Hildenbrandiales is an order of

crustose Crustose is a habit of some types of algae and lichens in which the organism grows tightly appressed to a substrate, forming a biological layer. ''Crustose'' adheres very closely to the substrates at all points. ''Crustose'' is found on rocks and ...

forms

red alga Red algae, or Rhodophyta (, ; ), are one of the oldest groups of eukaryotic algae. The Rhodophyta also comprises one of the largest phyla of algae, containing over 7,000 currently recognized species with taxonomic revisions ongoing. The majority ...

which bear

conceptacle Conceptacles are specialized cavities of marine and freshwater algae that contain the reproductive organs. They are situated in the receptacle and open by a small ostiole.Boney, A.D. (1969). ''A Biology of Marine Algae''. Hutchinson Educational L ...

s and produce secondary pit-connections. They reproduce by vegetative gemmae as well as

tetrasporangia Red algae, or Rhodophyta (, ; ), are one of the oldest groups of eukaryotic algae. The Rhodophyta also comprises one of the largest phyla of algae, containing over 7,000 currently recognized species with taxonomic revisions ongoing. The majority ...

, which are produced inside the conceptacles. The way in which the

tetraspore Tetraspores are red algae spores produced by the tetrasporophytic (diploid) phase in the life history of algae in the Rhodophyta as a result of meiosis.Jones, W.E. Revised and reprinted 1964. A Key to the genera of the British seaweeds.''Field Stu ...

s are produced is unusual enough to justify the formation of this distinct order. Some members of the order are known from freshwater rivers as well .

Systematics

Hildenbradiales are difficult to discriminate on morphological grounds; tetrasporangia morphology is the only vaguely reliable character, but molecular techniques have indicated a

monophyletic In cladistics for a group of organisms, monophyly is the condition of being a clade—that is, a group of taxa composed only of a common ancestor (or more precisely an ancestral population) and all of its lineal descendants. Monophyletic gro ...

Apophlaea ''Apophlaea'' is a genus of thalloid algae that is endemic to New Zealand. This genus has two species, both from the high intertidal zone on New Zealand's coasts. Specimens can reach around 15 cm in size. The thalli take a crustose form, but ...

'' within a

paraphyletic In taxonomy (general), taxonomy, a group is paraphyletic if it consists of the group's most recent common ancestor, last common ancestor and most of its descendants, excluding a few Monophyly, monophyletic subgroups. The group is said to be pa ...

Hildenbrandia ''Hildenbrandia'' is a genus of thalloid red alga comprising about 26 species. The slow-growing, non-mineralized thalli take a crustose form. ''Hildenbrandia'' reproduces by means of conceptacles and produces tetraspores. Morphology ''Hildenb ...

'', with many ''Hildenbrandia'' species being recognized as non-monophyletic.

References

Florideophyceae Red algae orders {{Rhodophyta-stub