Peltigera hydrothyria was described as Hydrothyria venosa from eastern North America more than a century ago based on material from Vermont, U.S.A. When molecular studies placed the species in Peltigera, a new epithet was required which led to the current name P. hydrothyria. Subsequent to its description the species has been documented throughout mountain ranges of eastern North America and the name P. hydrothyria applies to material from this region.

Aquatic lichens similar to P. hydrothyria in appearance were also discovered in mountain ranges of western North America. While these disjunct occurrences were initially referred to P. hydrothyria, extensive study with molecular methods has demonstrated that they are distinct species with distinct cyanobacterial symbionts. The western North American occurrences previously referred to as P. hydrothyria are now treated as P. gowardii and P. aquatica. It is well-established that P. hydrothyria does not occur in western North America and as such can be assessed independently of these taxa (McCune and Stone 2022).

Peltigera hydrothyria is native to eastern North America. It is an unmistakable, rare aquatic species only found in unaltered streams in mature forests, and is a valuable indicator of water quality in its narrow riparian niche. As such, with the widespread destruction of suitable habitat in eastern North America, it has declined from formerly documented historical findings, and has a best estimate area of occupancy (AOO) of 444 km². There are threats to the remnant fragmented population by long-term effects of climate change and continued loss and degradation of habitat, and is thus assessed as Endangered under criterion B2ab(ii,iii,iv,v).

Peltigera hydrothyria is endemic to the mountain ranges of eastern North America, primarily occurring in the Appalachian mountains. It consists of scattered and isolated occurrences across a large area of the temperate eastern United States, the Canadian Maritime Provinces and Quebec. The species has been recorded from; in the United States (Connecticut, Georgia, Maine, Massachusetts, New Hampshire, New Jersey, New York, North Carolina, Pennsylvania, Rhode Island, Tennessee, Vermont and Virginia) and in Canada (Quebec, New Brunswick and Nova Scotia).

A minimum area of occupancy (AOO) of 300 km² is calculated by removing all historical records, while a maximum of 460 km² includes all historic records. The current best estimate of AOO is 444 km² removing historic sites thought to be extirpated.

Throughout its range the population is only found in isolated locations within mature riparian zones, and due to the ongoing loss of suitable habitat, is estimated to be in decline. It is intrinsically rare and sensitive due to its small range and specific and narrow habitat niche (Environment and Climate Change Canada 2021).

In the United States, many locations are only known from historical vouchers, but all extant individuals reside in mountainous areas that are managed on public lands, some within state or federally protected areas; most recently discovered localities tend to be found in the southern end of the range. Of the 82 records within the United States, over half are from before 1950. In New England states like New Hampshire and Connecticut, the majority of records are from before 1900, many occurring in what have become large urban areas with rivers and streams that are now altered to such an extent as to render them unable to support the species. Several searches conducted in 2023 to rediscover historic locations have been unsuccessful.

In Canada, the species was listed as Threatened by COSEWIC (2013), and extensive work has been done to locate all extant locations. It is only known from a limited number of streams: 26 in New Brunswick, one in Quebec, and 12 in Nova Scotia. Recent searches have discovered healthy individuals in brooks within Fundy National Park, but the overall population in Canada is considered to be in decline, with a limited number of individuals in vulnerable locations (COSEWIC 2013). The total number of colonies in Canada in 2019 was estimated to be 2,083.

The 119 occurrences are widely separated. They are found in riparian corridors with narrow opportunities for spore dispersal (COSEWIC 2013). Some locations sustain locally abundant subpopulations, but most contain a small number of individuals in a limited area of one stream (often in an area with other streams providing optimal conditions, but no individuals present). Overall, the population is considered to be severely fragmented due to the distribution of the area of occupancy, dispersal limitations, and low population densities in the majority of the locations.

This species is restricted to relatively small, cold mountain streams with high water quality and low turbidity. It requires a pH in the range of 5.8-7.6, and cannot thrive with temperatures above 18°C. It is a distinctive and unusual lichen, greatly impacted by changes to both the aquatic and terrestrial aspects of its habitat.

Peltigera hydrothyia can grow underwater or at the surface (even on the surface of large rocks with seeps) but requires constant moisture and a perhumid climate. It cannot survive siltation, and does not co-occur with filamentous green algae (COSEWIC 2013). It is usually found attached to larger, non-mobile rocks and boulders.

