Thalli are unbranched yellowish brown tubes, solitary or in small clumps, 2-5 mm wide and up to 35 cm tall (Image A). Thalli typically have constrictions (Image A), but there are exceptions (Image B). In section thalli are hollow with 2-3 medullary cell layers, cells 30-70 µm diam. near the central lumen decreasing in size to 15-30 µm diam. toward the 1-2 cortical layers (Image C). Where reproductive unicellular paraphyses (12-15 µm wide by 40-45 µm tall) are intermingled with and overtop the uni(+bi?)seriate plurilocular structures (4-6 µm wide by 35-40 µm tall; Image D).
Mitochondria from this species have introgressed into other species in our flora rendering identification by COI-5P unreliable for some specimens/locations. Consequently this species is better identified with ITS and or rbcL (McDevit & Saunders 2017). For specimens unequivocally assigned to this species, distribution ranges from RI to NL in the Atlantic, as well as in southern BC and Norway, with plants growing on rock from upper intertidal pools to subtidal (3 m). Including specimens for which we only have mitochondrial data, we can add Haida Gwaii, BC, northern Japan, Kamchatka and Nome, AK to the range, and extend deeper into the subtidal (6 m). Currently this species can be distinguished from others in our flora by the presence of unicellular paraphyses when reproductive, the absence of hairs on the surface, and typically constrictions along the tube. Definitive identification of Scytosiphon spp. requires a few molecular markers (McDevit & Saunders 2017).
Image A. Strongly constricted specimen from mid intertidal pools at Maces Bay, NB (ignore the preliminary field identification, which predates our molecular work on this group; GWS000199).
Image B. A specimen lacking localized constrictions along the thalli (mid intertidal pool on rock, Starboard, ME; GWS003656).
Image C. Section from a reproductive area of the thallus displaying the hollow centre, medullary and cortical layers, and surface reproductive development (Duck Road Beach, NB; GWS045125).
Image D. Closeup of the uni(+bi?)seriate plurilocular gametangia and associated unicellular paraphyses (GWS045125).