A confession is in order, the Featured Image above is actually from the west coast species Nemalion vermiculare Suringar; the general appearance is similar although Nemalion multifidum tends to be more branched (Le Gall & Saunders 2010b).
Gametophytic thalli (alternation of heteromorphic generations) up to 20 cm tall with one to many fronds arising from a basal holdfast; soft, gelatinous and flaccid, worm-like and dull reddish brown in colour; branching irregular to subdichotmous, at times trichotomous (Image A). Axes typically terete in transverse section, but becoming flattened at the branching nodes and at times contorted. The vegetative construction is easily revealed in a squash mount (tease some tissue from the thallus with forceps and squash under a microscope slide). The medulla is clearly filamentous and bears dichotomously branched cortical filaments that are 5-10 cells in height (Image B).
This species could possibly be confused with Tsengia bairdii (Farlow) K.C.Fan & Y.P.Fan (assuming that it is not extirpated from our flora), but axes in the latter are more irregularly terete and flattened and the cortex consists of only a few layers, the outer cells adhering well in a squash mount making them difficult to observe in anything other than surface view. The cortical filaments in Nemalion multifidum typically spread with pressure revealing their dichotomous habit (Image B).
All of our genetically verified collections were taken from the mid-low intertidal during the summer months from the warmer regions of the Maritime Provinces, as well as one specimen provided from RI by Craig Schneider. Confusion regarding this species and Nemalion vermiculare Suringar found on our west coast was resolved in Le Gall & Saunders (2010b).
Image A. Genetically verified specimen collected in the summer at RI; generously sent by Craig Schneider (CWS09-38-6).
Image B. Squash mount near the tip of a branch revealing the filamentous medullary filaments and dichotomously branched cortical filaments composed of 5-10 layers of elongate cells (CWS09-38-6).
Seaweed of Canada 