It is usually found in areas with mature mixed hardwood/conifer forests with a high continuous canopy along riparian corridors, providing shade in the warmer months, and thus keeping the water temperatures cool. It prefers strong elevational gradients and is most often found in tributaries with small waterfalls that create protected eddies and adequate aeration. Elevations of United States locations range from 150-2,000 metres above sea level but it is found as low as 10 m asl in Nova Scotia.

The threats to this aquatic species are diverse and significant. The major threats stem from 1) changes in habitat (macro- and micro- scales) resulting in deposition of pollutants and ecosystem alterations, 2) changes in habitat (macro- and micro- scales) that are likely to result from climate change, 3) changes in habitat (macro- and micro- scales) resulting conversion and deterioration of natural habitats both historically and ongoing.

Loss and degradation of the remaining suitable habitat for Peltigera hydrothyria is a past, present and future threat. The species is unable to survive in streams that are affected by anthropogenic damage and proximity to development of all kinds, including stream alteration due to residential development and subsequent waste water, roads for logging, energy production and mining, recreational ATV use, water table alteration, agricultural runoff and removal of tree canopy. Suitable water temperature and humidity levels are continually threatened by climate change, as well as changes in high and low water events. Viable pH levels and nitrate levels are affected by air pollution and wood harvesting. Siltation is a constant threat due to the erosion caused by the creation of roads and resource extraction.

In Canada the species was assessed as Threatened by COSEWIC in 2013 and was listed on Schedule 1 of SARA in 2018. A SARA Recovery Plan was published in 2021, and includes detailed maps showing critical habitat and action plans (Environment and Climate Change Canada 2021). It is also listed as Threatened in Nova Scotia (Nova Scotia Department of Lands and Forestry 2021). Approximately 95% of Peltigera hydrothyria in Canada occurs in areas that are managed for conservation.

There is no federal protection in the United States. At the state level, it is listed as a species of conservation concern in Virginia, Pennsylvania and North Carolina, and tracked as such, but there are no specific management plans in place. Most colonies are located in federally or state protected land, but there are many subpopulations in forests throughout the Appalachians where there is the possibility of loss due to development/logging/mining. Land managers within the states are often interested in managing lands for this species, but are hampered by lack of formally recognized conservation status.

There are many conservation actions that can be taken to maintain the population. In the United States: listing the species in the Natural Heritage programmes of more Appalachian states; educating and training land managers and local botanists to identify the species so we can monitor its health; federally listing the species as endangered in the United States; improving numerous regulations and policies that would safeguard the aquatic habitats where the species occurs; and providing increased protection for buffer forest stands along riparian areas where the species occurs and could occur in the future. In Canada, goals within the Recovery Plan include site planning and monitoring, outreach and land management, regulations for pollution and climate change, and further research, all with set dates for achievement. Maps have been created that identify critical habitat to be protected.

Research that will aid in the conservation of this species includes population assessments and monitoring (including checking the many historical sites in the United States), population genetics studies, and ecological studies that incorporate threats to the species. More information needs to be gathered about reproduction and dispersal. Additionally, a species recovery plan needs to be written for the United States.

A previously unknown occurrence of P. hydrothyria was found in the Kennedy Lakes Protected Natural Area in central New Brunswick in October 2022. Previous searching for P. hydrothyria in this area had been very limited. The location is intermediate between those in and near Fundy National Park in southeastern New Brunswick and on the Rivière Noire in the Laurentian hills of Quebec. The Kennedy Lakes area is a hilly upland with a cool humid climate. In retrospect, it seems that comparably humid uplands in northwestern New Brunswick and adjacent Quebec (south of the St. Lawrence River) could also have suitable habitats for P. hydrothyria. However, there has been little targeted searching in that area, or in other streams near the location in Kennedy Lakes PNA.

The Appalachian region of southeastern Quebec (Eastern Townships) and western and northern Maine might also have areas of potential habitat that has not been thoroughly explored. One specimen was collected from the Katahdin area of Maine in 2023 after a search of multiple areas of suitable habitat. These areas should be a priority for further targeted searches, and the few places with successful searches should be studied in further detail. There are many streams adjacent to the rare findings within this Appalachian region which have ideal habitat yet lack colonization. Funding is currently being pursued to address this question, and research is planned for summer of 2024.

Quantitative distribution modelling (e.g. with MaxEnt) is now a fairly routine part of assessments of the conservation status of lichens in Canada. It appears that no such modelling has been done yet for P. hydrothyria. It would be helpful to fill this gap.