Michael Guiry's seaweed site is a source of general information on all aspects of seaweeds. Seaweeds are marine algae: saltwater-dwelling, simple organisms that fall into the somewhat outdated, but still useful, category of "plants". Most of them are the green (about 1200 species), brown (about 1750 species) or red (about 6000 species) kinds illustrated on this page, and most are attached by holdfasts, which generally just have an anchorage function, although a particularly efficient one.