2020 - Lichens of Ireland

Ochrolechia parella

The thallus is white in colour. On close examination specimens can show a bluish tint to buff-white to grey colour. It forms regular circular or oblong growths where the substratum permits and when

there is no competition from other lichens. Sometimes growth bands or zones can be distinguished in the thallus. The edge or prothallus is much whiter than the main body. Sometimes its growth pattern is irregular. The thallus is lumpy, particularly near the center of a specimen.
The fruiting bodies or apothecia are abundant and have a margin the same colour as the thallus. When young or immature the apothecia are small and have a deeply convex disk. When mature they are quite large 2 mm - 3 mm in diameter and in conjunction with the cracked and lumpy thallus give this crustose lichen a rough appearance. The margins of the mature apothecia are very thick and ridged. The apothecia disk is pale grey to pink. The disc itself is flat and frequently covered with a white frost-like coating of minute calcium oxalate crystals. Sometimes the margin of the disk is swollen like an over-inflated tyre and pushes in over the disk surface.
O. parella likes direct sunlight. The thallus is thick, prolific, lumpy and contains numerous apothecia in these conditions. When growing in the shade the thallus is thin with fewer reproductive structures and is often fringed or fimbriate at the edges.

Distribution & Ecology

This is probably Ireland's most common white crustose coastal lichen. It is found in all coastal areas on smooth acid or siliceous rocks.
It favours slightly basic acid rocks such as basalt or gabbro. Gabbros are coarse grained basalts. In the sandstone hills of Kerry and West Cork, O. parella frequently forms large circular colonies. In 1915, the Irish lichenologist Matilda Knowles noted the regularity of a white lichen zone at the top of sea shores in the West of Ireland. She referred to this as the White Belt, with O. parella and Tephromela atra as characteristic species.
Although common around the coast, it is also found inland, particularly west of the river Shannon and in West Cork and Limerick.
It is rare on trees, although specimens have been recorded on mature Elm. The demise of this tree in the last century due to the Dutch Elm disease has likely deprived O. parella of a habitat here.
Ochrolechia parella is an aggressive colonizer of granite and an instrumental force in weathering by the production of calcium oxalate monohydrate and dihydrate, particularly during the formation of new apothecia.

The author has noted very large specimens along the West and South coast of Ireland. A particularly large specimen of 45 cm in length and 22 cm in width was seen near Brandon Head, Co. Kerry. O. parella's growth is sometimes stopped by other lichens. In such situations the O. parella's prothallus thickens to form a bounding wall at the interface between the competing individuals.

Other Names
Lecanora pallescens
Lecanora parella (L.) Ach.
Ochrolechia pallescens (Linnaeus) A. Massal.
Ochrolechia parella (Linnaeus) Massal.
Ochrolechia parella porinoides
Ochrolechia pallescens auct. brit. p.p.

Other Ochrolechia Recorded In Ireland
Ochrolechia subviridis (Heg) Erichsen
Ochrolechia szatalaensis Verseghy
Ochrolechia tartarea (L.) A. Massal.
Ochrolechia turneri (Sm.) Hasselrot

Text and Images © Paul Whelan, 2007, Biology.ie

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