Ramalina Usnea
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''Ramalina usnea'' is a species of
corticolous This glossary of mycology is a list of definitions of terms and concepts relevant to mycology, the study of fungi. Terms in common with other fields, if repeated here, generally focus on their mycology-specific meaning. Related terms can be found ...
(bark-dwelling),
fruticose lichen A fruticose lichen is a form of lichen fungi that is characterized by a coral-like shrubby or bushy growth structure. It is formed from a symbiotic relationship of a photobiont such as green algae or less commonly cyanobacteria and one, two or m ...
in the family
Ramalinaceae The Ramalinaceae are a family of lichen-forming fungi in the order Lecanorales. First proposed by Carl Adolph Agardh in 1821, the family now comprises 63 genera and about 750 species. Ramalinaceae lichens exhibit diverse growth forms, includin ...
. It grows on tree bark across the Americas and parts of East Africa. Growing up to long, it forms pale greenish-grey branching strands that darken when dry. The species is highly adaptable: it forms coarse, flattened branches in foggy environments and develops finer, more delicate forms in humid coastal areas. First described by
Carl Linnaeus Carl Linnaeus (23 May 1707 – 10 January 1778), also known after ennoblement in 1761 as Carl von Linné,#Blunt, Blunt (2004), p. 171. was a Swedish biologist and physician who formalised binomial nomenclature, the modern system of naming o ...
in 1767, it underwent a few taxonomic revisions until Reginald Heber Howe, Jr. established its current name in 1914. Unlike many lichens that grow primarily at their tips, ''R. usnea'' expands along its entire length by continuously rebuilding its cell walls. It serves as an important component of some ecosystems, particularly in the
Galápagos Islands The Galápagos Islands () are an archipelago of volcanic islands in the Eastern Pacific, located around the equator, west of the mainland of South America. They form the Galápagos Province of the Republic of Ecuador, with a population of sli ...
' transition zones, where it forms characteristic draperies on trees and is used as nesting material by certain birds.


