The Taxonomy of the Rhodophyta shows that it is an ancient division that branched off very early in the tree of life. There is only one class and two subclasses in the Division. In these subclasses, however, are over 10,000 species that have been described. Only two percent of these species are found in fresh water, and those are only found in very fast flowing streams. The other 98% are in the ocean.

The Rhodophyta are distributed worldwide (see, for example, the distribution of Plocamium,) but they grow best in waters between 10-15 ºC. In Monterey Bay, California, which is famous for the enormous brown algae, the total biomass (dry weight) of red algae outweighs the total biomass of brown algae (phaeophyceae: kelps etc.) put together (Goff, Lectures 1999).

In the warmer tropics grazing by fish and invertebrates (snails, etc.) is much more extreme, and algae are often restricted to cracks and crevices. Hard encrusting algae (calcified) are also very common in the tropics where grazing is severe.

Certain species in the Rhodophyta are particularly well adapted to living in the cold. Antarctica has two erect and one encrusting algae as far as 78 ºS of the equator. The Rhodophyta are particularly well adapted to low light levels, and the Antarctic species are adept at energy storage. Both of these factors allow the Rhodophyta to survive Antarctica's long months of near darkness.
 
 

Pigments:
Chlorophyll a	Chlorophyll a: The main pigment responsible for photosynthesis, which allows the alga to incorporate the carbon from carbon-dioxide, CO2, into sugars and also harness other forms of energy.  Chlorophyll d: Only in a few advanced red algae. There is no evidence that chlorophyll d is involved in photosynthesis.
b carotene	a and b carotenes: These give the alga an orange tinge, but do not usually play a direct role in photosynthesis.
Phycobilisome. (Goff, Marine Botany Web site 1999) Phycoerythrin. Phycocyanin	Phycobilisomes: Form tiny bumps that stud the membranes involved in photosynthesis (thylakoid membranes). These phycobilisome bumps, which are composed of phycobiliproteins, act like little antennae that catch as much light as possible. They shuttle that light energy to the photosynthetic centers where sugars are created and energy is harvested. The three principle phycobiliproteins are: Phycocyanin and Allophycocyanin Give some red algae a distinct blue/green appearance Phycoerythrin Phycoerythrin reflects red light and is therefore responsible for the color of most red algae. All the colors that are not reflected are absorbed, so phycoerythrin is good at absorbing blue/green light. Because that is the color of light that penetrates the deepest in the ocean, red algae can live at depths which receive as little as one thousand times less light than the surface does (0.1% surface irradiance).
Starch Products:
Glycogen: Floridean starch	Floridean starch: Most red algae store their sugars as glycogen, or Floridean starch, as it is often called. Glycogen is a long chain of glucose sugars with lots of side branches (a 1-4 linked glucans with 1-6 branches). This Floridean starch is very different from the starch that brown or green algae store. Greens have a grainy starch that reacts with iodine to create a black color (said to be IKI +). Brown algae have an oily starch that doesn't produce any color when put to the iodine test (IKI -). Reds, which lie somewhere between those two, produces a pinkish brown color in response to iodine-exposure (also considered IKI -).
Cell Motility:
	There is none. Firstly, there are very few single celled red algae. And secondly, the gametes (reproductive cells) of red algae cannot swim like human sperm can, because they lack flagella, or tails. Male reproductive cells, therefore, are not sperm, but instead are called "spermatia". They rely entirely on water movement to carry them to the female reproductive cell (which, in Rhodophyta, is called a carpogonium instead of an egg).
Cell Walls:
Sheets of agar gel are used in genetic research. Ice creams contain carrageenan as a thickener. Heavily calcified corallines are stony and hard for fish to eat.	Besides having the usual cellulose cell walls that most algae have, Rhodophyta also have 3 important chemicals in their cell walls. These chemicals are amorphous (shapeless) mucilages (mucuses) which are widely used in science and food preparation. Agar: The petri dishes that scientists use to grow bacteria and other such things usually have a layer of agar in them, on which the bacteria grow. Agar is also used widely in genetic research and in food preparation as a gelling agent/stabilizer. Pharmaceuticals use agar to regulate time-released medications. Carrageenan: Is used as widely as agar, but more in food preparation. Ice cream and instant puddings are stiffened with carrageenans. Algae often protect themselves from grazers (like snails) by producing different carrageenans that are indigestible to the herbivore. Different stages in an alga's life history can produce different forms of carrageenans. This helps ensure that an entire algal population is not eaten up - at least one phase of its life cycle will survive. Gelans: Are not made of chains of mucusy sugars, as agar and carrageenan are. Little is known about gelans. Calcium carbonate: Some red algae, like the corallines, deposit very hard calcium carbonate in their cell walls. This prevents them from being eaten and gives them structural support. Encrusting corallines are an important contribution to the growth of coral reefs. This is especially true in the tropics, where fish grazing is more frequent, and where the warmer water causes calcium to come out of solution more readily (precipitate).
Morphology:
	There are very few single celled red algae.  Most of the species are filamentous (single row of cells forming hairs). Many filamentous forms, however, have evolved in such a way as to create thick, fleshy thalli (bodies).  Parenchymatous Thalli: Some of the red algae from the subclass Bangiophyceae have a fleshy, sturdy body that is created from three dimensional cell division. Instead of only dividing along the length of the filament, cell division occurs sideways to form sheets, or in all three directions to form solid stacks of cells. This type of cellular growth, which is very similar to that of land plants, is called parenchymatous growth. Pseudoparenchymatous Thalli: Most of the red algae have filamentous growth that just appears to be parenchymatous. There are many ways in which a thallus can become thick and fleshy, using different configurations of filaments. In all cases, however, a close inspection of the tip of a branch or blade will reveal a single apical cell at the tip of a single filament. Uniaxial forms Have one central filament with lots of branching. As the branches keep branching, the outermost branches become extremely dense, forming what seems to be a sheet of cells on the surface. However, neighboring cells on the surface are not directly related to one another, as they are in parenchymatous tissue. Multiaxial forms A number of parallel hairs grow out from the base of the holdfast (horizontal filaments attached to the rock). Each filament has branches that tangle with each other, forming a dense web that appears solid. Articulated corallines, those hard, jointed algae, are multiaxial, with regions of branches which deposit calcium in their walls. Wherever there are no branches, a joint appears.  Pericentral forms A single filament divides at the tip, creating new cells at the tip which elongate the filament. As the older cells get left behind they start to divide away from the axis.
Life Cycles:
	The Bangean life history and Floridean life history are fairly complex, and are described separately.
	For more on Rhodophyta, visit UC Berkeley's "Introduction to the Rhodophyta"