Generally, variations of seaweed are grouped by their colouration, but biochemical features and cell structure plays an important role in classification, too. Many brown seaweeds, often called ‘wracks,’ have small, grape-like air sacs that enable them to stay afloat during high tides, whereas green and red seaweeds are part of the same classification which includes all the plants found on land. This offers an interesting insight into how different our seaweeds are and how each species plays an important role on our islands.

When we think of seaweed, sea lettuce (Ulva lactuca) is likely to be one of the first species that comes to mind. Green, slimy, and unmistakable, it is found attached to rocks or floating inside rock pools if detached. Much like its namesake, the fronds of sea lettuce, which grow up to 30cm, look like ruffled flaps of lettuce, surprisingly tough despite its tatty appearance. It is remarkably resilient, able to thrive in highly polluted areas; large quantities of sea lettuce are often used as an indicator for polluted water. When detached, it will continue to grow and form large colonies, which can have a detrimental effect on other species of seaweed as it can block out the light, thus killing the plants submerged below. Seaweeds, like other plants, require access to sunlight in order to photosynthesise, which is why they grow in intertidal zones (the area above water level at low tide). You can find sea lettuce washed up on the shore as well in the rocky areas where it thrives.

Large, dramatic and easily recognisable, oar weed (Laminaria digitata) is another familiar face in the shallow seas around Scilly's coast. It can grow in depths of up to 20m, anchoring itself to bedrock and other hard surfaces with its strong, root-like holdfasts. Its leathery fronds may be exposed at low tide, providing a safe microhabitat for marine life such as worms, brittle stars, and even sea spiders. Oar weed is brown in colour and has a thick, leathery appearance. Its long fronds split into finger-like ribbons, which are attached to a flexible stipe (or stalk) that can grow to lengths of 5ft. This allows the plant to bend at low tides, preventing the fronds from drying out. It also helps the plant to survive rough and stormy conditions — great for Scilly!

Gutweed (Ulva intestinalis) is a species of green seaweed that is closely related to sea lettuce, but lacks the frilled shape of its cousin, instead forming tube-like mats that look like intestines when inflated with oxygen (contributing to its common name). Gutweed is found on all UK shores and grows in a wide variety of habitats, including sand, mud, rock pools, shells, and even on other species of seaweed. When submerged, the air bubbles trapped inside the tubes allow the fronds to float upright underwater. Even when detached, gutweed will float to the surface and continue to grow. Some varieties of gut weed are known to survive in the mouths of rivers, too, and also in brackish waters, as it is very tolerant of low salinities (the saltiness of water). On Scilly, gutweed grows primarily on the upper shores of our beaches. This species is fast-growing anyway but has the potential to grow at an alarming rate if the water has a high concentration of nitrogen and phosphorous compounds.

There is a species of brown seaweed on our shores that is often better acquainted with our bare feet than our eyes. Bladder wrack (Fucus vesiculosus) is widely associated with the seashore and grows on Scilly’s rocky shores between the high and low watermarks, forming dense beds across the mid shore. It is recognised by its mixed shades of olive brown and round air bladders that allow the seaweed to float on the surface, helping it to absorb nutrients whilst submerged. Often paired with a similar seaweed called egg wrack, bladder wrack provides a home for many marine creatures; it is also a food source for creatures such grazing snails. I remember as a child wading through wet, slimly forests of bladder wrack, looking for crabs hiding under the rocks, detesting the feeling of those air ‘bladders’ between my toes. These days I have a better appreciation for the wracks — they are the favourite food source of the brightly coloured flat periwinkle.

Perhaps one of the only species of seaweed with an appetising name, sugar kelp (Saccharina latissima) is commonly found in shallow waters around our coastline. Similar to oar weed, sugar kelp is large and brown and attaches to rocks using strong, root-like holdfasts, growing in depths up to 30m and in deep rockpools. Sugar kelp is identified by its long, single strand that can grow up to 5m long, giving the seaweed the alternative name of sea belt. The edges of this ribbon-like strand are ruffled and dimpled, which helps with water movement and the exchange of gases on the surface. The name ‘sugar kelp’ comes from the sweet white power (mannitol) that is found on the surface as the seaweed dries. Sugar kelp has a long history of being used as a sweetener! Additionally, it has also been used to predict the weather; the strands were once hung outside to detect humidity, and if the strands remained dry, the day would be fine, but if it went soft and limp, rain was on the way. This contributed to its other common name of Poor Man's Weather Glass.

All seaweeds found on foot in the UK are non-toxic and edible, but not all of them are tasty, so it is recommended that you research the best species of edible seaweeds before you go foraging on our beautiful shores. Always seek professional advice before consuming wild plants.