Taxonomy

The lichen was first
scientifically described A species description is a formal scientific description of a newly encountered species, typically articulated through a scientific publication. Its purpose is to provide a clear description of a new species of organism and explain how it diffe ...
by
Carl Linnaeus Carl Linnaeus (23 May 1707 – 10 January 1778), also known after ennoblement in 1761 as Carl von Linné,#Blunt, Blunt (2004), p. 171. was a Swedish biologist and physician who formalised binomial nomenclature, the modern system of naming o ...
in 1767 as ''Lichen usnea''. In his protologue, Linnaeus characterised it as a filamentous, (hanging) lichen with a smooth, compressed
thallus Thallus (: thalli), from Latinized Greek (), meaning "a green shoot" or "twig", is the vegetative tissue of some organisms in diverse groups such as algae, fungi, some liverworts, lichens, and the Myxogastria. A thallus usually names the entir ...
. He described the species as thread-like and branching, reaching up to a foot in length, with obtuse branch angles and a thallus that could separate into two layers. He noted that specimens were whitish when fresh but became grey-brown when dry. Linnaeus based part of his description on observations by the Dutch-born botanist
Nikolaus Joseph von Jacquin Nikolaus Joseph Freiherr von Jacquin (16 February 172726 October 1817) was a scientist who studied medicine, chemistry and botany. Biography Born in Leiden in the Netherlands, he studied medicine at Leiden University, then moved first to Par ...
, who had collected extensively in the Caribbean and documented the species growing on trees in
Martinique Martinique ( ; or ; Kalinago language, Kalinago: or ) is an island in the Lesser Antilles of the West Indies, in the eastern Caribbean Sea. It was previously known as Iguanacaera which translates to iguana island in Carib language, Kariʼn ...
, as well as reports of its presence in the
East Indies The East Indies (or simply the Indies) is a term used in historical narratives of the Age of Discovery. The ''Indies'' broadly referred to various lands in Eastern world, the East or the Eastern Hemisphere, particularly the islands and mainl ...
,
St. Helena Saint Helena (, ) is one of the three constituent parts of Saint Helena, Ascension and Tristan da Cunha, a remote British overseas territory. Saint Helena is a volcanic and tropical island, located in the South Atlantic Ocean, some 1,874 km ...
, and
Madagascar Madagascar, officially the Republic of Madagascar, is an island country that includes the island of Madagascar and numerous smaller peripheral islands. Lying off the southeastern coast of Africa, it is the world's List of islands by area, f ...
. It underwent several
taxonomic 280px, Generalized scheme of taxonomy Taxonomy is a practice and science concerned with classification or categorization. Typically, there are two parts to it: the development of an underlying scheme of classes (a taxonomy) and the allocation ...
changes over time.
Erik Acharius Erik Acharius (10 October 1757 – 14 August 1819) was a Swedish botanist who pioneered the Taxonomy (biology), taxonomy of lichens and is known as the "father of lichenology". Acharius was famously the last pupil of Carl Linnaeus. Life Ac ...
reclassified it as ''Parmelia usneoides'' in 1803, and again in 1810 as ''Alectoria usneoides''. Reginald Heber Howe, Jr. finally established its current name, ''Ramalina usnea'', in 1914. The
lectotype In biology, a type is a particular specimen (or in some cases a group of specimens) of an organism to which the scientific name of that organism is formally associated. In other words, a type is an example that serves to anchor or centralizes ...
specimen ( Linnaean Herbarium 1273-278), collected from
Martinique Martinique ( ; or ; Kalinago language, Kalinago: or ) is an island in the Lesser Antilles of the West Indies, in the eastern Caribbean Sea. It was previously known as Iguanacaera which translates to iguana island in Carib language, Kariʼn ...
, was designated by
Henry Imshaug Henry Andrew Imshaug (1925–2010) was an American lichenologist notable for work on the genus ''Buellia'' and his "enormous and important collections from the Rocky Mountains, Great Lakes region, West Indies and subantarctic islands, together with ...
in 1972. Chemical analysis revealed that the type specimen contains usnic and divaricatic acids—a variant differing from earlier reports that identified only ramalinolic and sekikaic acids. It is also notable as the only confirmed historical collection of ''R. usnea'' from the
Lesser Antilles The Lesser Antilles is a group of islands in the Caribbean Sea, forming part of the West Indies in Caribbean, Caribbean region of the Americas. They are distinguished from the larger islands of the Greater Antilles to the west. They form an arc w ...
, where the related species '' R. peranceps'' is now more commonly found. A 1978 study revealed significant insights into the taxonomy of the ''R. usnea''
species complex In biology, a species complex is a group of closely related organisms that are so similar in appearance and other features that the boundaries between them are often unclear. The taxa in the complex may be able to hybridize readily with each oth ...
. Three distinct chemical races were identified within ''R. usnea'', each characterised by different substances in the medulla: a sekikaic acid race, a divaricatic acid race, and a race lacking diagnostic medullary compounds. This chemotaxonomic approach has been important in understanding the species' diversity. Research has shown that ''R. subanceps'' and ''R. bogotensis'' are synonyms of ''R. usnea'', representing different morphological expressions within the species' range of variation. The ''R. usnea'' complex has been found to comprise four distinct species: ''R. usnea'', '' Ramalina anceps'', '' Ramalina chilensis'', and a newly described species, '' Ramalina sharpii''. Some researchers have questioned the distinction between ''Ramalina usnea'' and ''R. anceps'', suggesting that ''R. anceps'' may represent a
chemotype A chemotype (sometimes chemovar) is a chemically distinct entity in a plant or microorganism, with differences in the composition of the secondary metabolites. Minor genetic and epigenetic changes with little or no effect on morphology or anatomy ...
of ''R. usnea''. The two taxa are morphologically similar, with ''R. anceps'' appearing thinner and less contorted, but these characteristics are considered unreliable for species delimitation.


