Rock Jigsaw Parasite, Opegrapha pulvinata
Opegraphaceae or Open Scribble Lichen Family

A parasite growing on rock jigsaw lichen (Willeya diffractella), penetrating the host and sucking up nutrients. It is visible through its fruiting bodies, which sit on top of the poor host. They are visible at left as small black lumps on top of the rock jigsaw thallus. The closeup at right shows that the parasite has rather shapeless fruiting bodies, resembling a pile of bear poop. The fruiting bodies are lirellae, as in the scribble lichens. Each lump is actually several small lirellae in a clump. Within the lirellae the parasite forms its spores inside sacs called asci (singular ascus); each ascus contains eight spores (bottom left). At first transparent, the spores turn dark brown as they mature (bottom right). Each spore is made of four cells and can infect other rock jigsaws or related lichens.

Common in the Park. Look for the unfortunate host, the rock jigsaw, growing on limestone rocks. Then look closely to see if the tiny (less than a millimeter) black fruiting bodies are present. Not all rock jigsaws are infected.

Interestingly, most members of the genus Opegrapha are true lichens in their own right, resembling the scribble lichen. This species, and some others, however, have evolved to parasitize other lichens.


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