The simplest of the seaweeds are among the cyanobacteria, formerly called the blue-green algae, and green algae (division Chlorophyta), found nearest the shore in shallow waters and usually growing as threadlike filaments, irregular sheets, or branching fronds. The brown algae (division Phaeophyta), in which brown pigment masks the green of the chlorophyll, are the most numerous of the seaweeds of temperate and polar regions. They grow at depths of 50 to 75 ft (15–23 m). The red seaweeds (division Rhodophyta), many of them delicate and fernlike, are found at the greatest depths (up to 879 ft/268 m); their red pigment enables them to absorb the blue and violet light present at those depths.
The Columbia Electronic Encyclopedia, 6th ed. Copyright © 2012, Columbia University Press. All rights reserved.