Individuals 10-25 cm tall, developing from a distinct discoid holdfast, and forming isolated clumps otor gregarious carpets in the upper intertidal. This species has regularly dichotomously divided, typically spiralling, fronds with a central midrib (Image A). When sexually mature the tips are terminated by swollen receptacles (Image A). Although relatively distinctive in our flora owing to the typically spiralled habit of the blades, this feature can be virtually absent in some habitats (Image B). Another distinguishing feature is the ridge that forms around the receptacle margin (Image C), but this can again be variable. Similarly, some specimens produce distinctive cryptostomata (sterile cavities similar to conceptacles, producing hairs, but lacking reproductive structures; Image D). This species produces a highly distinctive salt marsh morphology (F. spiralis var. lutarius (Kützing) Sauvageau), which we have observed at a few locations in the NW Atlantic (Image E).
This species is monoecious with antheridia (Image F; 15-17 by 42-50 µm) and oogonia (Image G; 130-150 by 150-165 µm when mature) produced in the same conceptacles.
DNA barcoding with standard markers (COI-5P, rbcL-3P) fails to distinguish between this species and Fucus vesiculosus Linnaeus (see Kucera & Saunders 2008). Morphological identification can be complicated as well because individuals intermediate in morphology with Fucus vesiculosus Linnaeus can occur in some locations, as well as in the intertidal where these two species overlap. Further, these species are reported to hybridize, which only further complicates field identification (see Kucera & Saunders 2008, Mathieson & Dawes 2017). Consequently ranges reported here are based on morphological species, but are in need of further study, especially in northern NW Atlantic populations. We have records from throughout BC in the Pacific, and from ME, throughout Atlantic Canada, extending to Makkovic in Labrador, NL in the (sub)Arctic. In the NW Atlantic F. spiralis tends to dominate the upper intertidal being typically higher in the zone than Fucus vesiculosus Linnaeus. Interestingly, whereas the salt marsh morphology in the NW Atlantic has been assigned to F. spiralis based on molecular analyses of our collections, this morphology is a variant of Fucus distichus Linnaeus in our NE Pacific collections.
Image A. Typical specimen in the upper intertidal zone on rock displaying the spiralled blades with terminal receptacles (Huxley I., BC; GWS012854).
Image B. Specimen lacking significant spiralling of the fronds from the upper intertidal on rock (Campobello I., NB; EGWS000117).
Image C. Receptacles with an evident sterile ridge (arrows) around the margins (upper intertidal on rock, Marco I., BC; GWS036019).
Image D. Cryptostomata (hair pits; arrows) obvious on an upper intertidal specimen from Anthony Cove, NB (EGWS000687).
Image E. Salt marsh morph (F. spiralis var. lutarius) from the uppermost intertidal growing unattached and tangled in the vegetation (Letete, NB; EGWS000695).
Image F. Squash mount of a mature conceptacle revealing a cluster of antheridia (upper intertidal on rock, Lepreau exposed biodiversity site; EGWS000701).
Image G. Squash mount of a mature conceptacle revealing a mature oogonium (containing eight eggs) among immature oogonia, antheridia, and sterile hairs (EGWS000701).
Seaweed of Canada 