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 Seaweeds are marine algae: saltwater-dwelling, simple organisms that fall into the somewhat outdated, but still useful, category of "plants".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.
Most people know two general categories of seaweeds: wracks (members of the brown algal order Fucales such as Fucus) and kelps (members of the brown algal order Laminariales such as Laminaria), and many have heard of Carrageen or Irish Moss (usually a red alga, Chondrus crispus) and Dulse or Dillisk (also a red alga, Palmaria palmata). Seaweeds make up the Sargasso Sea, a large ocean gyre in the western Atlantic where drift plants of several species of the genus Sargassum accumulate. Seaweeds are particularly important ecologically: they dominate the rocky intertidal in most oceans, and in temperate and polar regions cover rock surfaces in the shallow subtidal. Although only penetrating to 8-40 m in most oceans, some are found to depths of 250 m in particularly clear waters (Mediterranean, Caribbean, Brazil). The Giant Kelp (Macrocystis) is one of the largest plants in the world, which in western North America forms an important association with the newly rescued Sea Otter.
Seaweeds are algae that live in the sea or in brackish water. Scientists often call them "benthic marine algae", which just means "attached algae that live in the sea". They come in three basic colours: red, green, and brown. Brown as shown on the right on a New Zealand shore: red from Tasmania below; and green endemic sea Enteromorpha howensis on Lord Howe Island off Australia.
Red and brown algae are almost exclusively marine, whilst green algae are also common in freshwater (rivers and lakes), and even in terrestrial (rocks, walls, houses, and tree bark in damp places) situations.
Many of these algae are very ancient organisms, and although lumped together as "algae", are not really very closely related, having representatives in 4 of the 5 or 6 kingdoms of organisms.
Seaweeds are far more complex organisms than generally realised. Many have specialised tissues and growth forms. They may have very complicated sex, with many of them producing sex pheremones (chemicals that attract sperm), and with many different types of sex organs. Red algae have the most complicated sex known in plants.
Kelps are known to have quite rapid translocation, something that is not credited to algae in many textbooks. There is even growing evidence of root-like structures in some wracks that reach deep into rocks. Generally, seaweeds and many algae have holdfasts: basal structures that do exactly what the name suggests - hold fast to the rock. Seaweeds must produce some amazing adhesives as quite small holdfasts seem to be sufficient for quite lage plants. On the right you can see the small holdfast (about 1 cm across) of an Ascophyllum nodosum (Egg Wrack) clump about 2 m in length.
Read more at www.seaweed.ie