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.

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