Hildenbrandia is a genus of thalloid alga comprising 26 species. The slow-growing, non-mineralized thalli take a crustose form. Hildenbrandia reproduces by means of conceptacle

Conceptacle

Conceptacles are specialised cavities of marine and freshwater algae that contain the reproductive organs. They are situated in the receptacle and open by a small ostiole. Conceptacles are present in Corallinaceae, and Hildenbrandiales, as well as the brown Fucales. In the Fucales there is no...

s and produces tetraspores.

Morphology

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.

Growth

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

Coralline

Coralline means "resembling coral" and may refer to:* Coralline algae, whose fronds are covered with calcareous deposits.* Coralline rock, formed by the death of layers of coralline algae.* Sclerosponges, sometimes called "coralline sponges"....

 algae, the outer layer of the thallus is shed seasonally, presumably to avoid colonization by epiphytes.

Habit

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.

Reproduction

Sexual reproduction has never been observed in any Hildenbrandia species. It can reproduce by splitting into multiple colonies by fragmentation, or via stolon

Stolon

In biology, stolons are horizontal connections between organisms. They may be part of the organism, or of its skeleton; typically, animal stolons are external skeletons.-In botany:...

s (i.e. sending out lateral branches) or gemma

Gemma (botany)

A gemma is a single cell, or a mass of cells, or a modified bud of tissue, that detaches from the parent and develops into a new individual. This type of asexual reproduction is referred to as fragmentation. It is a means of asexual propagation in plants...

e.

Marine Hildenbrandia, on the other hand, reproduce by means of tetraspores that are produced within the thallus by conceptacles.

Systematics

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

External links

an excellent, accessible overview of the genus.

• Images of Hildenbrandia at Algaebase
  AlgaeBase
  AlgaeBase is a global species database of information on all groups of algae, as well as one group of flowering plants, the sea-grasses.AlgaeBase evolved from Michael Guiry's seaweed website, and has grown into a database of algae from throughout the world, and in freshwater, terrestrial, and...