Description

The
thallus Thallus (: thalli), from Latinized Greek (), meaning "a green shoot" or "twig", is the vegetative tissue of some organisms in diverse groups such as algae, fungi, some liverworts, lichens, and the Myxogastria. A thallus usually names the entir ...
is pendulous, reaching up to in length, with irregular branching and a solid (non-hollow) structure. Branches are typically flattened (though occasionally slightly cylindrical), often contorted, with apices that taper to a slender or rounded form, and measure 0.5–2.5 mm wide.
Soredia Soredia are common reproduction, reproductive structures of lichens. Lichens asexual reproduction, reproduce asexually by employing simple fragmentation and production of soredia and isidia. Soredia are powdery propagules composed of fungus, fung ...
are absent.
Pseudocyphella Pseudocyphellae (singular ''pseudocyphella'') are structures in lichens that appear as tiny pores on the outer surface (the cortex) of the lichen. They are caused when there is a break in the cortex of the lichen, and the medullary hyphae extend ...
e are linear in form, occurring both marginally and laminally. The
cortex Cortex or cortical may refer to: Biology * Cortex (anatomy), the outermost layer of an organ ** Cerebral cortex, the outer layer of the vertebrate cerebrum, part of which is the ''forebrain'' *** Motor cortex, the regions of the cerebral cortex i ...
is more or less distinct, about 10 
μm The micrometre (Commonwealth English as used by the International Bureau of Weights and Measures; SI symbol: μm) or micrometer (American English), also commonly known by the non-SI term micron, is a unit of length in the International System ...
thick; chondroid tissue clearly and heavily cracked.
Apothecia An ascocarp, or ascoma (: ascomata), is the fruiting body ( sporocarp) of an ascomycete phylum fungus. It consists of very tightly interwoven hyphae and millions of embedded asci, each of which typically contains four to eight ascospores. As ...
are common, marginal or laminal; the is flat, becoming convex with age; the is entire, without
pseudocyphella Pseudocyphellae (singular ''pseudocyphella'') are structures in lichens that appear as tiny pores on the outer surface (the cortex) of the lichen. They are caused when there is a break in the cortex of the lichen, and the medullary hyphae extend ...
e;
hymenium The hymenium is the tissue layer on the hymenophore of a fungal fruiting body where the cells develop into basidia or asci, which produce spores. In some species all of the cells of the hymenium develop into basidia or asci, while in oth ...
40–45 μm; subhymenium 20–30 μm; the is 50–70 μm thick.
Ascospore In fungi, an ascospore is the sexual spore formed inside an ascus—the sac-like cell that defines the division Ascomycota, the largest and most diverse Division (botany), division of fungi. After two parental cell nucleus, nuclei fuse, the ascu ...
s are long-, 2-celled, often with 3–6 additional
septa SEPTA, the Southeastern Pennsylvania Transportation Authority, is a regional public transportation authority that operates bus, rapid transit, commuter rail, light rail, and electric trolleybus services for nearly four million people througho ...
, typically 18–22 by 3–4 μm.


