Gametophytes of this cold water species form wiry tufts of dark black to red terete axes, 4-28 cm in height, which can bleach to yellow and white towards the tips (Image A), while the sporophytes are small brownish-red crusts commonly occurring on cobble (Image B). Gametophytes display irregular to loosely dichotomous branching, and are not complanate (i.e. not branching in a single plane) (Image A). In transverse section the cells of the longitudinal medullary filaments are 4-25 µm in diameter and bear the anticlinal filaments of the thick multi-layered cortex (Image C). Cystocarps form as surface pustules that extend past their margins over the thallus surface (Image D) and can be observed with the naked eye as swellings along the axes. Closer inspection reveals remnant carpogonia, a tangled zone of gonimoblast development, and ultimately straight gonimoblast filaments terminating in carposporangia (Image E). Spermatangia borne singly on mother cells form in extensive sori over the upper axes (Image F).
Genetically verified records extend from mid intertidal pools to subtidal (15 m) from CN to the Island of NL, although a few collections were made in Churchill, MB. This species grows on rock or cobble, rarely invertebrates, and is partial to sandy habitats.
Distinguishing the gametophyte stage of A. plicata from its congener Ahnfeltia borealis D.Milstein & G.W.Saunders can be challenging. They do typically differ in range with A. plicata being common from RI to the Island of NL, in the NW Atlantic, but we also have a few collections from Churchill, MB, in Hudson Bay where Ahnfeltia borealis D.Milstein & G.W.Saunders is common. Similarly, the predominantly Arctic Ahnfeltia borealis D.Milstein & G.W.Saunders with collections from Nome, AK, to northern Labrador, NL, also has a single drift record for the gametophyte stage on the north shore of PE, and more surprisingly one-off collections of its crustose sporophyte from Haida Gwaii, BC, and the north shore of Cape Cod, MA. Definitive identification, especially in colder areas of the NW Atlantic flora and in Hudson Bay, require molecular analyses. This species can also be confused with Gymnogongrus griffithsiae (Turner) Martius south of Cape Cod in the NW Atlantic flora. In addition to biogeography, Gymnogongrus griffithsiae (Turner) Martius has slightly compressed axes near the branch tips and a pseudoparenchymatous rather than filamentous medulla. Note that in A. plicata the central filaments run longitudinally and appear pseudoparenchymatous in transverse section (Image C) – they are best viewed in long section if confusion with Gymnogongrus griffithsiae (Turner) Martius is suspected. Further, the cortex in Gymnogongrus griffithsiae (Turner) Martius forms a single layer of anticlinal filaments, whereas in A. plicata multiple layers of anticlinal cortical growth can accumulate creating a ringed appearance (Image C). This species can be highly variable in morphology, some of which is recorded in the BOLD taxonomy browser.
Image A. Gametophyte stage subtidal (4 m) on rock (North Rustico, PE; GWS011606).
Image B. Sporophyte stage subtidal (5 m) on rock (Folly Cove, Gloucester, MA; GWS017286).
Image C. Transverse section mid thallus displaying the axially elongate medullary filaments in section, these bearing the anticlinal filaments of the multilayered cortex (subtidal (1.5 m) on cobble, Burnt Church, NB; GWS039109; rehydrated from press, aniline blue stained).
Image D. Transverse section displaying two cystocarps in different stages of development on an upper axis (lower mid intertidal pool on rock, Lepreau exposed biodiversity site, Bay of Fundy, NB; GWS011911; rehydrated from press, aniline blue stained).
Image E. Close up of a developing cystocarp with darkly staining remnant carpogonia (arrow), a zone of tangled gonimoblast filaments subtending straight gonimoblast filaments that terminate in carposporangia (double arrows) (GWS011911; rehydrated from press, aniline blue stained).
Image F. Surface sorus of spermatangia borne singly on spermatangial mother cells (GWS039109; rehydrated from press, aniline blue stained).