Seaweeds are actually multicellular algae. They can have quite complicated structures, though they’re not as complex as land plants. Whilst referred to here as plants, some scientists group them with single celled algae.

Seaweeds vary in structure and size, but all have a few things in common. The complete plant, whatever form it takes, is referred to as the thallus. Photosynthesis takes place in the flat, leaf-like parts (or blades) in those seaweeds that contain chlorophyll. There’s no difference between the upper and lower surfaces of the blades and there are no veins.

Some seaweeds have gas-filled bladders, or pneumatocysts, that keep the blades floating where sunlight penetrates the surface. An equivalent of the stem is the stipe which provides support, although remaining flexible. At the bottom of the plant is a holdfast which looks like a root system but does not feed the plant as true roots do. Its main function is to keep the plant anchored to the sea bed.

The colour of marine algae depends on the natural pigments and chlorophyll present. Green indicates the presence of chlorophyll or more correctly, chlorophyll that hasn’t had its colour hidden by other pigments. Most green algae live in fresh water and account for the bright green colour sometimes seen in ponds. About 10 per cent of green algae are found at sea, most often in bays, estuaries and rock-pools. They are the least complex structurally and are often small. Sea lettuce is an example found in many environments. Dead man’s finger (Codium) is found in temperate waters and has a sponge-like texture made of many filaments. Some green algae, known as endophytes, live on other types of plant.

Most of the 6000 species of marine algae are red - more than green and brown algae combined. Their colour is from a red pigment called phycobilin which hides the green of the chlorophyll. They actually come in a number of colour variations from pink through red to metallic purple, depending on how much light they get. The coralline red algae contains calcium carbonate deposits that are important in the formation of coral reefs.

There are 1500 species of brown seaweed, ranging in colour from olive green to deep brown. This colour comes from a yellow pigment that dominates the chlorophyll. The vast majority of these are marine algae and play an important part in the food web of temperate coasts. They may be simple and small, like the rookweeds or wracks (Fucus) found on temperate rocky foreshores and often seen when the tides goes out. Kelps are a type of brown algae and can be very large. The giant kelp can be as much as 100m long and its holdfast alone weighs several kilos.

In the case of Sargassum, a tiny piece of the plant can produce a new one, but many seaweeds reproduce by dispersing spores. This process can be extremely complex, involving male and female gametes from different plants that fuse to form tiny new plants. There’s some evidence to suggest that spore development may depend on the levels of nutrients, temperature or daylight.