Is Seaweed a Plant?

The common name of seaweed is generally applied to a great variety of marine plants and algae that grow in oceans, lakes, and rivers. Weeds are plants that spread easily and can be invasive and unwelcome. Seaweed can also spread easily and be invasive but is seaweed a plant?

Seaweed is not a plant.  It is a type of algae that is found mostly in salt water.  It differs from plants in two main ways:

  • Instead of absorbing nutrients through a root system and moving the nutrients through stems to leaves, seaweed can absorb food directly from the water into their tissues.
  • While both use photosynthesis to make food, seaweed can photosynthesize in all their tissues whereas plants only use their leaves.

Most seaweeds have holdfasts: basal structures that do exactly what the name suggests – hold fast to rocks. They fall into three main categories; red, brown, and green. Red and brown seaweeds are mostly marine whereas green species do occur in freshwater areas. Red seaweed (Rhodophyta) has over 6,500 different species, including dulse (Palmaria palmate), and irish moss (Chondrus crispus). Brown seaweed (Phaeophyceae) has over 1,800 species, including knotted wrack (Ascophyllum nodosum). This is also known locally as rockweed. Green seaweed (Chlorophyceae) has over 1,500 marine species, including sea lettuce (Enteromorpha sp). It is also known as sea grass.

Sea lettuce is a type of seaweed one can can’t help but notice on the shoreline at Burntcoat Head Park. It is bright green year round and slippery so care must be taken when walking on it. A local name is mermaid’s hair. Knotted wrack is also quite easy to notice on the shoreline. It can be identified by its many round bladders that run up and down its length. Clumps often wash up on the shoreline. Other species of wrack (Fucus) include toothed and bladder wrack.