Calopadia forms a thin, continuous or occasionally patchy
thallus (the body of the lichen), usually smooth and pale brownish to greyish in colour. The
apothecia (
fruiting bodies) are constricted at the base and brown to dark brown, but not black; their margins are prominent when young, later fading or disappearing. The —the tissue forming the rim around the apothecial disc—is (composed of small, brick-like cells), while the (the layer beneath the spore-bearing tissue) does not react with potassium hydroxide (K–). The
hymenium contains or only slightly branched
paraphyses (filamentous supporting cells). The asci are cylindric- (cylindrical to club-shaped) with a blue-staining (J+) , showing the
ascus structure characteristic of
Sporopodium. The ascospores are (divided by both transverse and longitudinal walls) and variable in number, from one to eight per ascus. The
asexual reproductive structures, or , are grey to dark brown and produce
conidia (asexual spores) that are (needle-shaped), curved or spirally coiled, and multi-
septate (divided by many cross-walls). The conidia arise in association with small algal cells near the thallus surface. In overall appearance and spore type,
Calopadia resembles
Tapellaria, but differs in having non-black apothecia, a K– hypothecium, and a continuous rather than patchy thallus. A field study of
Calopadia puiggarii documented the full
life cycle of a
Calopadia species on living leaves and showed how
sexual and
asexual reproduction can both contribute to thallus formation. In that study, campylidia contained photobiont cells among the conidiogenous tissue, and the filiform, septate macroconidia were released together with algal cells that they often encircled; the macroconidia could then
germinate and lichenize those dispersed photobionts. The same work reported
pycnidia in
Calopadia for the first time; these minute structures produce
microconidia that were interpreted as likely spermatia (male
gametes).
Chemistry Chemical studies using
thin-layer chromatography and
high-performance liquid chromatography have reported a range of
secondary metabolites in
Calopadia, including
pannarin and
atranorin, along with several chlorinated
xanthones (
lichexanthone derivatives). In some species, discrete
chemotypes have been reported; for example,
C. perpallida and
C. subcoerulescens have been described with more than one "chemical race" based on their detected compounds. ==Species==