Hildenbrandia rivularis is a species of freshwater red algae. It forms red, crusty thalli on stones submerged in water, typically in streams and rivers, less commonly in lakes and brackish parts of seas. It occurs in scattered locations on almost all continents. The species was formerly considered an indicator of clean or slightly polluted waters. The scientific genus name is sometimes spelled in various orthographic variants, especially as Hildenbrandtia. The life cycle of this species was described by the Polish hydrobiologist Karol Starmach.
Structure
Habit The thallus is in the form of a flat crust closely adhering to the rocky substrate, giving it the appearance of a coating. It grows radially, assuming a rounded, disc-like shape. It is composed of cells arranged in tightly fused vertical rows, with weakly branching. The lower layer of the thallus adheres to the substrate, without forming rhizoids. The upper layer of the thallus is smooth. The filaments of the vegetative thallus are strongly fused and difficult to separate. Forms similar to transections may develop at the edges of the thallus. The color is blood-red (from pink to burgundy), with a brownish hue in winter. Growth in thickness is limited, making it impossible to distinguish annual growth layers. Compared to other freshwater red algae, it exhibits relatively low phenological variability, while its eco-physiological parameters and reproductive intensity vary depending on seasonal conditions.
Anatomy Anatomical features are quite variable. Initially, it was thought to be geographic variability, but in reality, the same variability occurs within local populations. Cells are approximately cylindrical to spherical. Individual cells contain single chloroplasts. Cells in different parts of the thallus have uneven sizes, varying even twofold, especially in the case of branching. The average size of cells is 8.4 × 8.6 μm, the average length of filaments is 38.4 μm, and the average thickness of the basal layer is 5.5 μm in the typical form, while in the variety distinguished by slightly darker coloration – Hildenbrandia rivularis var. drescheri Lingelsh. 1922 – these dimensions are respectively: 6.0 × 6.4 μm, 52.8 μm, 9.2 μm. Despite these differences, molecular studies do not indicate distinctiveness of this variety, and specimens classified as the species H. rivularis (at least those from Europe and the Canary Islands) show relatively low genetic variability compared to other traditionally recognized species of this genus. Despite the vivid red color of the entire thallus, individual cells may appear light green.
Similar species The most similar species is another freshwater representative of the genus – Hildenbrandia angolensis. The range of anatomical dimensions of these two species overlaps, but on average, the cells of H. angolensis are smaller. A similar range of cell sizes is also found in the freshwater Hildenbrandia cuprea. Its colonies form a crustose mat and are brick-red, while colonies of H. rivularis form a crustose, vivid red biofilm. Sometimes, the thalli of both species can be very similar to each other. It may be confused with other algae forming crustose thalli on rocks, such as the brown algae Heribaudiella fluviatilis. In saline waters, including the entire Baltic Sea, its variants are usually other species of Hildenbrandia (especially Hildenbrandia rubra), although it may co-occur with them in the brackish Bothnian Bay.
… excerpt ends here. Continue reading the full article.
