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Conceptacles are specialized cavities of marine and freshwater
algae Algae (; singular alga ) is an informal term for a large and diverse group of photosynthesis, photosynthetic eukaryotic organisms. It is a polyphyletic grouping that includes species from multiple distinct clades. Included organisms range from u ...
that contain the reproductive organs. They are situated in the receptacle and open by a small
ostiole An ''ostiole'' is a small hole or opening through which algae or fungi release their mature spores. The word is a diminutive of "ostium", "opening". The term is also used in higher plants, for example to denote the opening of the involuted ...
.Boney, A.D. (1969). ''A Biology of Marine Algae''. Hutchinson Educational Ltd, London Conceptacles are present in Corallinaceae,Irvine, L.M. and Chamberlain, Y.M. (1994). ''Seaweeds of the British Isles''. Volume 1, Part 2B. Natural History Museum, London. and Hildenbrandiales, as well as the
brown Brown is a color. It can be considered a composite color, but it is mainly a darker shade of orange. In the CMYK color model used in printing or painting, brown is usually made by combining the colors orange and black. In the RGB color model used ...
Fucales The Fucales (fucoids) are an order in the brown algae (class Phaeophyceae). The list of families in the Fucales, as well as additional taxonomic information on algae, is publicly accessible at Algaebaseref name="Guiry and Guiry">Guiry, M.D. and ...
. In the Fucales there is no haploid phase in the reproductive cycle and therefore no
alternation of generations Alternation of generations (also known as metagenesis or heterogenesis) is the predominant type of life cycle in plants and algae. It consists of a multicellular haploid sexual phase, the gametophyte, which has a single set of chromosomes alte ...
.Fritsch, F.E. (1945). ''The Structure and Reproduction of the Algae''. Vol 2. Cambridge University Press, Cambridge The thallus is a
sporophyte A sporophyte () is the diploid multicellular stage in the life cycle of a plant or alga which produces asexual spores. This stage alternates with a multicellular haploid gametophyte phase. Life cycle The sporophyte develops from the zygote p ...
.Smith, G.M. (1938). ''Cryptogamic Botany. Algae and Fungi''. Second edition, Volume ''1'', McGraw-Hill Bok Company, Inc. The diploid plants produce male (
antheridia An antheridium is a haploid structure or organ producing and containing male gametes (called ''antherozoids'' or sperm). The plural form is antheridia, and a structure containing one or more antheridia is called an androecium. Androecium is also t ...
) and female ( oogonia)
gametangia A gametangium (plural: gametangia) is an organ or cell in which gametes are produced that is found in many multicellular protists, algae, fungi, and the gametophytes of plants. In contrast to gametogenesis in animals, a gametangium is a haploid s ...
by meiosis. The gametes are released into the surrounding water; after fusion, the zygote settles and begins growth.Flecher, R.L. (1987). ''Seaweeds of the British Isles''. Volume 3, Part 1. British Museum (Natural History), London. Two
taxa In biology, a taxon (back-formation from ''taxonomy''; plural taxa) is a group of one or more populations of an organism or organisms seen by taxonomists to form a unit. Although neither is required, a taxon is usually known by a particular nam ...
, the corallines and Hildenbrandiaceae, bear conceptacles, although the striking difference between their formation indicates that the conceptacles are not homologous. Similar structures also exist: cryptostomata are similar to conceptacles but differ having only hairs and are sterile; caecostomata, are found only in '' Fucus distichus'', in these the ostiole becomes blocked during development.


Morphology

Conceptacles can measure from 40–150 μm in diameter in some Hildenbrandiales. Some conceptacles are pits within the thallus, whereas others form raised domes protruding above it. Conceptacles open to the environment via one or many pores, depending on the species and the type of conceptacle.


Components

* Columella. This is a central pillar constituting central calcified cells covered by cellular debris. These sterile cells rise up from the conceptacle floor to form a peak. * Roof filaments. Some conceptacles form by the centripetal expansion of a hole near the thallus surface; in such cases, a roof forms by nearby filaments arching over and establishing themselves as short (often 1–9 cells long) filaments that cover the chamber, leaving a central pore through which the spores can escape. If these filaments are vicarious they may produce a beak-like opening. * Spores. In asexual conceptacles, the spores tend to be large, and so are squeezed in, filling the conceptacle chamber "like orange segments".


Types of conceptacle

There are a range of different conceptacles, classified according to the nature of the spores that they contain; some species may possess as many as four distinct types. For example, ''Lithophyllum incrustans'' bears asexual, male, female and cystocarpic (strictly, a type of female) conceptacles, discussed below. * Asexual conceptacles. These develop where weakly calcified cells break down to produce a cavity. In ''L. incrustans'', Their columella is central and obvious (see above); it is accentuated by the spores (which may be bispores, tetraspores, etc.), which squeeze against the columella and conceptacle walls. Asexual conceptacles may be uniporate or multiporate. * Sexual (male / female) conceptacles. These lack a columella. ''L. incrustans'' has distinct male and female plants; the two conceptacle types never co-occur on the same thallus. Male conceptacles tend not to sit as deep within the thallus as female conceptacles; their size is similar although their shape tends to differ. Cells within circular regions within the perithallus lengthen to form sexual conceptacles, leaving long, wefty cells around the walls of the conceptacle. In the male conceptacles of ''Austrolithon'', marginal filaments grow more quickly, bending in over the top of the conceptacle cavity and eventually setting up as distinct filaments that form the conceptacle roof. * Cystocarpic conceptacles. These are not associated with tissue demineralization; rather, they start to form at the centre (as a female conceptacle) and develop radially. Being mature female conceptacles, these too lack a columella.


Development

In most coralline algae, a cluster of reproductive cells forms in the middle layer of the alga, and is engulfed by the surrounding tissue, which grows up and over the reproductive cells to form a roof and a uniporate conceptacle. Caps may subsequently develop to protect the opening. However the conceptacle may originate at any depth within the thallus, at the surface layer or at the basal perithallus. Four different modes of asexual conceptacle formation exist. In ''Corallina'' and ''Bossiella'', In the coralline '' Bossiella'', the conceptacle instead forms in the outer layer. A thickening forms, which separates the outer
epithallium The epithallium is a layer of many algae which bears the majority of photosynthetic Photosynthesis is a process used by plants and other organisms to convert light energy into chemical energy that, through cellular respiration, can later be re ...
from the underlying cortex; this thickening and the overlying epithallium will end up being the cap of the conceptacle, and the underlying cells will develop to become
reproductive initials Reproductive initials are filaments below the cuticle surface of algae and fungi which give rise to the bulbs of spore-producing cells (in fungi, conidiophores A conidium ( ; ), sometimes termed an asexual chlamydospore or chlamydoconidium (), ...
. Once the cap is formed, the filamentous cells underneath begin to degrade. The tissue around the edge of the cap grows more quickly. This combination produces a chamber beneath the cap.


Evolutionary history

Conceptacles appear in the fossil record at least as early as the Silurian. Similar structures have been noted in ''
Prototaxites ''Prototaxites'' is a genus of terrestrial fossil fungi dating from the Middle Ordovician until the Late Devonian periods, approximately . ''Prototaxites'' formed small to large trunk-like structures up to wide, reaching in length, made up of ...
'', which would imply that this giant land organism was not a simple fungus (as most paleontologists assume today) but a lichen.


References

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Further references

*https://web.archive.org/web/20120207133912/http://www.mbari.org/staff/conn/botany/browns/Cystoseira/lifehistory.htm Algal anatomy