Specimens tentatively (discussed below) tied to this species name are thus far known from the low intertidal growing on rock or coarse algae, typically Fucus (Featured image above). They are dark green in color and vary in height from 1-8 cm (Image A). This species has a strong tendency to form rope-like strands (Image A), which is facilitated by the production of strongly curved hooks and or rhizoids (Image B). Branches are typically formed laterally near cell apices, coming off at varying angles up to ~45 degrees from the bearing axis (Image C). Vegetative cells 70-100 µm in width with a variable 1:1 to 1:6 width to length ratio. Chloroplasts are parietal and reticulate, and contain multiple pyrenoids (Image D).
Although only two or three species are reported in our eastern flora (NW Atlantic and into the Arctic; Mathieson & Dawes 2017; Sears 2002), we have uncovered six genetic groups (Saunders & Kucera 2010; unpublished data). Further, two of the currently recognized NW Atlantic species reportedly extend to the BC flora, but we have uncovered only a single rare genetic group common to these floras despite extensive sampling (159 sequenced specimens to date; in addition a specimen from Kamchatka matched a different Atlantic genetic group). Consequently this group only tentatively receives the name Acrosiphonia arcta. Considerable taxonomic and biogeographical work remains. Species of Acrosiphonia can be confused with species of Cladophora (e.g. Cladophora rupestris (Linnaeus) Kützing (A), however, the latter lack the hooks and or descending rhizoids that characterize species of the former and have branches borne both terminally and laterally on the bearing cell. In Acrosiphonia spp. branches are typically produced laterally. Spongomorpha aeruginosa (Linnaeus) Hoek has thinner (< 50 µm) filaments and fewer larger pyrenoids in each cell.
Image A. Specimen displaying the strong rope-like habit of this genetic group, which results from numerous strongly curved hooks and spines (low intertidal pool on Fucus, Letete exposed biodiversity site, NB; GWS042849).
Image B. Whole mount displaying the highly curved hooks that characterize this species (GWS042849).
Image C. Branches formed laterally near cell apices, coming off at varying angles up to ~45 degrees from the bearing axis (GWS042849).
Image D. Chloroplasts are reticulate and parietal with multiple distinct pyrenoids (GWS042849).
Seaweed of Canada 