Conceptacles are specialized cavities of marine and freshwateralgae that contain the reproductive organs. They are situated in thereceptacle and open by a smallostiole.[1] Conceptacles are present inCorallinaceae,[2] andHildenbrandiales, as well as thebrownFucales. In the Fucales there is nohaploid phase in the reproductive cycle and therefore noalternation of generations.[3] Thethallus is asporophyte.[4] Thediploid plants produce male (antheridia) and female (oogonia)gametangia bymeiosis. Thegametes are released into the surrounding water; after fusion, thezygote settles and begins growth.[5]
Twotaxa, thecorallines andHildenbrandiaceae, bear conceptacles, although the striking difference between their formation indicates that the conceptacles are not homologous.[6]Similar structures also exist: cryptostomata are similar to conceptacles but differ having only hairs and are sterile;[5] caecostomata, are found only inFucus distichus, in these the ostiole becomes blocked during development.[1]
Conceptacles can measure from 40–150 μm in diameter in some Hildenbrandiales.[7] Some conceptacles are pits within the thallus, whereas others form raised domes protruding above it.[8] Conceptacles open to the environment via one or many pores, depending on the species and the type of conceptacle.[8]
This is a central pillar constituting central calcified cells covered by cellular debris.[8] These sterile cells rise up from the conceptacle floor to form a peak.[9]
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)[10][11] filaments that cover the chamber, leaving a central pore through which the spores can escape.[12] If these filaments are vicarious they may produce a beak-like opening.[13]
In asexual conceptacles, the spores tend to be large, and so are squeezed in, filling the conceptacle chamber "like orange segments".[8]
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 andcystocarpic (strictly, a type of female) conceptacles, discussed below.[8]
These develop where weakly calcified cells break down to produce a cavity. InL. 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.[8] Asexual conceptacles may be uniporate or multiporate.[8]
These lack a columella.[8]L. incrustans has distinct male and female plants; the two conceptacle types never co-occur on the same thallus.[8] Male conceptacles tend not to sit as deep within the thallus as female conceptacles; their size is similar although their shape tends to differ.[8] Cells within circular regions within theperithallus lengthen to form sexual conceptacles,[8] leaving long, wefty cells around the walls of the conceptacle.[14] In the male conceptacles ofAustrolithon, 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.[12]
These are not associated with tissue demineralization; rather, they start to form at the centre (as a female conceptacle) and develop radially.[8] Being mature female conceptacles, these too lack a columella.[8]
In most coralline algae, a cluster of reproductive cells forms in themiddle 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.[13] However the conceptacle may originate at any depth within the thallus, at the surface layer or at the basal perithallus.[8]
Four different modes of asexual conceptacle formation exist. InCorallina andBossiella,
In the corallineBossiella, the conceptacle instead forms in theouter layer. A thickening forms, which separates the outerepithallium 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 becomereproductive initials.[13] 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.[13]
Conceptacles appear in the fossil record at least as early as the Silurian. Similar structures have been noted inPrototaxites, which would imply that this giant land organism was not a simplefungus (as most paleontologists assume today) but alichen.[15]
Similar structures also exist: cryptostomata are similar to conceptacles but differ having only hairs and are sterile;[5] caecostomata, are found only inFucus distichus, in these the ostiole becomes blocked during development.[1]
Cryptostomata (singular: cryptostoma) are structures found in some types ofbrown algae.
The anatomical structures are found in some species ofFucus, but not inPelvetia canaliculata. Cryptostomata are similar to conceptacles,[16] but they are sterile cavities producing only hairs.[5][17] They are found on the lamina ofFucus serratus. Colourless tufts of hairs can be seen issuing from them.[18]
They are also sometimes referred to by the German name ofFasergrübchen.[19]
That the cryptostoma and conceptacle are homologous cannot be doubted