Similar species

''Ramalina usnea'' can be distinguished from ''
Usnea ''Usnea'' is a genus of fruticose lichens in the large family Parmeliaceae. The genus, which currently contains roughly 130 species, was established by Michel Adanson in 1763. Species in the genus grow like leafless mini-shrubs or tassels anch ...
'' (bear lichen) species by its flattened, two-sided branches that lack a central cord of supporting tissue Several closely related species share morphological features with ''Ramalina usnea'', forming what has been called the ''R. usnea'' complex. '' R. anceps'' is primarily found in the West Indies, where it grows in moist forests up to 900 m elevation. While it can be difficult to distinguish from finely-branched forms of ''R. usnea'', it can usually be identified by its very shiny cortex, more terete (rounded) branches, general absence of striations, and more rigid, less twisted branch pattern. It also has distinctive chemistry, containing
norstictic acid Norstictic acid is a depsidone produced as a secondary metabolites in lichens. The compound contains both an aldehyde carbonyl group and an adjacent hydroxyl In chemistry, a hydroxy or hydroxyl group is a functional group with the chemical fo ...
with
salazinic acid Salazinic acid is a depsidone with a lactone ring. It is found in some lichens, and is especially prevalent in ''Parmotrema'' and '' Bulbothrix'', where its presence or absence is often used to help classify species in those genera. History In ...
. A variant, ''R. anceps'' var. ''peranceps'', has somewhat more flattened and striated branches and contains only salazinic acid in its medulla. '' Ramalina sharpii'' is restricted to pine-oak forests above 2000 m in the mountains of central Mexico. It is distinguished from ''R. usnea'' by its much broader branches, which can reach 2–30 mm in width, very prominent white striations, and longer thalli that can grow up to 100 cm. The species also differs in its medullary chemistry, containing norstictic acid with or without salazinic acid. '' Ramalina chilensis'' is an
endemic Endemism is the state of a species being found only in a single defined geographic location, such as an island, state, nation, country or other defined zone; organisms that are indigenous to a place are not endemic to it if they are also foun ...
species of central and northern Chile, found in coastal habitats and on seaward slopes of coastal mountains where it is influenced by sea fogs. It differs from ''R. usnea'' in its shorter length, rarely exceeding 20 cm, and its consistently broad, flat branches with prominent white striations. The species has an unusual chemistry combining sekikaic acid and norstictic acid, and is restricted to Mediterranean climate areas of Chile. ''
Ramalina menziesii ''Ramalina'' is a genus of greenish fruticose lichens that grow in the form of flattened, strap-like branches. Members of the genus are commonly called strap lichensField Guide to California Lichens, Stephen Sharnoff, Yale University Press, 201 ...
'' from
California California () is a U.S. state, state in the Western United States that lies on the West Coast of the United States, Pacific Coast. It borders Oregon to the north, Nevada and Arizona to the east, and shares Mexico–United States border, an ...
has sometimes been confused with ''R. usnea'' but can be readily distinguished by its distinctive net-like reticulate branching pattern. Additionally, ''R. menziesii'' is restricted to the
Pacific Coast Pacific coast may be used to reference any coastline that borders the Pacific Ocean. Geography Americas North America Countries on the western side of North America have a Pacific coast as their western or south-western border. One of th ...
north of
Sinaloa Sinaloa (), officially the (), is one of the 31 states which, along with Mexico City, compose the Federal Entities of Mexico. It is divided into 18 municipalities, and its capital city is Culiacán Rosales. It is located in northwest Mexic ...
, Mexico, while ''R. usnea'' is not found in this region.


Development

The development and growth patterns of ''Ramalina usnea'' have been studied through field observations and microscopic analysis. Research has shown that growth occurs throughout the entire thallus rather than being limited to specific zones, a pattern known as diffuse growth. When measured over approximately one year, thallus segments showed length increases roughly proportional to their initial size, with some variation possibly related to seasonal conditions.
Transmission electron microscopy Transmission electron microscopy (TEM) is a microscopy technique in which a beam of electrons is transmitted through a specimen to form an image. The specimen is most often an ultrathin section less than 100 nm thick or a suspension on a g ...
shows that diffuse growth in ''R. usnea'' is marked by a unique cell wall development: new layers are continually deposited near the protoplast while older layers become disrupted and mix with electron-transparent materials. This pattern differs significantly from typical fungal growth, where a single discrete cell wall is maintained. In ''R. usnea'', the cell walls are instead repeatedly disrupted and replaced during tissue expansion, with new structural components being deposited to their interior at the interface with the protoplast. This type of development has been previously documented in the lichens ''Ramalina menziesii'' and ''Usnea longissima''. New branches arise from small buds on mature tissue, typically emerging near pseudocyphellae and at fracture points along the thallus. These buds begin as minute swellings on the edges of existing branches and develop an inrolled apex (curved inward at the tip) that persists as the branch elongates. The species shows no evidence of apical branching through bifurcation, unlike some related species. The thallus cortex consists of densely packed fungal cells with very thick walls, oriented parallel to the branch axis. These cortical cells are distinctive in having cell walls that occupy significantly more volume than their cellular contents. When viewed in cross-section, the cells appear to be grouped in bundles, suggesting that new cells grow within the wall material of older neighboring cells. This pattern of continued cell wall accumulation may enable the mature tissue to maintain its capacity for elongation growth. Unlike many other lichens where growth is primarily restricted to the margins or tips, ''R. usnea'' exhibits growth throughout its structure. The thallus can fragment easily, particularly in its finer portions, with new branches often developing from the broken ends. This process can result in the formation of a sympodial growth pattern, where the main axis is built from a series of successive branches.


