How to identify seaweed

Seaweeds require enough light to photosynthesise, so are most abundant in the intertidal zone; especially along rocky shores. Here are 12 for you to spot and identify.

Bladderwrack. © Olga Mironova/Getty

A tremendously diverse and beautiful variety of multicellular marine algae are collectively called ‘seaweed’. Often they are grouped into red, brown or green species, but these are not rigid distinctions.

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Seaweeds require enough light to photosynthesise, so are most abundant in the intertidal, or littoral, zone – especially along rocky shores, which offer plenty of secure attachment points. They are extraordinarily tough, having adapted to survive being exposed and submerged by the tides. Some species have gas-filled sacs and float in surface waters, sometimes far out to sea.

For centuries, seaweeds have been harvested for food – several of the dozen species pictured here are edible (and delicious). Foraging ‘sea greens’ is great fun, but watch the tides and take care to collect them away from any potential sources of pollution.

The colonisation of British shores by introduced seaweeds, such as wireweed and wakame (Japanese kelp), is causing concern for native biodiversity. So if you spot any aliens, report your finds to the Big Seaweed Search to help biologists track their spread.

All illustrations by Dan Cole / The Art Agency 


Purple laver (Porphyra umbilicalis)

Purple laver seaweed. © Dan Cole/The Art Agency
Purple laver seaweed. © Dan Cole/The Art Agency

Small and purple, with broad, tough fronds. Found in upper zone of rocky shores. Edible; eaten as laver bread.

Oarweed (Laminaria digitata)

Oarweed seaweed. © Dan Cole/The Art Agency
Oarweed seaweed. © Dan Cole/The Art Agency

A glossy, golden-brown kelp up to 2m long, with distinctive ‘fingers’ on broad fronds. Exposed at low tide.

Furbelows (Saccorhiza polyschides)

Furbelows seaweed. © Dan Cole/The Art Agency
Furbelows seaweed. © Dan Cole/The Art Agency

A brown kelp with a frilled stem and large bulbous holdfast that attaches to rocks. Revealed at low spring tides.

Bladderwrack (Fucus vesiculosus)

Bladderwrack seaweed. © Dan Cole/The Art Agency
Bladderwrack seaweed. © Dan Cole/The Art Agency

Olive-brown and up to 2m long, with air bladders at the frond ends. Forms beds in mid-zone of rocky shores.

Channelled wrack (Pelvetia canaliculata)

Channelled wrack seaweed. © Dan Cole/The Art Agency
Channelled wrack seaweed. © Dan Cole/The Art Agency

Brown and up to 15cm long, with fronds that curve inwards to create a channel. Found at top of the shore.

Pepper dulse (Osmundea pinnatifida)

Pepper dulce seaweed. © Dan Cole/The Art Agency
Pepper dulce seaweed. © Dan Cole/The Art Agency

Small, red or brown, fern-shaped seaweed of sheltered rocky coves. Edible; used as a curry-like spice.

Coral weed (Corallina officinalis)

Coral weed seaweed. © Dan Cole/The Art Agency
Coral weed seaweed. © Dan Cole/The Art Agency

Small, pink or purple seaweed with a feathery structure. Forms a dense carpet in pools on rocky shores.

Carrageen (Chondrus crispus)

Carragheen. © Dan Cole/The Art Agency
Carragheen. © Dan Cole/The Art Agency

Purple, with flat fronds up to 20cm long. Found on rocks on lower shore. Edible; used as a thickening agent.

Sugar kelp (Saccharina latissima)

Sugar kelp seaweed. © Dan Cole/The Art Agency
Sugar kelp seaweed. © Dan Cole/The Art Agency

A brown kelp up to 2m long, with frilly fronds. Found below low-tide mark. Edible; used as a sugar substitute.

Gut weed (Enteromorpha intestinalis)

Gut weed. © Dan Cole/The Art Agency
Gut weed. © Dan Cole/The Art Agency

Green, grass-like seaweed with tubular fronds. Found on rocks, sand or mud at all levels of the shore.

Velvet horn (Codium tomentosum)

Velvet horn. © Dan Cole/The Art Agency
Velvet horn. © Dan Cole/The Art Agency

Green seaweed with spongy texture resembling deer antler ‘velvet’. Found in rockpools and on lower shore.

Sea lettuce (Ulva lactuca)

Sea lettuce. © Dan Cole/The Art Agency
Sea lettuce. © Dan Cole/The Art Agency

Small and green, with crumpled, lettuce-like fronds. Grows on rocks in mid-zone of shores. Edible.


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Main image: Bladderwrack. © Olga Mironova/Getty