On siliceous coastal rock. This mature thallus has many blackened apothecia.
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Some fungi are very poisonous so a mistaken ID could have serious consequences.
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This seems to be a common feature of C. crenularia. I have collected material that I am fairly sure is fungal attack (not yet looked at) but partial blackening of the apothecia sems common and without obvious cause. Maybe it is the early stage of fungal infection. Looking at this more is on my to do list.
I did have someone e-mail me to tell me the photographs on my site were wrong, this blackening supposedly being due to the underlying algal layer that characterises C. ceracea. Fortunately I had sectioned apothecia to check they were NOT C. ceracea! The blackening is not algal.
With no account of the blackening in Dobson or Smith et al., it was the mention on your site that drew my attention to it and made this thallus more easily recognisable (I saw plenty of younger, 'classic' crenularia thalli).
I'm aware of C. ceracea, but haven't knowingly seen it, and have very little knowledge of lichenicolous fungi, but what you say makes intuitive sense anyway: looking again at my photos, it seems clear that the black layer lies above the (dry) apothecial cortex.
I look forward to a name for it.
Lat/Lng: 56.2589844, -2.6231107
OS grid ref: NO614075