typically hollow even close to the tip and at least some of the assimilatory filaments are curved. In the (sub)Arctic sparsely and un-branched (admittedly rare) specimens look like Chordaria chordaeformis (Kjellman) H.Kawai & S.-H.Kim.
Image A. What is considered the typical morphology with secondary axes longer than the primary axis (arrow) (semi-exposed sites on rock, mid intertidal, Peggys Cove, NS; GWS001288).
Image B. Common cold water morphology with multiple but short secondary axes from the dominant primary axis (mid intertidal pool on rock, Gordon Point, Churchill Northern Studies Centre, MB; GWS005700).
Image C. View of multiaxial tip lacking a distinct apical cell (upper intertidal on Cladophora rupestris (Linnaeus) Kützing, Musquash Head Lighthouse, Saint John Harbour monitoring site, NB; GWS047866).
Image D. View of cells at surface near tips (GWS047866).
Image E. Squash mount revealing elongate medullary cells, 2-3 layers of subcortex and an outer layer of clavate assimilatory filaments (GWS047866).
Image F. Transverse section of younger axis with a mixture of small and large cells centrally, grading to smaller cells of the subcortex (GWS047866).
Image G. Transverse section of mature axis with small medullary cells centrally, surrounded by larger cells, which reduce to smaller cells of the subcortex and then the outer assimilatory filaments now modified as paraphyses (mid intertidal pool on rock, Riviere du Loup, QC; GWS008039; rehydrated from press, aniline blue).
Image H. Close up of the outer assimilatory filaments of a vegetative axis (GWS047866).
Image I. Assimilatory filaments in mature axes now modified as paraphyses (GWS008039; rehydrated from press, aniline blue)
Image J. Typically oblong to elliptical unilocular sporangia nested among the paraphyses (GWS008039; rehydrated from press, aniline blue)..