It forms red, crusty thalli on stones submerged in water, typically in streams and rivers, less commonly in lakes and brackish parts of seas.

The thallus is in the form of a flat crust closely adhering to the rocky substrate, giving it the appearance of a coating.

[6] Compared to other freshwater red algae, it exhibits relatively low phenological variability,[7] while its eco-physiological parameters[8] and reproductive intensity vary depending on seasonal conditions.

[2] Cells in different parts of the thallus have uneven sizes, varying even twofold, especially in the case of branching.

[9] Despite the vivid red color of the entire thallus, individual cells may appear light green.

[12] The explanation of the life cycle of Hildenbrandia rivularis is considered one of Karol Starmach's [pl] most important discoveries.

[13] Freshwater species are exceptional within the genus Hildenbrandia because they do not produce tetraspores and reproduce exclusively asexually.

[15] Karol Starmach hypothesized that the reproductive organs homologous to the tetrasporangia of other Hildenbrandia species are its gemmae,[3] but this hypothesis has not been confirmed.

[15] According to observations by Starmach from the late 1920s, bright spots appear on the upper side of the thallus of Hildenbrandia rivularis, which then grow into nodules and warts covered with a layer of mucus.

[6] In the late 1940s and early 1950s, Starmach conducted observations and experiments involving these structures, proving that they are gmmae.

Upon contact with a substrate, they settle on it, adhere with mucus, and after two days, develop rhizoids emerging from several basal cells.

Under favorable conditions, some rhizoids branch, and new chloroplast-containing cells develop at their tips, giving rise to new thalli.

After a few more days, the cells of the gemmae die, as do the older parts of the rhizoids, and the newly formed thalli grow radially on the substrate.

In such cases, a form resembling Chantransia [pl] may rarely appear, which over time may transform into a typical thallus.

[3] Gemmae and thallus fragments usually flow downstream, and upstream transport likely occurs through animals such as leeches[6] or Chironomidae's larvae.

[3] Another relatively rare form of vegetative reproduction is the formation of filaments resembling stolons, on which new thalli develop at the ends.

[20] Lithophyte, a component of the phytobenthos, has a relatively broad ecological tolerance,[20] although it prefers swiftly flowing rivers with relatively clean water.

It occurs over a wide range of specific electrolytic conductivity (a measure of dissolved mineral substances).

[25] In those types of watercourses where it is significant as an indicator, it is classified as a non-diatom phytobenthos species in category B, meaning it is less sensitive than taxa requiring the best environmental conditions.

[3] The habitat of Hildenbrandia rivularis on a hundred-meter stretch of stream in the Ślęża Landscape Park has been established as a natural monument called Red Algae (Polish: Krasnorost).

[6] Another hypothesis suggested that various subpopulations originated from multiple independent transitions of the marine form to inland water.

The parent species could be Hildenbrandia rubra, which inhabits transitional waters [pl] in the Baltic Sea, among other places.

With multiple invasions, genetic differentiation of European subpopulations could be expected, stemming from different parent populations.

Furthermore, genetic distinctiveness from marine species indicates that it has a history predating the end of the Weichselian glaciation.

[39] Hildenbrandia rivularis shows low genetic variability,[9] which may result from the fact that it reproduces only asexually.

[9] Based on morphological and ecological differences, a subspecies occurring in the Mecsek mountains, Hildenbrandia rivularis subsp.

[42] Another freshwater species, Hildenbrandia ramanaginae, occurs in India but is poorly described and is not included in molecular analyses allowing for the determination of relatedness.

The term Hildenbrandia was proposed in 1840 by Giovanni Zanardini and over time, especially since the 1950s, it became dominant (after identifying the person commemorated by this name).

[48][47] The species was first scientifically described by Frederik Michael Liebmann in a work from 1838 based on holotypic specimens collected by Statsraada Hornemann in 1826 in a stream at Kongens Møller in Zealand (type locality).

[53] By covering flintstones with a durable pink color, Hildenbrandia may have influenced the perception of places where this phenomenon occurs (e.g., Stonehenge) as magical.