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Images of British Lichens

Caloplaca oasis (A.Massal.) Szat.

Thallus thin, areolate, greyish-yellow, but often poorly developed or almost absent and apothecia seated on the blackened remains of Verrucaria, soredia absent; apothecia generally crowded, orange-yellow to deep orange or orange-red, thalline exciple scarcely developed but 'proper' margin conspicuous when young, less visible as the apothecia mature and become convex. Widespread and common on limestone and calcareous stonework, often on concrete wall-cappings, frequently parasitising Verrucaria species before continuing as independent thalli.

Refs: Arup (2009), 120 & 124-6 (photos); Wilk (2012), 24 (photo), 73-5; Wirth (1995), 1: 215 (photo, as C. holocarpa, identity fide Arup, 2009); Wirth et al. (2004), 242 (photo, as C. holocarpa, same photo as Wirth, 1995, printed with better colour); Wirth, Hauck & Schulz (2013), 1: 281, 323 (photo). The identities of the photos in Dobson (2011) are maybe uncertain.

C. oasis is closely similar to and confused with C. holocarpa, though genetically well separated. Arup (2009) clarified the group, confirmimg there are several morphologically very similar species, of which two are now known to be very common in the UK: C. oasis and true C. holocarpa. C. oasis generally has a more continuous, yellowish thallus and the apothecia tend to be more deeply red-orange, tightly clustered, rather uniform in size, with thinner proper margins that more quickly become indistinguishable from the apothecial discs. Measurements, including slight differences in the ascospores, are tabulated by Arup (page 115).


Caloplaca oasis
Caloplaca oasis, close view
Caloplaca oasis, close view
On limestone gravestone, parasitising Verrucaria hochstetteri, Eriswell, Suffolk, April 2012
Caloplaca oasis with C. aff. austrocitrina
C. oasis (left), with C. aff. austrocitrina (upper right), on limestone church wall, Great Chrishill, North Essex (vice-county), July 2009

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Uploaded September 2013, updated January 2016