Sexual reproduction is highly developed and normally exceedingly complex in red algae. It is based on the fertilization of egg cells by small, nonmotile, male gametes (spermatia). In many cases it involves the transfer of fertilized nuclei into specialized auxiliary cells and filaments (gonimoblasts) prior to their inclusion in spores. Alternation of cytologically different generations (usually isomorphic or identical in structure) is found in the more highly evolved genera. The reserve foodstuff is a polysaccharide known as Floridean starch.
The single class Rhodophyceae is divided into two subclasses, Bangiophycidae and Florideophycidae. Members of the former are more primitive and lack the complicated postfertilization processes found in those of the latter. They are also less highly organized morphologically. A well-known representative is laver (Porphyra).
Florideophycidae's orders—which include Ceramiales, Compsopogonales, Corallinales, Cryptonemiales, Gigartinales, Nemaliales, and Rhodymeniales—have been established based on features of the life cycle and postfertilization development of their species. The typical life cycle of one of the higher Florideophycean red algae, such as Polysiphonia, may be briefly summarized as follows.
The first generation, the gametophytes, produces spermatia and egg cells, commonly on different individuals. From the fertilized egg a small organism called the carposporophyte develops parasitically on the female gametophyte. It is diploid (having a double number of chromosomes in its nuclei) and consists of only a few rows of cells or filaments. The carposporophyte gives rise to a cluster of spores, usually inside a protective sheath (cystocarp). These spores are released and germinate to give rise to the second (ordinarily isomorphic) generation of organisms, which, unlike the gametophytes, has diploid nuclei. The number of chromosomes is reduced by half in the developing tetraspores (spores that develop in structures called tetrasporangia, which form on the second-generation algae). On being liberated, the tetraspores germinate to produce a new generation of gametophytes. Thus the life cycle is completed.
Certain genera of the order Corallinales deposit lime in the thallus, making the body rigid and stony. The body can be either solid (as in Lithothamnium) or in articulated segments (as in Corallina).
Some of the better-known representatives of the subclass Florideophycidae are Chondrus (Irish moss), Rhodymenia (dulse), and Ceramium (lobster claws). All are common on northern shores.
