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Home > Is Seaweed a Plant?

Is seaweed a plant?

No, seaweed is not a plant. Although it looks like a plant, it's really a type of algae.

Seaweeds are found only in salt water. They live along ocean coastlines. You'll often find them washed up on the beach, or waving in tidal pools near the shore. Wade into the ocean and you might feel some slippery against your feet and legs.

A boy plays with seaweed on a beach.

A brown algae called sea palm (Postelsia palmaeformis).There are many types of seaweeds, but most of them have the same parts. And it's true that those parts often look similar to parts of a plant.

  • A structure called a holdfast helps seaweeds grip the bottom.
  • The stipe looks like a plant stem.
  • Many seaweeds have blades or membranes on the stipe that resemble leaves.
  • Some have air bladders like balloons that help them float near the surface.

Seaweeds (like all algae) have another similarity to plants: photosynthesis. Photosynthesis is a process that produces the organism's own food with the help of light.

Can You Spot the Differences?

Seaweeds differ from plants in important ways:

  • Seaweeds absorb water and nutrients in all their tissues, directly from the surrounding water. They don't have the complex system of roots, specialized tissues and leaves that help plants move water and nutrients in their body.

  • Seaweeds can do photosynthesis in all their tissues; most plants photosynthesize only in their leaves.

Because of these abilities, scientists sort seaweeds into the algae group, not with plants. All algae have these same abilities.

Creating Food from Light

Although they are not plants, seaweeds share one important thing with trees, shrubs, grasses and other plants: they make their food in the same way.

Seaweeds and plants make food by a process called photosynthesis. They use energy from sunlight to convert carbon dioxide from the air into food energy.

Underwater view of a bull-kelp forest (Nereocystis luetkeana) bathed in light.

Because seaweeds use photosynthesis to make food, they need to grow in shallow water where light penetrates.

Most seaweeds live within about 100 metres of the surface, although some types can live more than 300 metres down.

The Longest Seaweed

How fast does a human hair grow? Just over 1 centimetre a month, on average.

Imagine your hair growing as fast as bull kelp (Nereocystis luetkeana), which can grow 25 centimetres in a day!

A strand of bull kelp (Nereocystis luetkeana) washed up on a beach.

Over its life, a bull kelp can grow to 20 metres in length, or more!

This seaweed is found in the Pacific Ocean.

An image from a video showing a kelp forest.
Watch This Video!

Some of the most impressive examples of seaweeds can be found off the west coast of North America.

There, the seaweeds grow so tall and thick that a patch is called a forest! The kinds of seaweeds that make such forests are kelps. Take a virtual dive through a kelp forest by watching this video.
Video (23 sec.)


Types of Seaweeds

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