| TAXON: | Circinaria hoffmanniana (S. Ekman & Fröberg ex R. Sant.) A. Nordin (2016) |
| RECENT SYNONYMS: | Aspicilia contorta subsp. hoffmanniana S. Ekman & Froberg ex R. Sant. (1993) |
| FAMILY: | Megasporaceae |
| GROWTH FORM: | Crustose, heteromerous |
| SUBSTRATES: | Calcareous rock, including soft limestone, concrete, cement |
| PHOTOBIONT: | Chlorococcoid alga |
| REPRODUCTION: | Apothecia; pycnidia rare |
| ASCUS: | Clavate; apex with K/I- tholus and blue thin outer coat; (2-)4(-6) spores |
| SPORES: | Ascospores globose to subglobose, aseptate, (16-)18-28(-35) x (14-)17-26(-30) µm; conidia when present filiform, sometimes curved |
| NOTABLE FEATURES: | Thallus grey to grey-green, rimose-areolate, sometimes scabrid or pruinose; prothallus conspicuous, white, effuse; apothecia cryptolecanorine (aspicilioid), black, immersed to subsessile |
| CHEMICAL TESTS: | Epithecium K+ yellow-brown (in section) |
| HABITAT: | Wherever suitable rock outcrops or worked stone available, including urban areas; tolerant of nitrogen pollution |
| DISTRIBUTION: | Throughout Britain; common in southern and eastern England |
| CONSERVATION STATUS: | Least Concern |
| LICHENICOLOUS FUNGI: | Muellerella lichenicola, Opegrapha parasitica |
| IDENTIFICATION DIFFICULTY: | Green 1: Field identification possible |
| CONFUSION SPECIES: | Circinaria calcarea, C. contorta |
FIELD NOTES
Circinaria hoffmanniana is one of three common Circinaria species in Britain, alongside C. calcarea and C. contorta. Formerly known as Aspicilia contorta subsp. hoffmanniana, it is now considered a separate species in its own right thanks to molecular analysis. It’s tolerant of nitrogen-pollution, so you’ll find it on calcareous substrates even in agricultural and urban areas, including concrete and cement.
At first glance, C. hoffmanniana can be confused with both of its common cousins. All three are saxicolous, with pale, areolate thalli, and characteristically abundant, black ‘aspicilioid’ apothecia that remain more-or-less immersed in the thallus.
Fortunately, distinguishing between these three common Circinaria species is fairly straightforward. You must examine the margin of the thallus closely. Considering the lichen as a whole, if the thallus margin is:
- Delimited, and cracked into contiguous areoles in a radial pattern to the edge, your lichen is C. calcarea
- Effuse, and disaggregating into more-or-less dispersed and circular areoles, your lichen is C. contorta
- Effuse, and neither cracked to the edge nor disaggregating, your lichen is C. hoffmanniana
This method is not completely foolproof, however. Specimens intermediate between C. contorta and C. hoffmanniana can exist, and these two species cannot otherwise be distinguished. As always with lichens, it may not be possible to identify every single thallus without e.g. DNA sequencing!
If you’re a bit unsure, you could try to build further evidence that you have C. hoffmanniana. Relative to C. calcarea in particular, the thallus tends to be thinner, and a bit darker or even greenish rather than chalky white. Indeed, there seems to exist a particular form of C. hoffmanniana looking very similar to C. calcarea but for its grey-green colour and more effuse margin, as in the specimen below. C. hoffmanniana also tends to be somewhat pruinose or scabrid, whereas C. calcarea is smooth.
It’s also worth noting that, unlike C. contorta and C. hoffmanniana, C. calcarea doesn’t tolerate nitrogen-pollution well, and it prefers ‘hard’, i.e. well cemented, limestone. So you’re more likely to have one of the former two species if you’ve found your specimen e.g. on concrete in an urban or eutrophicated environment.
LAB NOTES
Circinaria hoffmanniana can normally be identified without microscopy.
SPECIMENS
Circinaria hoffmanniana : England : VC25 East Suffolk : TM3389 : September 2024 : On limestone chest-tomb : First record for hectad
