Trapelia forms a low-profile
crust that adheres tightly to its
substrate.
Colonies may begin as a thin, continuous film with a smooth outer edge; over time this film either cracks into a mosaic of tiny plates () or arises directly as discrete areoles that later press together to form an almost seamless crust. The uppermost layer of fungal tissue () is only weakly differentiated and comprises rounded cells, while a delicate film of dead cell remnants often leaves a fine frost-like sheen (). A narrow, sometimes inconspicuous —an initial growth of colourless
hyphae—may fringe the colony. The internal algal partner is a minute, spherical
green alga ( photobiont). Reproductive structures appear as rounded
apothecia that sit flush with, or slightly raised above, the
thallus surface. When young they are partly embedded; mature apothecia may display a thin, brown, often ragged margin formed from disrupted thallus tissue, giving the impression of a "pseudothalline" rim. The ranges from pale pink-brown to almost black and is frequently uneven or roughened. Internally, the is brown and may dissolve to an orange-brown solution in
potassium hydroxide solution (a common diagnostic test), whereas the underlying is pale to light brown. Slender
paraphyses thread the
hymenium; these filaments branch and fuse profusely near their tips, but their ends remain narrow rather than swollen. Each
ascus is slightly club-shaped,
stains weakly blue in
iodine and carries eight single-celled, colourless
ascospores that are
ellipsoidal and 9–25
micrometres (μm) long. In many species tiny, immersed
pycnidia generate asexual
conidia—straight or slightly curved rods—that help the lichen spread. Chemical analyses detect one or more of three common
secondary metabolites—
gyrophoric acid,
lecanoric acid and 5-
O-methylhiascic acid—which can assist with species-level identification. ==Species==