Lichenes

Although the majority of lichens are terrestrial, some grow in areas subject to salt spray and are a conspicuous feature of the upper reaches of many rocky shores. Some species are able to withstand daily submersion by the tide and are found in the eulittoral zone where they compete for space with other organisms, for example, barnacles and seaweeds. Lichens are complex plants comprised of fungal and algal components existing in what is believed to be a mutually beneficial association known as symbiosis. The exact nature of the relationship is still debated, but photosynthesis of the algal cells provides the fungus with organic nutrients, while the fungus provides the alga with support and protection.

The main part of the lichen, known as the thallus, is typically seen in one of three forms; crustose, foliose and fruticose. The flattened, crust-like thallus of crustose lichens is firmly attached to the sub­stratum and very difficult to remove without damage to the lichen. Foliose lichens have a horizontal, leaf-like thallus usually loosely attached to the substratum and often divided into lobes. In the fruticose lichens the thallus stands upright or hangs downwards from the base.

Reproduction of lichens is poorly understood but it is known to vary within the group. Reproductive bodies, known as ascocarps, are seen on the surface of some lichens as saucer-shaped structures (PI. If). In other species, the ascocarps are embedded in the thallus and open onto the surface via a pore. They are known as perithecia. The ascocarps contain fungal spores and their structure together with that of the spores is important in taxonomy. The spores are dispersed by wind and after germination must combine with suita­ble algal cells in order to re-establish the partnership, a process known as lichenization. Vegetative reproduction has also been described in lichens. Pieces of the thallus become detached and are dispersed by wind, insects and birds and grow into new lichens.

Lichens grow best on stable surfaces and do not flourish on friable slates and shales or in areas where there is atmospheric pollution. They are affected by the chemical nature of the substratum, different species growing on acidic rocks compared with calcareous rocks. Lichens are generally regarded as slow-growing and long-lived, and while some show almost no growth, others attain growth rates of

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LICHENES

10--30 mm per year. Longevity in excess of 50 years has been recorded for some species. Lichens found intertidally and at the littoral fringe-terrestrial boundary often form distinct zones accord­ing to their ability to withstand submersion in sea water and salt spray. On rocky shores, the most striking of these is a zone of the black lichens Verruca ria maura (p. 67) and Lichina (p. 69), above which is a zone of the orange lichens, Caloplaca marina (p. 69) and Xanthoria parietina (p. 69) together with the grey-green coloured Ramalina siliquosa (p. 68).

Although some lichens are difficult to identify, those described in this book can be recognized on the basis of external characters seen under a good quality hand lens. For more advanced studies, examination under a microscope is essential and chemical testing may be necessary. Only the most common and widespread species are included here.

Verrucaria maura Wahlenberg (PI. lc) Thallus crustose, smooth; adheres closely to substratum and is difficult to remove. Surface of thallus crossed by meshwork of fine cracks. Black in colour. Perithecia visible as black spots.

V. maura is a very common lichen, widely distributed in north-west Europe. Where conditions are favourable it forms a broad, black band on the upper reaches of the littoral fringe, contrasting sharply with the grey-white barnacles and the bright orange-yellow Xanthoria (p. 69) and Caloplaca (p. 69). It is often described as being similar in appearance to an oil stain on the rocks. The upper limit of Verruca ria forms the upper limit of the littoral fringe. In the sheltered sea lochs of the west coast of Scotland, it is one of the most characteristic organisms of the upper shore.

";NVerrucaria mucosa Wahlenberg (PI. Id) ; Thallus crustose, smooth, in irregular patches up to 300 mm across; adheres closely to surface of substratum. Green, olive in colour, becoming darker in bright sunlight.

V. mucosa is widely distributed and often abundant on the west coast of Britain on rocks and boulders. On sheltered shores large patches are very conspicuous because of the bright green colour. It extends from the littoral fringe to the middle shore, but does not form a distinct band so characteristic of V. maura (p. 67).

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Leeanora atra (Hudson) Acharius (PI. Ie) Thallus crustose, thick with cracked, warty surface; grey in colour. Ascocarps with black centres and pale, irregular 11Ulrgins.

L. atra is a common terrestrial lichen of north-west Europe, found on stones and walls. It extends into the lower reaches of the terrestrial fringe where it withstands salt spray.

A second species of Lecanora, Leeanora gangaleoides Nylander, is common on rocks close to the sea. It is similar in appearance to L. atra but the margins of the ascocarps are smooth and when the ascocarp is cut open it is green-brown in colour, compared with the purple-brown colour of L. atra.

Ochroleehia parella (Linnaeus) Massalongo (PI. If) Thallus crustose, thick and warty; grey in colour, edged with white. Ascocarps with pinkish-grey centres and thick margins; with powdery surface.

