Arthothelium hymeniicola

This fungus is non-lichenised and forms no thallus of its own. Instead, it lives endohymenially: it develops within the hymenium (the spore-producing layer) of the host lichen's apothecia. The infection does not visibly deform or discolour the host fruiting bodies, so it may be undetectable without microscopy. Microscopically, A. hymeniicola consists of loosely clustered, nearly spherical asci embedded mainly in the upper part of the host hymenium. The asci are 8-spored, about 35–40 × 24–30 μm, with a thickened upper wall (about 5 μm) and a small ocular chamber; in iodine tests the wall is I− but turns orange in Lugol's iodine (IKI). The ascospores are hyaline, ovoid to ellipsoid, and muriform (divided by several transverse septa plus a longitudinal septum), measuring 14–20 × 6–8 μm; they lack a distinct gelatinous sheath. Paraphysoids and conidiomata have not been observed.

Arthothelium hymeniicola is known only from its type collection on Campbell Island, from north of Beeman Station. It was collected on a branch in Dracophyllum scrub, where it was strictly confined to the hymenia of its Bacidia host. There are no confirmed records from elsewhere.

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