Graphis crebra

Graphis crebra is a species of corticolous lichen in the family Graphidaceae. It has a pantropical distribution. Like other script lichens, it grows on bark and resembles calligraphy. It can be distinguished from several other similar species by the white pruina (powder) on its apothecial discs.

Taxonomy
The lichen was described as new to science in 1899 by Finnish lichenologist Edvard Vainio from a collection made in Gourbeyre, Guadeloupe.

Description
Graphis crebra has a corticate thallus that is dull to somewhat shiny, and pale grey. The lirellae (an ascoma with a long, narrow disc resembling dark squiggly lines) are (bursting through the surface) with a lateral, short, unbranched to rarely one-branched, straight to slightly curved. The apothecial becomes exposed very early, with a distinct white  (a crystalline or powdery surface covering). The (a layer of sterile tissue that contains the hymenium) is black. Graphis crebra produces ascospores that are 5–9-septate, and measure 20–30 by 5–8 μm.

The main secondary compound in Graphis crebra is norstictic acid.

Similar species
Similar species include Graphis cincta, which lacks open pruinose discs, and G. handelii, which does have open discs but is not pruinose. Another lookalike species is the Australian G. streimanii, which has larger spores and lirellae that are longer and more branched. The species G. manipurensis and G.sirohiensis, both known only from Manipur, India, are also similar in appearance to Graphis crebra. The Indian species, however, do not have the white pruinose disc characteristic of G. crebra. Another pantropical, norstictic acid-containing species, Graphis handelii, can be distinguished from G. crebra by its epruinose apothecial discs.

Habitat and distribution
In addition to the type locality in Guadeloupe, Graphis crebra is also known from St. Helena and the Galapagos. In 2011, it was reported from Florida in North America. In 2016, G. crebra was reported from Portugal, which was also a new occurrence for Europe, and in 2017 from the Seychelles.