Peltigera hydrothyria, commonly known as the waterfan, is a relatively rare aquatic lichen in the family Peltigeraceae, native to North America. It grows in cold, clean mountain streams, where it attaches to rocks and bedrock in shaded, riparian habitats. First described in 1856 as Hydrothyria venosa, it was initially placed in its own genus due to its distinctive gelatinous thallus and aquatic lifestyle. Molecular studies later demonstrated its affinity with the genus Peltigera, leading to its reclassification in 2000. The lichen forms small, blackish rosettes with ruffled margins and prominent veining, features that help it thrive in submerged or semi-aquatic habitats.
Three genetically distinct lineages are now recognized within the species, corresponding to eastern North America (var. hydrothyria) and western North America (vars. gowardii and aquatica). The eastern variety, var. hydrothyria, is listed as Endangered on the IUCN Red List due to significant population declines across its range, driven by habitat loss, pollution, and climate change. Western North American populations (vars. gowardii and aquatica) face similar threats, including logging, land development, and watershed disruption, although these varieties are generally less studied and monitored.
Peltigera hydrothyria plays an ecological role in nutrient-poor environments through its symbiotic relationship with cyanobacteria, which enables nitrogen fixation. Its distinct morphology and ecological preferences differentiate it from other aquatic lichens, such as Leptogium rivale. Ongoing research seeks to clarify the species’ population genetics, habitat requirements, and response to changing environmental conditions, providing critical insights for its conservation. This research has underscored the importance of preserving riparian habitats, not only for this species but also for maintaining broader biodiversity in freshwater ecosystems.
Taxonomy Historical taxonomy
Peltigera hydrothyria was originally described as Hydrothyria venosa by John Lewis Russell in 1856. Before its publication, Russell had circulated the species as Leptogium fontanum in Edward Tuckerman's 1854 Lichenes Americanae Septentrionalis Exsiccati (no. 150), but without a formal description, it remained a nomen nudum. Russell presented his findings at the Essex Institute in 1853, though they were not formally published until three years later. He first encountered the aquatic lichen in 1851 on Bald Mountain in Vermont while exploring with Charles Christopher Frost, remarking on its "grateful aroma" and distinctive morphology. Subsequent collections in Vermont and New Hampshire included one from Wantastiquet Mountain, which later served as the source of the lectotype. This specimen, distributed in the Reliquiae Tuckermanianae exsiccatae (no. 51), illustrated the defining features detailed in Russell's protologue.
Initially placed in its own genus Hydrothyria and assigned to the Collemataceae due to its gelatinous, unstratified thallus and aquatic habitat, the lichen was considered a rare, monotypic genus endemic to North America. Russell compared Hydrothyria to Collema and Leptogium, emphasizing the submarginal apothecia, thin membranous thallus, and fan-shaped () veining—traits he regarded as bridging algae and lichens. Despite sharing structural similarities with Peltigera (notably vein and rhizoid characteristics, as well as ascus morphology), he considered that the distinctive ecology and morphology initially justified a separate genus.
Molecular findings and genus reassignment
Molecular phylogenetics analyses in the late 20th century revealed that Hydrothyria venosa was nested within the genus Peltigera, prompting its reclassification as P. hydrothyria in 2000. To accommodate this newly integrated aquatic lineage, a new section, Hydrothyriae, was established within Peltigera. Historically, P. hydrothyria was regarded as rare and associated with mature riparian ecosystems in the Pacific Northwest, but molecular data showed it formed a strongly supported monophyletic group alongside Solorina, clarifying its evolutionary position within the family.
Infraspecific diversity and varieties
Subsequent molecular studies using multiple genetic markers (ITS, β-tubulin, and EFT2-1) uncovered cryptic diversity within western populations. Researchers identified three monophyletic lineages: an eastern lineage (P. hydrothyria s.str.) retaining methylgyrophorate and methylecanorate, and two western lineages (P. gowardii s.s. and P. gowardii s.l.) lacking detectable secondary metabolites. The western epithet gowardii honors the Canadian lichenologist Trevor Goward, who collected the type specimen of P. gowardii in the Trophy Mountains of British Columbia.
Although initially recognized as separate species based on genetic, chemical, and geographic data, their morphological similarity proved challenging. By 2022, these lineages were reclassified as three varieties of P. hydrothyria: var. hydrothyria (eastern), var. gowardii, and var. aquatica (both western). This revision underscores the importance of integrating molecular, ecological, and chemical evidence to clarify species boundaries within cryptic lichen groups.
Description
Peltigera hydrothyria is an aquatic foliose lichen forming small rosettes of variable size attached to the by one to several holdfasts. The thallus is characterized by a gelatinous, nonstratified () structure that is fully corticated. The thallus appears black when wet and slate gray when dry, with lobes wide. In dry conditions, it becomes thin, papery (about 100 μm thick), and ruffled. Submerged thalli appear translucent dark green or brown, aiding underwater light absorption.
