Seaside centipede lichen (Heterodermia sitchensis) COSEWIC assessment and status report: chapter 3

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Species Information

Name and classification

The Seaside Centipede Lichen, Heterodermia sitchensis Goward & Noble (Figure 1), was described in 1984 from the west coast of Vancouver Island, British Columbia (Goward 1984). The holotype specimen is on deposit at UBC, with an isotype each at the Canadian Museum of Nature (CANL), and the University of Helsinki (H), Finland. Though the taxonomic distinctness of this species has not been challenged since its description, H. sitchensis is an asexually reproducing lichen, and for this reason could represent the secondary counterpart of a fertile primary species such as H. podocarpa (Bél.) Awas. Lichenologists are not agreed on how best to treat secondary "species", i.e., whether as mere forms or subspecies of the primary species, or as distinct species in their own right. Given, however, that H. sitchensis is chemically, morphologically and geographically distinct from H. podocarpa, most lichenologists would unhesitantly accord it species status.

Morphological description

Heterodermia sitchensis is a semi-erect, cushion-forming, foliose (leaf) lichen about 2 cm across (Figure 1). The lobes are thin, stiff, short to elongate, separate to loosely overlapping, 1-2 mm wide, and have long thin cilia (eyelashes) along the margins. The upper surface is strongly convex, pale greenish white (but readily discolouring to bluish black), and often bears scattered whitish maculae (spots). The lower surface is white, appressed-cottony and unevenly thickened, with strut-like extensions of the lower cortex protruding into the medulla. Mature thalli bear apothecia (sexual fruiting structures) in urn-shaped outgrowths near the lobe tips. These have prominent flaring rims that in turn bear ring-shaped soredia (powdery asexual reproductive propagules) on their inner surface. Among the chemical substances present in H. sitchensis are atranorin, zeorin, and various fatty acids. Atranorin reacts K+ yellow both with the cortex and with the medulla. Goward (1984) reported a PD+ yellowish to pale orange medullary reaction; but this has not been confirmed in more recent material. A more detailed description is given in Goward (1984).

Generic description

Heterodermiais a genus of small to medium-sized lichens with narrow lobes almost always fringed with cilia ("eyelashes"). The upper cortex consists of elongate cells oriented lengthwise, giving the thallus a "stretched" appearance at close inspection, while the lower surface either bears a cortex or lacks a cortex, in the latter case with the white medullary hyphae exposed. The apothecia are dark brown with conspicuous pale margins coloured like the upper cortex. The spores are brown, 8 per ascus, 2-celled, and thick-walled. In the related genus Physcia, the cells of the upper cortex are unoriented, confering a more even appearance at close inspection.

Within Heterodermia, H. sitchensis belongs in the H. podocarpa group. This is an assemblage of about 27 foliose species (Trass 1992), all of which have erect or semi-erect lobes, terminal or subterminal apothecia, and a non-corticate lower surface (Kurokawa 1962). The H. podocarpa group has two obvious centres of distribution: one in southeast Asia (12 species), and the other in Central America, north to Mexico (15 species).  In the United States and Canada, this group is represented by only five species: H. echinata (Taylor) Culb., H. erinacea (Ach.) W. Weber, H. galactophylla (Tuck.) Culb, H. podocarpa (Bél.) Awasthi, and H. sitchensis.

Similar species

With its loose habit, cottony lower surface, marginal cilia and sorediate urns, H. sitchensis is a distinctive species not readily confused with any other lichen. Most similar, perhaps, is Physcia tenella, another tree-dwelling species with pale greenish lobes lined with cilia. In that species, however, the soredia are located on the undersides of the lobe tips, and the lower surface is corticate (hard and skinlike), not cottony. Cavernularia hultenii can also sometimes be similar, though in that species the lower surface is black and pocked with tiny cavernulae ("pits"). See Goward et al. (1994) for keys to these and other similar lichens.

Genetic description

No information available.