Catillaria gerroana

The thallus of Catillaria gerroana forms a thin, paint-like crust (to about 80 μm thick) that adheres tightly to sandstone and spreads in patches up to 10 cm across. Its surface is pale to medium grey-green, smooth to faintly cracked, and lacks a true cortex—meaning the green algal layer sits directly beneath a thin skin of fungal tissue. Calcium oxalate crystals are abundant within the medulla and give the lichen a gritty feel when cut. The partner alga (photobiont) consists of spherical chlorococcoid cells 6–15 μm wide, interwoven with short fungal hyphae.^[2]

Apothecia (fruiting bodies) are extremely numerous, initially flat but soon becoming markedly convex and finally almost hemispherical; each measures roughly 0.23–0.48 mm in diameter. The disc is dull dark green to greenish-black and non-powdery, while the narrow margin is the same colour and quickly disappears as the disc expands. Microscopically, the excipulum (rim) shows an outer brown zone that turns deep red-brown in nitric acid, and an inner clear zone. The colourless hymenium is 40–50 μm tall and capped by a dark brown epihymenium. Paraphyses are slender but swell abruptly at their tips, each tip containing a dark pigment cap. Asci are of the Catillaria-type and hold eight colourless, thin-walled ascospores that are narrowly ellipsoid to short-fusiform, usually slightly constricted at the septum, and typically measure around 11.5 × 4.5 μm. Minute flask-shaped pycnidia immersed in the thallus release narrowly ellipsoid to oblong conidia 2–3 × 1–1.5 μm, providing an additional asexual dispersal route. Standard spot tests are all negative and thin-layer chromatography detects no secondary metabolites, a profile typical for many Catillaria species.^[2]

As of its original publication, the species was known only from its type locality: weather-beaten sandstone outcrops on the foreshore at Black Head, Gerroa, New South Wales. It grows just above the high-tide zone where surfaces receive regular sea spray and occasional runoff, forming mosaics with other maritime crusts such as Buellia cranwelliae, various Caloplaca species, Rinodinella fertilis, Pertusaria melanospora var. sorediata and Verrucaria aff. fusconigrescens. No additional populations have been located despite targeted searches along comparable coastal cliffs.^[2]