Seaweeds are many-celled marine algae. They are classified according to their pigment type:

  • green algae
  • red algae
  • brown algae

If you find a white seaweed, it is probably a red algae that has been bleached by the sun. A seaweed that feels crisp is a species that is calcified (lime-coated).

You can read more in Dr Anne Brearley’s article on Seaweeds and seagrasses of the Cottesloe area.

What are algae?

(singular = alga, plural = algae)

Algae are simple plants.

  • Micro-algae are tiny, single-celled plants. We may see a mass of them as a green scum in water or slime on the side of a pool. At sea they may form part of the plankton.
  • Macro-algae are many-celled plants, but they have no roots, stems or leaves, and they have no vascular system to carry water or sap around the plant. They have no flowers, pollen, fruit or seed, but reproduce by spores. They are the seaweeds.
  • Blue-green algae (cyanobacteria) are a different group altogether, related to the bacteria.

Click on an image for a bigger picture (opens in new window).

Green algae

 Common nameScientific nameDescription
Sea LettuceUlva lactuca This is a worldwide species of green alga. It is seen here just south of Mudurup Rocks forming a bright green band along the rocky shore. The lamina or “leaves” are only two cells thick. It is sometimes used as a food.
Velvet Golf Ball Codium mamillosumLike all species of the genus Codium, this spherical one has a velvety surface made up of tiny tubes, which are the swollen ends of the entwined filaments that make up the interior of the plant.
Dead Man’s FingersCodium galeatum This aptly-named alga regularly branches into two and can grow up to a metre long. All species of Codium have a similar spongy or velvety surface although their overall shape differs greatly.
Velvet Sponge WeedCodium spongeosumThe various species of Codium have very different shapes but all have the same spongy or velvety outer surface. Each plant is composed of entwined filaments, without cross-walls, so that a Codium plant is essentially an enormous single cell.

Red algae

 Common nameScientific nameDescription
Red algaeRhodophytaThere are many species of red algae growing along our coast. Many are small plants forming the “algal turf” on the reef platforms. Their red pigment is able to trap filtered light under the water and use it for photosynthesis. Some are calcified – they have a lime-coated surface and feel crisp. Stranded red algae are often bleached white by the sun.
Slimy BagsGloiosaccion brownii Despite their varying colours, these are red algae. Their Latin generic name means “glue sack” meaning that they are filled with mucilage. The function of this glue is not well understood.
Jelly Weed Betaphycus speciosum (formerly Eucheuma)In colonial days women collected this red alga to make jelly and blancmange as they had done with Irish Moss in Britain. It contains the gelling compounds agar and carrageenan. It is a thick, tough plant with short, spiny branches.
Epiphytic Algae These red algae are growing as epiphytes on the stem of the wireweed Amphibolis. Many algae, as well as animals such as hydroids and barnacles, grow attached to seagrasses without being parasitic – it is simply a base or substratum for them to hold onto.

Brown algae

 Common nameScientific nameDescription
Sargasso WeedSargassum sp. The many species of Sargassum have little ball-like floats. They have a holdfast (like roots), a stipe (like a stem) and laminae (like leaves). However these are not true roots, stems and leaves because algae do not pump water up from the roots to the leaves as the vascular land plants do – they simply absorb nutrients directly from the water.
Warty WeedScaberia agardhii The wiry stems of this brown alga are crowded with stumpy branches and floats, giving it a warty appearance. It grows in water up to 40 m deep.
Common Kelp Ecklonia radiata The kelps are large, tough, leathery brown algae. Common kelp can grow up to two metres high, attached to the reef by a holdfast. It has a tough, cylindrical stipe and spiny, corrugated laminae. It grows commonly on limestone reefs, down to depths of 40 metres, and is often the dominant feature of the underwater landscape on rough-water coasts.
Common Kelp - holdfastEcklonia radiata This is the holdfast of the common kelp Ecklonia radiata described above. Seaweeds, or algae, do not have true roots because they absorb all their nutrients directly from the water. The holdfast is merely to hold it to the rock in rough seas. This one, however, came away with the piece of rock or coral that is was attached to.
This picture of Asher McCarthy was a winner in our 2007 Flotsam photo competition.
Oyster Thief Colpomenia sinuosa This is a brown seaweed shaped like a hollow cushion. It is normally fixed to a substratum such as a rock or the shell of a mollusc. Oxygen from photosynthesis fills the hollow and gives it buoyancy. Its common name arose in the oyster industry, when large individuals of this species floated to the surface carrying their attached substratum with them!
Ornate TurbinariaTurbinaria gracilisThis is a tough, gristle-like brown seaweed with characteristic “crowns”.