Rhizocarpon reductum

7 February 2025

TAXON:	Rhizocarpon reductum Th. Fr. (1874)
RECENT SYNONYMS:	–
FAMILY:	Rhizocarpaceae
GROWTH FORM:	Crustose, heteromerous
SUBSTRATES:	Siliceous rock, often pebbles
PHOTOBIONT:	Chlorococcoid alga
REPRODUCTION:	Apothecia
ASCUS:	Slightly clavate; apex Rhizocarpon-type; 8 spores
SPORES:	Ascospores colourless, muriform, (6-)8-13(-17) cells in optical section, 25-35 x 10-15 µm
NOTABLE FEATURES:	Thallus pale grey to brown, areolate; prothallus black, normally conspicuous, sometimes fimbriate; apothecia small, (0.3-)0.4-0.6(-0.8) mm, black, mainly innate, typically with flat, rough surface
CHEMICAL TESTS:	Thallus typically K+ yellow, Pd+ orange (stictic acid); rarely all negative
HABITAT:	Wherever suitable substrate is available, including gravestones, stable pebbles
DISTRIBUTION:	Common and widespread in Britain
CONSERVATION STATUS:	Least Concern
LICHENICOLOUS FUNGI:	Endococcus perpusillus, E. propinquus, Muellerella erratica, M. pygmaea, Phaeospora parasitica, Polycoccum arnoldii.
IDENTIFICATION DIFFICULTY:	Green 2: Field identification possible with care
CONFUSION SPECIES:	Other ‘white’ Rhizocarpon species (see field notes)

FIELD NOTES

Rhizocarpon reductum is the most common and widespread Rhizocarpon species in Britain. You’ll find it more or less anywhere on suitable substrate, including for example on flint pebbles and sandstone gravestones as shown below. It’s sometimes subdivided into varieties based on form, which may one day be reassessed as subspecies or species in themselves:

R. reductum var. cinerea, with a pale-grey thallus and more-or-less concentrically arranged apothecia
R. reductum var. dispersa, with dispersed areoles on a conspicuous black prothallus, and occurring more often on vertical surfaces
R. reductum var. fimbriata, with a small thallus fringed by a conspicuous thread-like, black prothallus, and occurring more often on pebbles

The first and third of these varieties are illustrated in the galleries below. The three varieties aren’t recorded separately in Britain, but it would be good practice to note the relevant variety as a comment on any R. reductum records you submit to the British Lichen Society or other biological recording authority.

R. reductum requires a bit of care to identify, as some other Rhizocarpon species look similar. If you’ve encountered a ‘white’ rather than a ‘yellow’ Rhizocarpon, and you think it might be R. reductum, you should first be sure of the substrate and location. Your lichen isn’t R. reductum if it’s on:

Calcareous rock such as limestone, in which case it’s likely to be R. petraeum or R. umbilicatum
Base-rich siliceous rock such as basalt, in which case it’s likely to be R. distinctum or R. petraeum
Damp rocks close by streams or lakes, or in wet oceanic woodland or upland habitat, in which case it’s likely to be R. infernulum or R. lavatum

But if your lichen is on hard, dry, ‘acidic’ siliceous rock, however, it’s a good candidate for R. reductum. Some confusion species still remain, though, and you must continue to rule them out as follows:

R. hochstetteri, which you might encounter in montane habitat, but whose thallus tends to be browner, with larger apothecia 0.6-1.0(-1.3) mm in diameter
R. polycarpum, which you might encounter in coastal or montane habitat, but whose thallus tends to be dark brown, and whose medulla is more likely to test K- and Pd-
R. richardii, which you might encounter in coastal habitat, but whose thallus tends to be brown with a grey-violet tinge, and whose medulla often tests C+ red from gyrophoric acid

If you can confidently rule out these species, you can conclude your lichen is almost certainly R. reductum. Any remaining small doubt would relate to Rhizocarpon species that are much rarer in Britain, and which you most probably wouldn’t encounter unless you’re specifically seeking them out.

That said, if your remaining doubt relates instead to uncertainty about one or more of the confusion species above, your only recourse is microscopic examination. In that case, see the lab notes below.

LAB NOTES

Rhizocarpon reductum can normally be identified without microscopy, if you take good care. However, in some cases you may find you need to examine ascospores to confirm conclusively that you have R. reductum, rather than another similar-looking ‘white’ Rhizocarpon.

Fortunately, the spores of R. reductum are highly distinctive. Mature spores are colourless, and they have many cells arranged like bricks in a wall. These ‘muriform’ spores have usually at least eight and as many as a dozen or more cells visible at once under the microscope.

By contrast, the only confusion species above that have truly muriform spores are R. lavatum and R. petraeum. And these two species can be fairly readily ruled out by substrate or location. The other confusion species tend to have 1-septate spores, or at best ‘submuriform’ spores without as many cells as the spores of R. reductum. The spores of some confusion species also turn brown with age.

It’s imperative, however, to ensure that you examine mature ascospores. Immature spores of R. reductum won’t appear muriform, as their cells haven’t yet fully divided. Both mature and immature spores of R. reductum are illustrated below.

SPECIMENS

Rhizocarpon reductum var. fimbriata : England : VC25 East Suffolk : TM4468 : September 2024 : On flint pebble