Gomphillus lichens are characterised by their crustose
thallus, which is very thin, semi-transparent, and sub-gelatinous when wet. The thallus can either spread irregularly across the () or have well-defined edges. The primary photosynthetic partner () in
Gomphillus species is a green alga, meaning the algal cells are spherical or nearly so. The reproductive structures (
ascomata) are
apothecia, which are fruiting bodies that can appear irregularly spherical () or often resemble a top shape. These structures are brown-black on the upper surface but gradually transition to pale or colourless at the lower portion. The apothecia lack a (a rim derived from the lichen's body) but have a well-developed , a cup-like structure that extends below into a stalk-like extension. The exciple is pale brown at the upper edge and pale within, consisting of thin, interconnected
hyphae embedded in a gelatinous matrix; it is not (hardened into a black, coal-like consistency). The uppermost layer of the apothecium () is reddish-brown and smooth, lacking . The internal structure of the apothecia () consists of numerous thin, interconnected filaments called
paraphyses, each less than 1 μm wide, with tips that are not swollen. The
asci, or spore-producing sacs, contain eight and are long and narrow—up to 50 times longer than they are wide. These asci are , meaning they have a specialised double-wall structure that aids in spore release, and they do not react to
iodine staining (K/I–). The ascospores themselves are thread-like, extremely elongated, multi-
septate (divided into many compartments), and colourless.
Gomphillus species also produce
conidiomata, small structures for
asexual reproduction, which appear as black on the upper surface and pale below. These structures release
conidia, which are tiny,
ellipsoidal cells that can disperse and form new colonies. In some related species outside the genus (extralimital species), specialised spore-producing structures called are present. No lichen-specific
secondary metabolites have been detected in
Gomphillus species using
thin-layer chromatography, a common method for identifying lichen substances. ==Habitat and distribution==