Habitat and distribution

''Ramalina usnea'' has a broad distribution throughout the Western Hemisphere. In North America, it occurs from southern
Florida Florida ( ; ) is a U.S. state, state in the Southeastern United States, Southeastern region of the United States. It borders the Gulf of Mexico to the west, Alabama to the northwest, Georgia (U.S. state), Georgia to the north, the Atlantic ...
and
Texas Texas ( , ; or ) is the most populous U.S. state, state in the South Central United States, South Central region of the United States. It borders Louisiana to the east, Arkansas to the northeast, Oklahoma to the north, New Mexico to the we ...
in the United States, extending south through the
West Indies The West Indies is an island subregion of the Americas, surrounded by the Atlantic Ocean, North Atlantic Ocean and the Caribbean Sea, which comprises 13 independent island country, island countries and 19 dependent territory, dependencies in thr ...
, Mexico, and Central America into northern South America. Along the South American continent, it is found from the northern regions southward along the east coast through Brazil, Paraguay, and Uruguay to northern Argentina. The species has also been reported from several locations in the Old World, with populations documented in
East Africa East Africa, also known as Eastern Africa or the East of Africa, is a region at the eastern edge of the Africa, African continent, distinguished by its unique geographical, historical, and cultural landscape. Defined in varying scopes, the regi ...
that closely resemble the American specimens. The species shows considerable morphological variation in response to its environment. For example, in foggy, high-elevation areas it forms coarse thalli with broad, flattened branches, while in humid coastal regions it develops finer, more delicate structures. These striations may serve an adaptive function in water uptake from fog. In contrast, populations in tropical coastal areas of Central America and insular habitats in the West Indies and Galápagos Islands, where fog is rare but relative humidity remains consistently high, tend to develop finer, more delicate branching patterns. In the Galápagos Islands, ''R. usnea'' is especially abundant in the transition zone, thriving in semi-arid conditions influenced by coastal fog. It is commonly found on shrubs and trees, forming dense, pendulous masses. The species is particularly associated with ''
Psidium galapageium ''Psidium oligospermum'', the Galápagos guava or guayabillo, is a small tree or shrub native to the tropical Americas, ranging from Mexico through the Revillagigedo Islands, Central America, Cuba, Puerto Rico, the Windward Islands, the Galápago ...
'' forests, where it drapes over branches, creating a characteristic feature of this habitat. Three main chemical variants (races) of ''R. usnea'' exist across its range. The sekikaic acid race (usually with homosekikaic acid) is the most abundant and occurs throughout the species' distribution. The divaricatic acid race is most common in Colombia and Venezuela, occurs in the Galápagos Islands, and is found through the West Indies to Florida, but appears absent from Central America and eastern South America. A race lacking diagnostic medullary compounds is found over most of the geographic range and is particularly abundant in eastern South America.


