name commonly used for the multicellular marine algae. Simpler forms, consisting of one cell (e.g., the diatom) or of a few cells, are not generally called seaweeds; these tiny plants help to make up plankton. The more highly developed types of seaweed usually have a basal disk, called a holdfast, and a frond of varying length and shape, which often resembles a plant in having stemlike and leaflike parts.
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.
Seaweeds reproduce in a variety of ways. Lower types reproduce asexually. More advanced kinds produce motile zoospores that swim off, anchor themselves, and grow into new individuals, or they reproduce sexually by forming sex cells (gametes) that, after fusing, follow the same pattern. Sometimes pieces of a seaweed break off and form new plants; in a few species there is a cycle of asexual and sexual reproduction foreshadowing the alternation of generations characteristic of plants.
The largest of the green algae, Ulva (sea lettuce), grows to a ribbon or sheet 3 ft (91 cm) long. It provides food for many sea creatures, and its broad surface releases a large amount of oxygen. Fucus, called rockweed or bladderwrack, is a tough, leathery brown alga (though it often looks olive-green) that clings to rocks and has flattened, branched fronds buoyed by air bladders at the tips.
Seaweeds, especially species of the red algae Porphyra (nori) and Chondrus, form an important part of the diet and are farmed for food in China and Japan; other species (often called laver) are eaten in the British Isles and Iceland. Commercial agar (vegetable gelatin) is obtained from species of red algae and is the most valuable seaweed product. Irish moss or carrageen (Chondrus crispus), a red alga, is one of the few seaweeds used commercially in the United States. After being bleached in the sun the fronds contain a high proportion of gelatin, which is used for cooking, textile sizing, making cosmetics, and other purposes. In Japan it is made into a shampoo to impart gloss to the hair.
The kelps generally include the many large brown seaweeds and are among the most familiar forms found on North American coasts. Some have fronds up to 200 ft (61 m) long, e.g., the Pacific coast Nereocystis and Macrocystis, found also off the Cape of Good Hope. Common Atlantic species include Laminaria and Agarum (devil's apron). The kelps are a source of salts of iodine and potassium and, to a lesser extent, other minerals. When the seaweed is burned, the soluble mineral compounds are removed from the ashes (also called kelp) by washing. They are used chiefly as chemical reagents and for dietary deficiencies in people and in livestock. Kelp is also a commercial source of potash, fertilizer, and medicines made from its vitamin and mineral content. Kelps are especially abundant in Japan, and various foods known as kombu are made from them.
The brown algae of the genus Sargassum are called gulfweed. They inhabit warm ocean regions and are commonly found floating in large patches in the Sargasso Sea and in the Gulf Stream. Gulfweed was observed by Columbus. Although it was formerly thought to cover the whole Sargasso Sea, making navigation impossible, it has since been found to occur only in drifts. Numerous berrylike air sacs keep the branching plant afloat. The thick masses of gulfweed provide the environment for a distinctive and specialized group of marine forms, many of which are not found elsewhere.
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Large multicellular red, brown, or green marine algae that are generally found attached to the sea bed, rocks, or other solid structures by...
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