noun, plural: seaweeds
Seaweeds are marine plants thriving particularly near the seabed. Thus, they are one of the benthic organisms found near or in marine sedimentary habitats, e.g. along the foreshore and abyssal depths. Some of the fundamental requirements for seaweeds to grow are a seawater habitat, light that can penetrate seabed since it is essential for photosynthesis, and a substrate to attach to. Nevertheless, there are species that are considered as seaweed yet they are freely floating, e.g. Sargassum species.
Seaweeds include members of the algal species, e.g. red algae, brown algae, and green algae. The term seaweed is therefore a colloquial term for some algal species in marine waters. Because of this seaweeds are used sometimes as a synonym for algae. There are many other algae though which are not considered as seaweeds, such as the unicellular diatoms.
Seaweeds typically have a thallus body, which is undifferentiated vegetative tissue. They therefore do not have true stems, leaves, and roots. A true stem, leaves and roots would have a vascular system as found in higher plants. They rather have lamina, stipe, holdfast, and haptera plant body parts.
Ecologically, seaweeds are essential as food for some marine organisms. Humans utilize seaweeds various purposes such as for medicine, research, food, and fertilizers.