Individuals range from 0.5-15 cm in height and can form localized clusters on various substrata (Image A) to extensive carpets on exposed rock (Image B) or in areas where freshwater streams run through the intertidal at low tide (Image C). Specimens can form fluffy tufts of apparently free filaments (eg. Image C), but more typically appear as rope-like strands (Image D), which is enhanced by the production of rhizoidal filaments in this species (Image E). Branching is typically opposite with one in each pair more developed than its partner and tapering towards the tips (Image F), although short ‘varia-type’ (Pylaiella varia Kjellman) branches can occur (Image G), as well as individuals with thallus regions of predominantly unilateral branching (Image H). Cells are variable in shape from squat to elongate rectangular, 32-57 µm wide by 40-125 µm long and contain discoid plastids (Image I). Axes are typically uniseriate, but can also be biseriate. Plurilocular structures can be terminal (Image J) or intercalary (Image K) in position, but are typically restricted to fewer than 12 segments.
In our experience this is the most widespread and common species of Pylaiella in Canadian waters with genetically verified records throughout all three regions. Collections in hand range from WA, throughout BC, from Nome through the Beaufort in AK, widely in the Canadian Arctic and Atlantic extending south to northern MA. Collections range from upper intertidal pools to the deep intertidal (18 m) and grow on a wide variety of solid (eg. rocks, mussels, etc.) substrate, as well as other algae (in particular fucoids). We have not encountered this species in our limited European collections. This appears to be the only species to produce rhizoids in our waters, which distinguishes it from Pylaiella littoralis (Linnaeus) Kjellman (‘rhizoids’ are modifications of branch tips in this species) and Pylaiella varia Kjellman in the NW Atlantic and Arctic. Branching is typically opposite with branches characterized by a leading and lagging branch, whereas in Pylaiella littoralis (Linnaeus) Kjellman opposite branches are in more even pairs, although some degree of unilateral branching can also occur. Furthermore, the number of contiguous segments converted to reproductive structures seems limited to fewer than 12 while in Pylaiella littoralis (Linnaeus) Kjellman they routinely exceed 30. Confusion with Spongonema tomentosum (Hudson) Kützing, which also produces rhizoids is possible but this species has ribbon like plastids (a feature only clearly observed in fresh material). Confident identification among Pylaiella spp. requires molecular data.
Image A. Specimens growing as localized clumps on mid intertidal Ascophyllum nodosum (Linnaeus) Le Jolis as is common to the Bay of Fundy (Letete, NB; GWS045101).
Image B. Specimens forming extensive carpets on semi-exposed rock (mid intertidal, Nome Breakwater, AK; GWS041864).
Image C. Specimens forming expansive mid intertidal carpets in the freshwater outflow of a stream (Island Bay, Gwaii Haanas, BC; GWS012861).
Image D. Specimens typically develop into rope-like strands (upper intertidal on Fucus, Point NW of Lighthouse, Makkovic, Labrador, NL; GWS040369).
Image E. Rhizoids descending along a vegetative filament (mid intertidal on Ascophyllum nodosum (Linnaeus) Le Jolis, Letete, NB; GWS045102).
Image F. Leading and lagging opposite branches in a specimen from Lepreau, Bay of Fundy, NB (GWS032357).
Image G. Short side-branches developing at right angles to the axes (varia-type) can be common in some specimens (mid intertidal pool on Ascophyllum nodosum (Linnaeus) Le Jolis, Letete, NB; GWS045100).
Image H. Region of specimen with predominantly unilateral branching (GWS045100).
Image I. Parietal discoid plastids (GWS045100).
Image J. Terminal plurilocular sporangia (mid intertidal on Ascophyllum nodosum (Linnaeus) Le Jolis, Letete, NB; GWS045102).
Image K. Intercalary plurilocular sporangia (GWS045102).