This thin crustose red algal species was recently described from a one-off collection in northern Labrador growing on kelp stipe at 6 m (Featured Image above). The crusts form extensively over the stipe such that individual crusts are difficult to discern (Image A) and their presence was only revealed by the slight reddish hue to the stipe. Crusts are predominantly monostromatic, the basal layer consisting of cells that are 4–6 μm wide by 7.5–15.0 μm long by 8–12 μm tall, which are linked by cellular fusions that are obvious in both surface view (Image B) and section (Image C). Crusts have localized regions of 1-2 celled erect filaments, which are coherent but apparently lack cellular fusions (Image C). Reproduction is uncertain, but likely by asexual tetrasporangia as is typical in the Meiodiscaceae (See Saunders 2020).
Red crusts can be notoriously difficult to identify. Fortunately, if our experience is any indication, you are unlikely to encounter this one. It may be confused with Meiodiscus spetsbergensis (Kjellman) G.W.Saunders & McLachlan if only basal portions are viewed, but this species has free erect filaments that can reach 4mm in length in strong contrast to the 1-2 celled coherent filaments in T. varicrassum. Confusion with Rhodophysema elegans (P.Crouan & H.Crouan ex J.Agardh) P.S.Dixon is possible as erect filaments are also coherent, but these are typically longer than two cells and can have cellular fusions between adjacent filaments. Finally, confusion with Rhodophysemopsis hyperborea (Rosenvinge) Masuda is possible as it also has compact (not as strongly coherent) erect filaments lacking cellular fusions, but in this case the filaments are typically more extensive in development both in being longer (more cells) and in developing across the entire surface of the crust versus the predominantly monostromatic nature of T. varicrassum.
Image A. Thin red crusts of T. varicrassum growing on subtidal kelp stipe (6 m) (N Head of Evans Bight, Torngat Mountains National Park, Labrador Peninsula, NF; GWS040284).
Image B. Cell fusions in surface view (GWS040284).
Image C. Cell fusion in section view of a bistromatic region of the thallus (stained with aniline blue; GWS040284).