Encrusting coralline algae flourish where waves break on the reef, forming an elevated bank known as the algal ridge. As water from the waves returns seaward, its load of suspended sand and gravel scours a series of narrow ridges. The alternating ridges and valleys are known as spurs and grooves.

Corals growing on top of the spurs are stoutly formed and compressed in shape to withstand the extreme force of breaking waves.

Being algae, the plant body in corlallines is referred to as a thallus (pl. thalli). Most coralline algae have an internal anatomy of the thallus that is based on the aggregation of branching filaments, each of which is formed through the action of its own single meristematic cell. This type of thallus anatomy is said to be pseudoparenchymatous

Most nongeniculate corallines show some degree of dorsiventral arrangement, that is, an arrangement in which a distinct upper and lower surface can be distinguished. There are two different types of organization in dorsiventral thalli:

In dimerous thalli, there are two distinct groups of filaments oriented more-or-less at right angles to one another, with a ventral layer of filaments (called basal filaments) that is uually one cell thick, and an erect layer in which filaments (called erect filaments) vary from one to many cells in length and comprise most of the thallus thickness. Dimerous thalli arise when a spore germinates to produce a unistratose layer of radiating, repeatedly branched filaments, that grow via meristematic cells that are located at the apices of the branches. The growth of this layer of basal filaments contributes to the areal expansion of the thallus. Erect filaments are formed in most dimerous species when cells of the basal filaments divide periclinally to form erect filaments. The first cell that is cut off remains meristematic and forms new cells of the erect filament ventrally, and epithallial cells dorsally. Since it is usually overlain by one or more epithallial cells, the meristematic cell is usually referred to as a subepithallial initial. Dimerous thalli occur in Lithophyllum, Titanoderma, Exilicrusta, Melobesia, and some species of Hydrolithon and Pneophyllum.

In monomerous thalli, there is but a single pseudoparenchymatous system of repeatedly branched filaments in which some derivatives contribute to a medulla which runs more-or-less parallel to the thallus surface (or substratum in dorsiventral, attached species) and some derivatives curve outwards and collectively form a cortex. In most dorsiventral thalli, downwards-curving filaments are suppressed, and only occur vestigially. The cortical filaments terminate at the upper surface in one or more epithallial cells, that are subtended by a subepithallial initial. Continued division of the subepithallial initial increases the thallus girth, and may give rise to surface protuberances which may themselves branch. Monomerous thalli occur in Clathromorphum, Leptophytum, Lithothamnion, Phymatolithon, Mesophyllum, and many species of Hydrolithon, Spongites, and Pneophyllum. In some species, the primary thallus is dimerous, but monomerous regions may develop secondarily from the cells of erect filaments (e.g. Lithophyllum incrustans).

In both monomerous and dimerous forms, the cells of adjacent filaments below the merismetic cells often fuse together, or form pit connections, allowing for cellular interchange throughout the plant in spite of the calcified cell walls. Cell fusions and secondary pit connections are used for taxonomic purposes at the sub-family as well as species level.

In dimerous thalli, there are basal and erect filaments. There is a primary terminal
initial (meristmatic cell) located at the margin of the thallus, and from which the thallus
expands laterally.

To emphasize this commonality with other multiaxial red algae, the ventral
layer of filaments are referred to here as the medulla, and their peripheral
derrivatives are referred to as the cortex. We can also speak of medullary
filaments and cortical filaments, to refer to the filaments which make up
the medulla and cortex respectively

. Monomerous thalli occur in Clathromorphum, Leptophytum, Lithothamnion, Phymatolithon, Mesophyllum, and many species of Hydrolithon, Spongites, and Pneophyllum. In some species, the primary thallus is dimerous, but monomerous regions may develop secondarily from the cells of erect filaments (e.g. Lithophyllum incrustans).

In both monomerous and dimerous forms, the cells of adjacent filaments below the merismetic cells often fuse together, or form pit connections, allowing for cellular interchange throughout the plant in spite of the calcified cell walls. Cell fusions and secondary pit connections are used for taxonomic purposes at the sub-family as well as species level.

Reproduction

Reproduction in the coralline algae is as kinky as it is in most of the other red algae. Three distinct phases occur in the life cycle, two of which are free-living, and one of which is attached to and dependent on the female plant. Both of the free-living stages look almost identical, differing only in their reproductive organs, which are usually borne inside small flask-shaped or elliptical cavities or depressions called conceptacles.

The stage which produces gametes is called the gametophyte. After fertilization, the zygote nucleus undergoes a special type of development to give rise to a tiny, uncalcified plant called the carposporophyte. The carposporophyte has twice the chromosomes number of the gametophyte generation, and develops inside the conceptacles of the female gametophyte. The carposporophyte produces spores, called carpospores, which are released through a pore in the female conceptacle. These spores, which still have twice the chromosome number as the gametophytes, germinate and grow into a third generation called the tetrasporophyte.

Female reproductive structures consist of
a carpogonium borne on a hypogynous
cell. The carpogonium is extended into
a receptive process (received spermatium
during fertilization) called a trichogyne.
Sometimes a sterile cell is also present.

The carposporophyte develops from the zygote nucleus following fertilization.
It is thus diploid. It remains within the female conceptacle, which enlarges to
accomodate it. In the diagram below, the carposporophyte is seen to consist of
the gonimoblast filaments, each of which terminates in a carposporangium
which produces a single carpospore. The filaments illustrated in pink are haploid
filaments of the gametophyte which surround the casposporophyte and enclose it
within the conceptacle. The conceptacle and its contents are sometimes referred to
as a cystocarpic conceptacle.

Diploid: having twice the haploid chromosome number. Following fertilization, a
diploid carposporophyte develops within the female conceptacle. The diploid
carposporophyte produces diploid carpospores by mitosis, and these are released and
grow into diploid tetrasporophyte

Meiosis is the process of reduction division, whereby the chromosome number in the nucleus is halved. The
fusion of gametes restores the chromosome number to the full complement. Following meiosis cells are said to
be haploid. Following the fusion of gametes, cells are said to be diploid. In coralline algae, meiosis takes
place in diploid tetrasporangia and results in haploid tetraspores. See also tetrasporophyte

Haploid: having half the diploid chromosome number. In coralline algae meiosis takes place
in the tetrasporophyte to gice rise to haploid tetraspores. The haploid tetraspores grow into
haploid gametophytes. The tetraspores and the gametophytes are the only haploid stages
in the coralline algae

Halydictyon arachnoideum from Victoria, Australia, a multinucleate red alga with giant cells. Stained with DAPI, a fluorochrome specific to DNA.

Link to Algal Ridge (http://www.cyberlearn.com/ridge.htm)