The Carlisle Mosquito Online

Friday, January 18, 2008


Crater Lichen

(Photo by Kay Fairweather)
On a winter walk when the ground is covered with snow you can still find lichens on vertical surfaces like tree trunks, tombstones, fence railings and the sides of stonewalls. I have many lichen friends living in the Green Cemetery. It is easy to visit them without skis or snowshoes even after heavy snow because the roads in the cemetery are kept plowed.

Name. Some lichens do not have common names and the scientific name is often a mouthful. The crater lichen is twice blessed; its scientific name, Diploschistes scruposus, rolls off the tongue rather nicely and it also has an excellent descriptive common name.

When and where found. There is a nice colony of crater lichen on the stone wall in the open area that divides the newer part of the Green Cemetery from the older part. Lichens grow slowly and are long-lived. I have been visiting this colony since I first saw it in 2005. I also have a small colony on the stone wall in front of my property on School Street. There are other crater lichens but this is the most common one occurring in North America. As lichens go, it is aggressive and unstoppable, growing over other species in its path. It grows only on non-calcareous rock in exposed sites. I have never found it on stone walls or rocks in the woods.

Identifying characteristics. The body of the lichen, called the thallus, is a very pale gray with a hint of green and it grows like a thick lumpy crust on the rock. Lichens that are closely bonded to their substrate like this are called crustose. (In the upper left and lower right corners of the photo you can see a foliose lichen, i.e., one with a leaf-like thallus.) There are a lot of species of gray crustose lichens growing on rocks. This one is distinguished by its "craters," each of which contains a spore-bearing structure called an apothecium. The crater opening is only about half to one and a half millimeters wide but despite the small size it is possible to recognize the crater formations with the naked eye.

Once you start looking closely at the lichens you will notice the apothecia of many lichens are sitting fully exposed on top of the thallus rather than being embedded in a crater. (Check the photos of the Peppered Rock Shield lichen in the Mosquito archive for December 15, 2006, for a comparison with the crater lichen.)

Chemistry. Lichens produce two major types of chemical compounds: the primary metabolites are intracellular and generally not unique to a particular lichen; the secondary metabolites, of which there are 630 different kinds, are extracellular so they are more accessible and their presence, or absence, is useful in lichen identification. A common reagent for lichen chemical testing is common household bleach. A tiny drop of bleach on the crater lichen will turn red.

Trails Committee night-time walk. Some lichen compounds (but not from the crater lichen) are fluorescent when viewed under long wave ultraviolet light. I will bring my UV light to the Trails Committee walk at Foss Farm on Saturday, January 19,

References. Lichens of North America, by Brodo, Sharnoff and Sharnoff; Lichen Biology, by Thomas H. Nash.

Submissions for the Biodiversity Corner are encouraged from everyone. You can write the column or tell me what you saw and I will write it. The organism doesn't have to be unusual. The only requirements are that it exists in the wild and was seen in Carlisle. Send a note to

2008 The Carlisle Mosquito