Hildenbrandia is a genus of thalloid red alga comprising 26 species. The slow-growing, non-mineralized thalli take a crustose form. Hildenbrandia reproduces by means of conceptacles and produces tetraspores.
Hildenbrandia cells are around 3–5 μm in diameter and the filaments are around 50–75 μm in height.
The thallus comprises two layers; the hypothallus, which attaches to the rock, and the perithallus, a pseudoparenchymous layer comprising vertical filaments, which unlike coralline red algae is not further differentiated.
Hildenbrandia comprises orderly layers of vertical oblong cells with thick vegetative cell walls, occasionally connected by secondary pit connections with pit plugs in the septal pores. It grows at its margins, away from the centre, and is able to quickly repair any gaps arising by regenerating from a basal layer of cells. As plants become more mature, they become multi-layered and strongly pigmented near their centres, whilst their single-layered margins begin to grow more slowly. Multi-layered areas may develop in the margins; these will detach and float away as gemmae to form new colonies, leaving a single layer of cells beneath them once they separate from the host plant.
Newly settled gemmae form rhizoids.
Conceptacles develop in a haphazard manner; cells in conceptacle regions deform one another and become less regularly shaped as they grow larger.
In a similar fashion to the coralline algae, the outer layer of the thallus is shed seasonally, presumably to avoid colonization by epiphytes.
The freshwater species H. rivularis and H. angularis seems to form a clade, and require an alkaline pH and hard water, preferring clean water. Unlike most other freshwater red algae (which prefer running water), H. rivularis prefers still water, particularly shady lakes or ponds. H. rubra and other marine species are found in brackish waters, but freshwater / gemma-bearing species cannot tolerate even moderate salinities. The genus is often found in a symbiotic partnership with fungi. Hildenbrandia has a remarkable tolerance to stresses including extreme temperatures, desiccation, and Ultra-violet light; it can be up and photosynthesizing near full capacity just minutes after being cooled to −17 °C or subjected to extreme salinities.
Sexual reproduction has never been observed in any Hildenbrandia species. It can reproduce by splitting into multiple colonies by fragmentation, or via stolons (i.e. sending out lateral branches) or gemmae.
Marine Hildenbrandia, on the other hand, reproduce by means of tetraspores that are produced within the thallus by conceptacles.
The genus contains these species (this list is out of date):
- H. angolensis
- H. arracana
- H. canariensis
- H. crouanii
- H. crouaniorum
- H. dawsonii
- H. deusta
- H. expansa
- H. galapagensis
- H. kerguelensis
- H. lecannellieri
- H. lithothamnioides
- H. nardiana
- H. occidentalis
- H. pachythallos
- H. patula
- H. prototypus
- H. ramanaginaii
- H. rivularis
- H. rosea
- H. rubra
- H. sanjuanensis
- H. yessoensis
The presence of H. rivularis near Stonehenge has been put forward as a reason for the site's perceived mystical properties. Flint in pools near the henge takes on a pink hue a couple of hours after being taken out of water due to the presence of the algae. It is assumed that ancient hunter-gatherers would have seen the rocks as having magical properties and would have deemed the site worthy of interest.  
^ Dethier, M. (1994). "The ecology of intertidal algal crusts: variation within a functional group". Journal of Experimental Marine Biology and Ecology. 177: 37–71. doi:10.1016/0022-0981(94)90143-0.
^ Sherwood, A.; Sheath, R. (2000). "Biogeography and systematics of Hildenbrandia (Rhodophyta, Hildenbrandiales) in Europe: inferences from morphometrics and rbcL and 18S rRNA gene sequence analyses". European Journal of Phycology. 35: 143. doi:10.1080/09670260010001735731.
^ "Hildenbrandia Ben: Morphology". washington.edu.
^ Cabioch, J.; Giraud, G. (1982). "La structure hildenbrandioïde, stratégie adaptative chez les Florideés". Phycologia (in French). 21 (3): 308–315. doi:10.2216/i0031-8884-21-3-307.1.
^ a b Pueschel, C. (1982). "Ultrastructural observations of tetrasporangia and conceptacles in Hildenbrandia (Rhodophyta: Hildenbrandiales)". European Journal of Phycology. 17 (3): 333–341. doi:10.1080/00071618200650331.
^ a b c d e Wayne Nichols, H. (1965). "Culture and development of Hildenbrandia rivularis from Denmark and North America". American Journal of Botany. 52 (1): 9–15. doi:10.2307/2439969.
^ a b c Sherwood, A. R.; Sheath, R. G. (2000). "Microscopic analysis and seasonality of gemma production in the freshwater red alga Hildenbrandia angolensis (Hildenbrandiales, Rhodophyta)". Phycological Research. 48: 241. doi:10.1046/j.1440-1835.2000.00208.x.
^ Pueschel, C. (1988). "Cell sloughing and chloroplast inclusions in Hildenbrandia rubra (Rhodophyta, Hildenbrandiales)". European Journal of Phycology. 23: 17–23. doi:10.1080/00071618800650021.
^ Sherwood, A. R.; Sheath, R. G. (2003). "Systematics of the Hildenbrandiales (Rhodophyta): gene sequence and morphometric analyses of global collections". Journal of Phycology. 39: 409–422. doi:10.1046/j.1529-8817.2003.01050.x.
^ a b Eloranta, P.; Kwandrans, J. (2004). "Indicator value of freshwater red algae in running waters for water quality assessment" (PDF). International Journal of Oceanography and Hydrobiology. XXXIII (1): 47–54. ISSN 1730-413X. Archived from the original (PDF) on 2011-07-27. Retrieved 2010-10-27.
^ a b Sherwood, A. R.; Shea, T. B.; Sheath, R. G. (2002). "European freshwater Hildenbrandia (Hildenbrandiales, Rhodophyta) has not been derived from multiple invasions from marine habitats". Phycologia. 41: 87. doi:10.2216/i0031-8884-41-1-87.1.
^ Saunders, G. W.; Bailey, J. C. (1999). "Molecular systematic analyses indicate that the enigmatic Apophlaea is a nember of the Hildenbrandiales (Rhodophyta, Florideophycidae)". Journal of Phycology. 35: 171. doi:10.1046/j.1529-8817.1999.3510171.x.
^ Garbary, D. (2007). "The Margin of the Sea". ALGAE AND CYANOBACTERIA IN EXTREME ENVIRONMENTS. Cellular Origin, Life in Extreme Habitats and Astrobiology. 11. pp. 173–191. doi:10.1007/978-1-4020-6112-7_9.
^ Guiry, M.D.; Guiry, G.M. (2008). "Hildenbrandia". AlgaeBase. World-wide electronic publication, National University of Ireland, Galway. Retrieved 2009-04-18.
^ "Stonehenge was the 'LONDON of the Mesolithic': River Avon acted like an 'A-Road' for ancient log boats, expert claims".
^ "Mesolithic settlement near Stonehenge: excavations at Blick Mead, Vespasian's Camp, Amesbury" (pdf). www.silversaffron.co.uk.
^ Jacques, David (2014). "Mesolithic settlement near Stonehenge: excavations at Blick Mead, Vespasian's Camp, Amesbury". Wiltshire Archaeological and Natural History Magazine. 107: 7–27.