Rhodophyta - Red Algae
Unicellular or multicellular PLANTAE of diverse morphology: endolithic minute filamentous epiphytes, branching filaments or foliose. Without motile structures or flagellated cells. Several species deposit calcium carbonate, either from certain regions of the plant (articulated forms) or from the whole thallus (encrusting forms). Size ranges from microscopic to several decimeters. Among the larger red algae two more or less distinct groups are recognizable: the more conspicuous, fleshy forms (filamentous, foliose or bushy), and the hard, often brittle encrusting corallines. Although shades of red, from almost black, to pink, to almost white, predominate, shades of yellow, olive, green or greenish brown are common as well.
Slightly under 4000 species of marine red algae are known, and placed in a single class, Rhodophyceae, which is divided into two subclasses, Bangiophycidae and Florideophycidae. Over 200 species representing six orders are reported from Bermuda. Six species are included in this account.
In all latitudes, but their numbers and conspicuousness markedly increase in temperate and tropical regions where they outnumber both green and brown algae. Although many intertidal species are known, most appear to be subtidal. All reds appear to originate as attached forms, but because of wave-action and grazing, they are often found floating. Larger brown and green algae often serve as hosts for epiphytic red algae. Epiphytic red algae usually do not penetrate their hosts but are superficially attached. In many cases a single host may harbour a dozen different species of epiphytes. Encrusting forms grow on every type of substrate below the surface of the water. Because of their deposition of calcium carbonate, these algae contribute substantially to the maintenance and stability of reef structures by filling in and cementing spaces and crevices between old coral heads. Factors influencing abundance of red algae: day length, relative light conditions, temperature, and periodic and continual grazing by herbivorous invertebrates and vertebrates, especially starfish and sea urchins.
Usually distinguished from green and brown algae by colour, which alone, however, is not a reliable characteristic.
Red algae are widely distributed bathymetrically as well as geographically. The depth to which the algae grow is determined by the availability of light penetrating the water and their possession of specific photosynthetic pigments. Red algae in high intertidal zones absorb the full light spectrum, especially red wavelengths, which enhance the production of phycocyanin. Under these conditions, the predominant colour is green, olive or dark brown. At greater depths, the absence of red light enhances the production of phycoerythrin, which results in a rosy-red or pink colour.