Peltigera aquatica

Peltigera hydrothyria is an aquatic foliose lichen forming small rosettes of variable size attached to the substrate by one to several holdfasts. The thallus is characterized by a gelatinous, nonstratified (homoiomerous) structure that is fully corticated. The thallus appears black when wet and slate gray when dry, with lobes 0.5–1 cm (3⁄16–3⁄8 in) 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, epruinose, and glabrous, 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. …

Peltigera hydrothyria displays a distinct biogeographic pattern across North America. The nominate variety (var. hydrothyria) is endemic to the Appalachian Mountains in eastern North America, while var. gowardii ranges from northern California through the Pacific Northwest to southern Alaska, with isolated populations in Idaho. In the eastern United States, it has been recorded in Appalachian states and New England, though many historical records—particularly those near urbanized regions—are now considered extirpated. In Canada, P. hydrothyria s.str. 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. This climatic requirement explains its patchy distribution, as suitable conditions exist along the Appalachian chain and coastal eastern Canada but not in intervening areas. Canadian populations, confirmed by genetic analysis as P. hydrothyria s.str., inhabit elevations of 68–723 m (223–2,372 ft) under birch and balsam fir canopies. Population sizes vary widely, from as few as 12 to over 484 mature individuals at a single site. …

Peltigera hydrothyria favors conditions with dissolved oxygen around 8.22 mg/L, nitrogen at 0.03 mg/L, and phosphorus at 0.013 mg/L, reflecting its need for clean, nutrient-poor environments. Low nitrate levels can enhance growth, but concentrations above 40 mmol damage the thallus and inhibit growth. Stream pH typically ranges from 6.0 to 7.0, and sinuous stream configurations that create protected pockets are preferred. Humidity is critical where thalli remain partially above water during low-flow periods; surrounding wet bryophytes help maintain moisture and prevent desiccation. The species frequently co-occurs with other aquatic lichens, such as Leptogium rivale, although P. hydrothyria generally occupies smaller, shaded streams, whereas L. rivale tolerates larger watercourses. Morphologically, L.rivale forms smaller, tightly attached rosettes with elongate lobes, in contrast to the sub-erect thallus and pronounced venation of P. hydrothyria.