Ecology

Research indicates that ''R. usnea'' shows morphological
plasticity Plasticity may refer to: Science * Plasticity (physics), in engineering and physics, the propensity of a solid material to undergo permanent deformation under load * Behavioral plasticity, change in an organism's behavior in response to exposur ...
—modifying its form in response to fog and wind exposure—to enhance water capture and gain a competitive edge in its habitat. When growing on columnar substrates in fog-influenced environments, the lichen's thalli show distinct variations in form depending on their position. On
leeward In geography and seamanship, windward () and leeward () are directions relative to the wind. Windward is ''upwind'' from the point of reference, i.e., towards the direction from which the wind is coming; leeward is ''downwind'' from the point o ...
(sheltered) sides, ''R. usnea'' thalli become significantly more pendulous and longer, reaching lengths of nearly 6 cm, while windward-facing thalli remain shorter at around 3 cm and more erect. This morphological variation appears to be an adaptation for harvesting water droplets from foggy air, with the more pendulous forms better suited to capturing moisture in turbulent
airflow Airflow, or air flow, is the movement of air. Air behaves in a fluid manner, meaning particles naturally flow from areas of higher pressure to those where the pressure is lower. Atmospheric air pressure is directly related to altitude, temperat ...
conditions. This adaptability may give ''R. usnea'' a competitive advantage in certain microhabitats. Research in fog-influenced desert environments has shown that ''R. usnea'' tends to dominate the leeward sides of its substrate, where its pendulous growth form is better suited to the local airflow patterns. These morphological responses occur over very small spatial scales, demonstrating the species' ability to adapt to fine-scale environmental gradients. In addition to its morphological plasticity, ''R. usnea'' plays a role in avian ecology by serving as a nesting material for certain bird species. In northeastern Argentina, the tropical parula (''Setophaga pitiayumi'') constructs its nests within masses of ''R. usnea'', utilising the lichen's filamentous and pendulous structure as a secure attachment point. Some researchers suggest that this preference may not only be due to structural support but also to the potential antiparasitic properties of the lichen, as it contains usnic acid, a secondary metabolite with
antimicrobial An antimicrobial is an agent that kills microorganisms (microbicide) or stops their growth (bacteriostatic agent). Antimicrobial medicines can be grouped according to the microorganisms they are used to treat. For example, antibiotics are used aga ...
and
antifungal An antifungal medication, also known as an antimycotic medication, is a pharmaceutical fungicide or fungistatic used to treat and prevent mycosis such as athlete's foot, ringworm, candidiasis (thrush), serious systemic infections such as ...
activity. In the
Galápagos Islands The Galápagos Islands () are an archipelago of volcanic islands in the Eastern Pacific, located around the equator, west of the mainland of South America. They form the Galápagos Province of the Republic of Ecuador, with a population of sli ...
, ''R. usnea'' is a dominant
epiphyte An epiphyte is a plant or plant-like organism that grows on the surface of another plant and derives its moisture and nutrients from the air, rain, water (in marine environments) or from debris accumulating around it. The plants on which epiphyt ...
in the transition zone, where arid lowlands merge into humid interior zones at elevations between 80–250 m. The species frequently coexists with other fog-adapted epiphytes such as ''
Usnea longissima ''Dolichousnea longissima'' ( syn. ''Usnea longissima''), commonly known by the names old man's beard or Methuselah's beard lichen, is a fruticose lichen in the family Parmeliaceae. Description This lichen is fruticose, with very long stems and ...
'' and ''
Teloschistes flavicans ''Teloschistes flavicans'', also known as the golden hair-lichen, is a lichenized species of fungus in the genus ''Teloschistes'' (meaning "split-ends"; a reference to the plant's finely divided thallus and dense coils which appear almost like Br ...
''. It often forms extensive, pale-green pendulous growths on trees such as ''
Psidium galapageium ''Psidium oligospermum'', the Galápagos guava or guayabillo, is a small tree or shrub native to the tropical Americas, ranging from Mexico through the Revillagigedo Islands, Central America, Cuba, Puerto Rico, the Windward Islands, the Galápago ...
''. The lichen's abundance in fog-exposed environments suggests it plays a role in moisture capture. Some species of
Darwin's finches Darwin's finches (also known as the Galápagos finches) are a group of about 18 species of passerine birds. They are well known for being a classic example of adaptive radiation and for their remarkable diversity in beak form and function. They ...
incorporate ''R. usnea'' into their nests, possibly for structural support or antimicrobial benefits.


References

{{Taxonbar , from1=Q10648740 , from2=Q107994955 , from3=Q108133491 , from4=Q107477762 , from5=Q108312095 , from6=Q108311221 , from7=Q108311999
usnea ''Usnea'' is a genus of fruticose lichens in the large family Parmeliaceae. The genus, which currently contains roughly 130 species, was established by Michel Adanson in 1763. Species in the genus grow like leafless mini-shrubs or tassels anch ...
Lichens described in 1767 Taxa named by Carl Linnaeus Lichens of Africa Lichens of Europe Lichens of the Caribbean Lichens of the United States Lichens of Mexico Lichens of Central America Lichens of South America