The red algae are small, mostly marine plants that grow in the lower intertidal regions along the coast (Fig.1). Many are free living, usually attached to rocks. Others grow on or in other marine plants. A few are microscopic, but most are filamentous, membranous, or pseudoparenchymatous (where the plant consists of numerous interwoven filaments). Cell structure in the group is eukaryotic. Cells in many species are linked by a structure called a pit connection. Photosynthetic pigments in plastids include chlorophyll a (+ chlorophyll d in some), and various accessory pigments, the most notable being phycobilins. Food is stored in the form of floridean starch. Cell walls contain a network of cellulose fibrils embedded in mucilaginous polysaccharides like agar and carrageenan. These polysaccharides help prevent drying when the algae are exposed during low tides. Some species of red algae are commercially important as sources of wall polysaccharides to be used as jelling, emulsifying and stabilizing agents (Fig.2). Many red algae have calcium carbonate (lime) in their walls. These algae contribute to the formation of coral reefs.

Sexual reproduction in red algae is oogamous, with female gametes produced in carpogonia (with associated female receptive hairs called trichogynes) and non-flagellated male gametes released from spermatangia. No flagellated cells of any kind are to be found in this group. The vegetative plant of simpler red algae is haploid, the only diploid stage being a zygote. The life history of advanced species is very complex, often consisting of three separate stages with free living or parasitic haploid and diploid plants. In Polysiphonia, there is a small filamentous 1n gametophyte (Fig.3) that produces spermatangia (Fig.4) and carpogonia. When a male gamete fertilizes an egg, a small 2n carposporophyte results (Fig.5). The carposporophte remains attached to the gametophyte and produces 2n carpospores when mature. Carpospores grow into 2n filments that resemble the 1n gametophte filaments. When mature these filments produce 1n tetraspores by meiosis (Fig.6). The second stage sporophyte is therefore called a tetrasporophyte. When tetraspores germinate, they become gametophyte filaments and thus complete the sexual cycle.

Fossil evidence suggests that the red algae evolved in tropical oceans, possibly from blue-green bacteria about 450 million years ago. Reef forming species were abundant in the tropics 100 million years later. Over 3500 species have been described.