The upper surface is smooth, dull, , and , while the lower surface has cylindrical veins of parallel, compact fungal hyphae that converge into a central rhizoid, anchoring the lichen to its substrate. The thallus of var. gowardii is typically dark gray when dry but takes on a purplish-brown, translucent appearance when fully submerged, giving it a texture and coloration reminiscent of seaweed. In the Pacific Northwest, western varieties (vars. gowardii and aquatica) grow as loose, ruffled clumps on submerged rocks, maximizing surface area for light capture and gas exchange in streams. In addition to rocks, occasional growth on submerged wood or aquatic plant stems, such as Darmera peltata, has also been documented. The lobes, typically medium-sized and up to wide, exhibit pronounced veining, a characteristic feature that aids in structural integrity under submerged conditions. The cortex is , thin, hyaline, and approximately 8–10 μm thick. Under even closer microscopic examination, the internal cellular organisation reveals additional complexities. Young fungal cells contain a full complement of cellular components distributed throughout their volume, with the notable exception of specialized structures called dictyosomes. As these cells age, they develop an intricate network of internal membranes that eventually form into specialized compartments called vacuoles. These vacuoles, which are fluid-filled spaces within the cells, may serve multiple functions: helping maintain consistent internal conditions within the lichen, storing metabolic products, and potentially playing a role in secreting substances. The cell contents between these vacuoles appear to be held in place by limiting membranes, suggesting a highly organized internal structure that helps the lichen maintain its functions even in challenging aquatic environments.
Peltigera hydrothyria forms submarginal apothecia (fruiting bodies), setting it apart from most Peltigera species with marginal apothecia. These reddish-brown structures are plane to concave. The asci are 8-spored, producing unornamented, hyaline, 3-septate spores measuring 24–33 by 6.6–7.8 μm. In immature apothecia, the margins are slightly raised and often exhibit a reddish tint, becoming more convex and darkened with age.
Peltigera hydrothyria is a bimembered lichen, forming a symbiotic association between an ascomycete fungus and cyanobacteria (Nostoc) as the photobiont. This photobiont was identified to species in 1964 as Nostoc sphaericum. The thallus structure internally resembles that of the cyanomorph form of P. venosa, with which it shares several morphological features including the presence of a distinct rhizoid and veins with compact internal structure.
The vein structure resembles that of section Peltigera, with a compact central core of parallel, conglutinated hyphae. This anatomical feature represents one of the key characteristics linking it to the genus despite its unusual aquatic habitat and overall morphology.
The hyphae of Peltigera hydrothyria have a characteristic structural feature—multiperforate septa. Unlike the single-pore septa commonly observed in most ascomycetes, these multiperforate septa contain multiple pores, as confirmed by electron microscopy. The multiple pores likely improve nutrient and water transport, aiding the lichen's survival in its aquatic habitat by supporting efficient symbiotic exchange.
Photobiont
Peltigera hydrothyria forms a symbiotic relationship with cyanobacteria from the genus Nostoc, which serves as its photobiont. Early studies identified the cyanobiont as Capsosira lowei. Molecular evidence, including 16S rRNA and ITS comparisons, later confirmed its placement within Nostoc, a genus widely associated with lichens and plants.
The photobiont shows distinct structural adaptations when lichenized. Compared to free-living forms, the Nostoc cells within P. hydrothyria develop more numerous photosynthetic structures called thylakoids, which arrange themselves in distinctive spiral whorls toward the cell center. This increased density and organized arrangement of photosynthetic machinery likely enhances the lichen's ability to capture light in its shaded aquatic habitat. The cell wall of the lichenized Nostoc maintains a characteristic four-layered structure similar to its free-living relatives, suggesting that certain fundamental features remain stable despite the symbiotic relationship.
Phylogenetic studies indicate that Nostoc strains in aquatic Peltigera species, including P. hydrothyria, form a distinct lineage. This lineage may also include cyanobacteria from other aquatic lichens, such as Leptogium rivulare, pending further sampling. Aquatic Nostoc strains show morphological plasticity, with pseudodichotomous filament branching in lichenized states but more variable forms in culture. This plasticity suggests that the photobiont is highly responsive to its symbiotic environment. Multipore septa in the fungal hyphae may further improve nutrient and water transport within the thallus, facilitating efficient exchange between partners in nutrient-poor environments. Though P. hydrothyria depends on its Nostoc symbiont, which appears specifically adapted to aquatic habitats, the extent of these adaptations remains under investigation. At Hen Wallow Falls in the Great Smoky Mountains, the lichen thrives in acidic splash zones with pH as low as 4.7, suggesting selective pressures that strengthen the partnership.
Nitrogen fixation by the photobiont allows P. hydrothyria to grow in nutrient-poor, shaded aquatic habitats. The cyanobiont produces hormogonia, specialized structures that enhance resilience and regeneration in fluctuating aquatic ecosystems.
Chemistry