Trapeliopsis grows as a thin, granular to minutely leaf-like crust that sits close to its
substrate. In some species the enlarge into tiny, overlapping scales (squamules) that can give the colony a miniature
rosette appearance; in others the
thallus remains a loose dusting of coarse grains. Only those distinctly forms develop a true upper —an organised skin of tangled fungal
hyphae—while the purely granular
morphs have no differentiated surface layer. Throughout the thallus the
photosynthetic partner is a
green alga of the
Chlorella or
Pseudochlorella type, often dividing into clusters of two to four cells whose flattened faces make them appear slightly asymmetrical. Reproductive structures present as low, button-like
apothecia that are yellow-brown to almost black. They are slightly pinched in at the base and lie flush with the thallus surface from an early stage, expanding outward without splitting. Some species show a pronounced rim cut from thallus tissue (a ), whereas others reveal only the internal —a ring of colourless fungal
hyphae set in a faintly pigmented gel that never darkens to the deep brown seen in some related genera. Inside the apothecium, delicate
paraphyses thread the
hymenium; these filaments branch and fuse near their tips but remain narrow and usually colourless, though an external pigment coating can make the upper layer appear slightly swollen. Each thin-walled
ascus holds eight smooth, ellipsoidal
ascospores that stay colourless and single-celled,
staining weakly or not at all with
iodine (a "
Trapelia-type" response).
Asexual reproduction occurs in immersed
pycnidia that release rod-shaped to thread-like
conidia. Chemical analyses typically detect
gyrophoric acid, with traces of
lecanoric acid also common. While sterile material may be difficult to separate from the superficially similar
Trapelia, molecular studies show that
Trapeliopsis forms a distinct evolutionary
lineage, and its colourless exciple plus uniformly narrow paraphyses help distinguish it from dark-rimmed genera such as
Placynthiella. ==Species==