The lichen forms small, blackish rosettes with ruffled margins and prominent veining, features that help it thrive in submerged or semi-aquatic habitats.

Ongoing research seeks to clarify the species’ population genetics, habitat requirements, and response to changing environmental conditions, providing critical insights for its conservation.

[6] Before its publication, Russell had circulated the species as Leptogium fontanum in Edward Tuckerman's 1854 Lichenes Americanae Septentrionalis Exsiccati, but without a formal description, it remained a nomen nudum.

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.

[9][10] Russell compared Hydrothyria to Collema and Leptogium, emphasizing the submarginal apothecia, thin membranous thallus, and fan-shaped (flabelliform) veining—traits he regarded as bridging algae and lichens.

Historically, P. hydrothyria was regarded as rare and associated with mature riparian ecosystems in the Pacific Northwest,[11] but molecular data showed it formed a strongly supported monophyletic group alongside Solorina, clarifying its evolutionary position within the family.

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.

gowardii and aquatica) grow as loose, ruffled clumps on submerged rocks, maximizing surface area for light capture and gas exchange in streams.

[13] The lobes, typically medium-sized and up to 1 cm (3⁄8 in) wide, exhibit pronounced veining, a characteristic feature that aids in structural integrity under submerged conditions.

Young fungal cells contain a full complement of cellular components distributed throughout their volume, with the notable exception of specialized structures called dictyosomes.

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.

The multiple pores likely improve nutrient and water transport, aiding the lichen's survival in its aquatic habitat by supporting efficient symbiotic exchange.

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.

[18] 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.

[9] 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.

[8] These substances are often difficult to detect using standard spot tests and thin-layer chromatography techniques, sometimes requiring multiple analyses of the same specimen using different solvent systems.

[8] This chemical difference between the eastern and western populations was one of the key factors, along with molecular and biogeographic evidence, that led to the recognition of these taxa as distinct varieties.

[10] Another aquatic lichen, Leptogium rivale, occurs in the Cascades, Sierra Nevada, and Colorado mountain ranges, where it often overlaps in habitat with western North American populations of P. hydrothyria.

occurs in small numbers of forested streams in Quebec, New Brunswick, and Nova Scotia, where the climate is perhumid, with year-round wetness due to precipitation exceeding evaporation and transpiration.

[21] This climatic requirement explains its patchy distribution, as suitable conditions exist along the Appalachian chain and coastal eastern Canada but not in intervening areas.

[21] In 2022, the discovery of a previously undocumented population in the Kennedy Lakes Protected Natural Area, New Brunswick, suggested that additional suitable habitats may exist in the humid uplands of that region and adjacent Quebec.

[21] Humidity is critical where thalli remain partially above water during low-flow periods; surrounding wet bryophytes help maintain moisture and prevent desiccation.

[19] Northern California populations occur at elevations up to 3,900 m (12,800 ft), where snowmelt-fed streams maintain stable flows and minimal sedimentation, essential for sustaining suitable habitat.

[8] In the Southern Appalachians, elevated levels of aluminum, iron, and manganese, along with periodic low pH, degrade stream habitats, endangering both aquatic lichens and other organisms like brook trout and macroinvertebrates.

[21] Historical accounts, such as William G. Farlow's 1884 observation of streams "fairly carpeted" with P. hydrothyria, underscore the species' sensitivity to pristine conditions and highlight how easily these habitats can be degraded.

[8][19] Strategies include maintaining or expanding riparian forest buffers to stabilize temperature, humidity, and stream bed conditions, as well as regulating water quality standards to reduce nutrient runoff, sedimentation, and chemical pollution.

[9] Conservation practitioners recommend establishing permanent monitoring sites and employing benthic diatom indices or predictive habitat models to identify priority areas for protection.

[20][8][21] Although the U.S. Northwest Forest Plan's "survey and manage" program is no longer active, its initial efforts to preserve sensitive habitats highlight the importance of integrated, landscape-level approaches.

[11] By addressing these multifaceted threats through collaborative land-use planning, improved environmental regulations, and focused research, conservation efforts aim to safeguard the long-term survival of P. hydrothyria and the biodiversity of the freshwater ecosystems it inhabits.