Subphylum Rhodellophytina

The simpler taxa occupy two subphyla: Rhodellophytina and Metarhodophytina, each with a single class. Members of the Rhodellophytina are simple unicells (Figure 2) or pseudofilaments, cells held in a linear array by the common gelatinous covering (Figure 3). They have no sexual reproduction. However, some taxa, like Porphryidium (Figure 2) can produce large amoeboid forms whose particular function is unknown (Bold and Wynne 1978).

Subphylum Metarhodophytina

Members of the Metarhodophytina tend to be filamentous or pseudoparenchymatous. For example Composogon (Figure 4) develops a pseudoparenchymatous thallus at the base from which branched uniseriate filaments emerge. They tend to have a biphasic life cycle (that is, alternation of sporophyte and gametophyte stages) in which the spermatia are simple and derived from vegetative cells.

Subphylum Eurhodophytina

The subphylum Eurhodophytina is the most speciose of the three and contains two classes: Bangiophyceae and the Floridiophyceae. The defining synapomorphy is the occurrence of pit plugs in at least one of the phases of the life history. The pit plug, sometimes called a pit connection, is a lens-shaped mucilaginous structure in the walls of adjoining cells. The plug fills an aperture, which otherwise would allow the flow of cytoplasm from cell to cell.

Class Bangiophyceae

Members of the Bangiophyceae have a simple alternation of heteromorphic generations in which the sporophyte is a small, prostrate filament called a conchocelis that releases meispores called conchospores. The sporophyte is the stage that has pit connections. The gametophyte can be variable in this group and range from filamentous (Figure 5) to foliose (Figure 6). Porphyra is the source of Nori, the black seaweed that wraps sushi; so, the discovery of its history opened the door for its culture and the global availability of Nori. Specialized cells in the foliose gametophyte of Porphyra form the spermatia, and other large cells function as eggs. Following syngamy, the zygote settles down on a mollusk shell and develops into the sporophyte (see Figure 7).

Class Floridiophyceae

The Floridiophyceae contains most of the taxa in the phylum. These plants tend to be complex, either filamentous or pseudoparenchymatous and tend to be seaweeds of warmer waters. The polysaccharides common in the cell walls of many in this group are the sources of agar, agarose, and carrageenin, common food additives. Chondrus crispus (Figure 8) is the red most commonly harvested on the coast of the eastern US as a source of agar. Corallina (Figure 9) is a taxon that impregnates its cell walls with calcium carbonate forming filaments that appear armored and segmented.

Sexual reproduction is generally triphasic, such that isomorphic gametophyte and sporophyte generations are separated by the carposporophyte, a very different sporophyte that emerges from the development of the zygote. Following syngamy and karyogamy, the zygote nucleus typically moves to another cell, the auxiliary cell, from which the carposporophyte begins to develop. In general, the carposporophyte is a set of small filaments that terminate in diploid spores, carpospores. These disperse and germinate to form the sporophyte. This is generally pseudoparenchymatous and identical to the gametophyte. Certain cells develop as sporangia in which meiosis occurs. In the case of Polysiphonia (Figures 10 and 11), the axial cells of the corticated filaments (text with tooltip) A corticated filament has a large axial cell with smaller appressed branching cells so that the axis appears to be striped. function as sporangia. Before this was recognized as a sexual cycle, the four meiospores that were produced in each sporangium were just referred to as tetraspores (text with tooltip) Tetraspores are meiospores produced by certain red algae, especially those that exhibit isomorphic alternation of generation. (4 spores), and this second sporophyte was called the tetrasporophyte (text with tooltip) A tetrasporophyte is a certain type of sporophyte in red algae that exhibit isomorphic alternation of generation. The tetraspores (actually meiospores) produced by the tetrasporophyte were named before the demonstration of a sexual life cycle in the red algae. . A gametophyte that is identical to the tetrasporophyte emerges from the tetraspore following its germination. The gametophytes have separate sexes, one produces spermatia in specialized spermatangia. The other produces eggs (called carpogonia (text with tooltip) Carpogonium (carpogonia, pl.) is the oogonium-producing structure (gametangium) of red algae. ), each with an elongate hair like extension called a trichogyne (text with tooltip) A trichogyne is a thread-like extension of the carpogonium (oogonium) in the red algae. A similar structure occurs in certain ascomycete fungi. . When a spermatium encounters a trichogyne, it transfers the nucleus, which fuses with the egg nucleus and it travels to an auxillary cell to begin the cycle again. As the carposporophyte develops in Polysiphonia, a cup-like envelope, the cystocarp, develops around the carposporophyte (see Figure 11). The life histories of the other red seaweeds are variations on the same theme. In some cases, they vary primarily in the location of the auxillary cell and the ploidy of the carposporophyte.

The ability of nuclei and other organelles to move through the thallus of the red algae has given rise to a number of parasitic taxa. These have specialized spores that fuse with cells of a target host plant. Then, they inject their nuclei, which direct the mitosis and proliferation of more parasitic nuclei. Then, they direct the development of spores and the formation of a sporangium.