Acrothrix gracilis Kylin (BC/A)

Light brown sparsely to moderately branched thalli with terete axes, 0.2-1.5 mm wide (Image A). Near the tips observation should reveal a terminal hair that extends from the trichothallic meristem of the single central axial filament (to date I have not been able to view this in rehydrated material), which is surrounded by 1-3 layers of inflated cells that decrease rapidly toward the surface where 5-6 celled straight to curved assimilatory filaments are formed (Image B). Transverse section of older thallus regions reveals a hollow interior in which the central axial filament is lost, the 1-3 layers of inflated medullary and at best single layer of subcortical cells that decrease rapidly toward the surface with the 5-6 celled straight to slightly curved assimilatory filaments (Image C). Hairs can be rare to common, are colourless, and appear to replace assimilatory filaments (Image C). Unilocular sporangia are oval, 16-21 µm wide by 26-34 µm tall, and are typically borne on the basal cell of the assimilatory filaments (Image D).

Our collections for this species are from BC, New England and Atlantic Canada growing on a variety of hard substrata, other algae and eelgrass from the low intertidal to 20 m subtidal. Possibly confused with Chordaria flagelliformis  (O.F.Müller) C.Agardh, that species is darker in colour, multiaxial, older axes remain solid and the assimilatory filaments are straight.

Image A. Specimen from the subtidal (3 m) on cobble, Garbage Beach Breakwater, Woods Hole, MA (GWS027867).

Image B. Straight to slightly curved and weakly clavate assimilatory filaments (GWS027867; rehydrated, aniline blue stained).

Image C. Transverse section revealing hollow centre, medulla of 1-3 large cell layers, a single layer of subcortex and covering of assimilatory filaments. Multicellular base (arrow) of pheaophycean hair evident (GWS027867; rehydrated, aniline blue stained).

Image D. Close up of the oval unilocular sporangia nested among the assimilatory filaments (GWS027867; rehydrated, aniline blue stained).