This crustose species can range in colour from yellowish through reddish to dark brown, the margins typically lighter than central regions (Image A.). It is characterized by unmistakably distinct overlapping lobes, but shares with other species concentric rings and striations on its surface (Image A). In section the thallus is unistratose (Image B) with the elongate cells of the basal layer (in the centre of the thallus) producing ascending and descending curved erect filaments (Image C). Surface cells of the descending erect filaments produce multicellular rhizoids, which aid in attachment (Image D), while those of the ascending filaments end in anticlinal rows (5-7 cells deep) of heavily pigmented cells (Image E).
This species is widely distributed throughout Canadian waters (genetically verified records from throughout BC, Canadian Arctic, Atlantic Canada and Maine). In the NW Atlantic it is easily distinguished as the only Ralfsia spp. to display overlapping lobes (Image A). It is found predominantly in the intertidal, typically in pools, but also extends into the shallow subtidal (8 m), which helps to differentiate it from the predominantly subtidal Ralfsia unimaculata Parente & G.W.Saunders found in BC (this species is also typically not lobed in habit, but molecular data may be necessary for an unequivocal identification). Populations of this species in the Canadian Arctic are typically subtidal in their habitat.
Image A. Specimen from a low intertidal pool at Lawn Point, BC, displaying the characteristic overlapping lobes (GWS044195).
Image B. Section showing overtopping unistratose lobes (upper intertidal pool on rock, Musquash Head Lighthouse, NB; GWS044469).
Image C. Elongate basal cells bear descending and ascending erect filaments (GWS044469).
Image D. Multicellular rhizoids formed where the descending filaments attach to the substratum (low intertidal on rock and algae, Lepreau exposed biodiversity site, Bay of Fundy, NB; EGWS001123).
Image E. Ascending erect filaments terminate in 5-7 rows of heavily pigmented cells (EGWS001123).