O. parella is a common terrestrial lichen of north-west Europe, found on stones, walls and trees. It extends into the lower reaches of the terrestrial fringe where it is 'sometimes abundant on rocks exposed to salt spray.

Ramalina siliquosa (Hudson) A.L. Smith (PI. Ig) Thallus fruticose, upright or hanging downwards and 11Uly be sparsely branched; strap-shaped; up to 100 mm in length. Light grey-green colour. Ascocarps usually at distal ends of branches, white or pale brown in colour.

R. siliquosa is widely distributed in north-west Europe. It is found in patches at the upper limit of the littoral fringe and above, and in some situations forms a distinct band above Xan~horia (p. 69) and Caloplaca (p. 69). It often grows with the moss, Crimmia maritima (p. 69).

Anaptyehia fusea (Hudson) Vainio (PI. Ih) Thallus foliose, thick, with branching, overlapping lobes. Dark green and spongy when wet; brown and stiff when dry. Ascocarps with black centres and brown, wavy margins.

A. fusca is widely distributed in north-west Europe, growing on rocks at the littoral fringe-terrestrial boundary.

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LICHENES

Caloplaca marina (Weddell) Zahlbruckner (PI. 2a) Thallus crustose, with many small granules, particularly in centre of thallus, together with ascocarps. Thallus and ascocarps rusty-orange colour. May be confused with Xanthoria parietina (p. 69) but thallus flatter and not leafy.

C. marina is widely distributed and common in north-west Europe. Along with Xanthoria parietina it often forms a broad, yellow-orange band just above the littoral fringe.

Caloplaca thallincola (Weddell) Du Rietz often occurs with C. marina. They are difficult to separate but the lighter colour of C. thallincola is a useful character.

Xanthoria parietina (Linnaeus) Th. Fries (PI. 2b) Thallus foliose, lobed; broad and leafy. Loosely attached to substratum. In patches up to about 100 mm across. Bright orange colour changing to greenish-yellow in shaded, humid conditions. Ascocarps deep orange in colour. May be confused with Caloplaca marina (p. 69) but thallus leafy.

X. parietina is widely distributed in north-west Europe growing in profusion on rocks, walls, trees and roofs, especially in areas enriched by nitrogenous compounds. It is also abundant on rocks on the high shore and along with Caloplaca marina often forms a distinct orange-yellow band just above the littoral fringe. In some places it is covered by the highest spring tides.

The maritime moss, Grimmia maritima Turner, often occurs with Xanthoria parietina and Ramalina siliquosa especially on the west coast of Britain, but rarely on calcareous rocks. It is dark, brownish-green in colour and grows as small, compact clumps up to 40 mm in height.

Lichina confinis (O.F. Muller) CA. Agardh (not illustrated) Thallus fruticose, formed· of upright, branched lobes which are less flattened than in L. pygmaea (p. 70). Up to 5 mm in height. Dark brown, black. Rounded ascocarps at ends of branches.

L. confinis is widely distributed and common in north-west Europe. It occurs higher on the shore than L. pygmaea (p. 70) and is found on the upper littoral fringe and above, often with Caloplaca marina (p. 69). It is most abundant on sheltered shores.

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Lichina pygmaea (Lightfoot) c.A. Agardh (PI. 2c) Thallus fruticose, formed of upright, flattened, branching lobes. Up to 10 mm in height. Dark brown, black; brittle when dry. Rounded ascocarps at ends of branches. Grows in tufts. Not to be confused with the red seaweed, Catenella caespitosa (p. 55).

L. pygmaea is widely distributed and common in north-west Europe. It has been recorded from the lower littoral fringe to about the middle shore where it is regularly covered by the tide and tolerates exposure to wave action. In some situations it grows in tufts covering several square metres of rock surface. The lichen harbours large numbers of invertebrates, particularly the isopod, Campecopea hirsuta (p. 306) which feeds on Lichina, and the bivalve, Lasaea rubra (p.252).

Arthopyrenia balodytes (Nylander) Arnold (not illustrated) Thallus crustose. On hard rocks seen as small, greenish-brown or black patches; on calcareous rock and shells is embedded in the substratum and appears as small black dots.

A. halodytes has a worldwide distribution. It occurs on the upper shore on rocks and on the shells of a number of common intertidal invertebrates, such as barnacles, mussels and limpets.

Alvin, K.L. 1977. The Observer's book of lichens. London: Frederick Warne.

Dobson, F.S. 1981. Lichens. An illustrated guide. (2nd revised edition). Richmond: Richmond Publishing.

Duncan, U.K. 1970. Introduction to British lichens. Arbroath: T. Bunde.

Fl~tcher, A. 1975. Key for the identification of British marine and maritime lichens. I. Siliceous rocky shore species. Lichenologist 7, 1-52.

Fletcher, A. 1975. Key for the identification of British marine and maritime lichens. II. Calcareous and terricolous species. Lichenologist 7, 73-115.

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