Which “weed” is not actually a weed but a name for a countless number of species?
And the answer: seaweed.
Photo credit: Grubio--1

Seaweed is the common name for a variety of marine plants and algae that grow in the ocean as well as other bodies of water. It is not technically a “weed” because it is both necessary and beneficial to its environment. A weed is classified as any plant that is either a nuisance or harmful to its environment.

Due to the fact that algae species evolved separately, it can be difficult to pin down a specific definition of seaweed. Generally speaking, however, seaweed (or "macroalgae") is a term applied to thousands of species of macroscopic, multicellular marine algae. Seaweeds are also sometimes referred to as "non-vascular plants" due to the fact that, despite their plant-like appearance, seaweeds lack the specialized vessels and cells that plants use to intake water and produce sap. Accordingly, seaweeds also lack structures such as stems, roots and leaves, instead using the nutrient-rich water to carry out photosynthesis.

Although seaweeds are not plants, they do occupy the same role in the ecosystem. Their ability to photosynthesize means that they are the primary producers of the food chain, and many marine ecosystems rely on seaweeds to survive.

The three separate categorizations of seaweeds are based on their varying pigments. Green algae make up the plylum chlorophyta, and get their bright green color from their dominant pigment: chlorophyll. Green algae is mainly found in freshwater environments, with only about 13% of green algae species being marine. This algae prefers bright sunlight, and can most often be found in shallow water (so make sure to keep a look out next time you go tide-pooling!).

The second phylym, rhodophyta, is comprised of red seaweeds. Thought to be the most diverse group of the three, red seaweeds can be found in the ocean all over the world in deeper waters. In fact, red seaweeds can survive at the greatest depths of any other algae due to their ability to absorb and convert light. One Caribbean species has even been found at depths of over 200 meters!

The last main group of macroalgae are the brown seaweeds, or orchophyta. These multicellular seaweeds are those which typically come to mind when picturing "seaweed"— they contain many recognizable species such as rockweeds and kelp. Brown seaweeds occupy colder, tidal zones in oceans across the planet.

Learn more about seaweeds and algae here.

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