What exactly is seaweed? Is it a plant and how does it grow? Lara Howe, Marine Officer at Manx Wildlife Trusts, explains the mysteries of this slimy water plant
Yes, seaweed grows in the sea, but it’s not a plant. While both seaweed and plants can photosynthesise using chlorophyll, that’s where the similarities end, says Lara Howe, Marine Officer at Manx Wildlife Trusts. Seaweed is definitely not a plant – here’s why.
What is seaweed?
Found in the sea, seaweeds is actually algae. Algae are made up of thousands of species that can be divided into three main groups – red, brown and green.
Instead of roots, seaweeds have holdfasts that anchor them to the rock to stop them getting washed away by the waves or current. As it has no roots seaweed takes the nutrients it needs from the sea.
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Seaweeds also have a stipe rather than a stem, although it doesn’t provide structural support for the seaweed like a stem does with a plant – in fact seaweeds often rely on gas filled bladders to keep them upright in the water. Instead of leaves they have blades, and this is where the majority of photosynthesis takes place.
Because light diminishes rapidly with increasing water depth, seaweeds typically inhabit shallow waters. However, various seaweed groups possess additional pigments enabling them to utilise different wavelengths of light. Consequently, they can thrive at varying depths, showcasing a diverse array of colours.
They can also possess a slippery coating that allows them to move freely in the water without suffering too much damage.
Some species are calcified and can encrust rocks giving them a pretty pink, purple or dark red colour.
Where can you see seaweed?
Seaweeds can be seen anywhere around the UK on rocky shores and shallow seas, where they anchor to rocks or stones. Just go exploring when the tide is out or go snorkelling or scuba diving.
If you walk along a beach at low water, you’ll see that different species of seaweed will be seen at different heights on the shore. For example, channel wrack will be found at the top of the shore whilst kelp species will be found at the lower end of the shore at the low water mark. This is called zonation and reflects the difference conditions the seaweeds face at different heights on the shore and each is brilliantly adapted to copy with these differing conditions.
Are any seaweeds rare?
Some seaweeds are rare, and others are super common. For example, many of our wrack species such as bladder wrack or spiral wrack are very common, whilst other species such as maerl, a calcified red seaweed, is declining as a result of its historic extraction for soil conditioning and harmful fishing practices and other damaging activities. And it takes a long time to grow back, 1mm a year!
Is seaweed edible?
Seaweeds are grazed by other organisms and often have defence mechanisms to stop them, such as noxious chemicals that make the algae taste bad or could even be toxic.
However, many seaweeds are edible and have been cultivated and traded for centuries, providing a wide range of trace minerals and vitamins.