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Plants living on the seashore have to cope with the action of the waves and the drying effects of sea breezes. One flowering plant that manages to cope with these conditions is the eel grass. However, the most successful plant colonisers of rocky shores are the seaweeds. Large brown seaweeds dominate sheltered shores. Red seaweeds flourish on the lower shore while green seaweeds are often abundant on the upper shore. Instead of roots, seaweeds have holdfasts that attach them securely to the rock. The main part of the seaweed, called the frond, may also be attached to the holdfast by a tough, flexible stalk or stipe.

Oarweed Laminaria digitata (Hudson)

Unlike the wracks, kelps such as oarweed can not survive the drier high shore and are restricted to the lower shore or deeper water. Their flexible stipe bends over to lie flat on the surface during extreme low water and this helps to prevent the plant from drying out. Growth takes place at the base of the frond with the top of the frond being continuously worn away by wave action. The reproductive organs are scattered over the fronds. In autumn, oarweed from 1 m2 has been estimated to produce 20 000 million spores in a year.

Many animals and plants use the branched holdfast of this plant as a refuge. The blue rayed limpet (Helcion pellucidium) is often found on the stipe or frond and can do considerable damage to the plant. However, animals and plants rarely attach themselves to the shiny stipe.

The young stipes of this kelp were at one time a popular food in Scotland and Ireland.

  • Habitat

    • Attached to rocks on the lower shore and just below.

  • Key Identification Features

    • Glossy golden-brown colour.

    • Smooth, long, rounded, bending stalk.

    • Broad frond with finger-like blades.

    • Branched, dome-shaped holdfast.

    • Up to 2 m in length.

Sugar kelp Laminaria saccharina (Linnaeus)

Sugar kelp may be found amongst oarweed (Laminaria digitata) on sheltered shores. The plant often lies horizontal, even when it is under water. Its undivided, wrinkled frond may be covered in a whitish powder. This powder is sweet to taste and it is from this that the seaweed has acquired its name.

Seasonal growth of the stipe results in alternate rings of light and brown tissue being laid down during periods of fast and slow growth. These rings can be used to age the plant. Spores are produced in the winter. The seaweed lives for about three years.

The sugar kelp is sometimes referred to as ‘poor man’s weather glass’ because in the past it was hung up to forecast the approaching weather. Brittle fronds meant that the weather would be dry. Soft, limp fronds meant rain was approaching.

  • Habitat

    • Abundant on the lower shore where sheltered.

  • Key Identification Features

    • Simple ribbon-like frond.

    • Wrinkled surface.

    • Yellowish-brown in colour.

    • Up to 4 m in length.

Knotted wrack Ascophyllum nodosum (Linnaeus)

The knotted wrack is the largest of the wracks and is abundant on sheltered shores. It is only found on firmly anchored rocks on account of its size and buoyancy. Unlike the bladder wrack (Fucus vesiculosus) the knotted wrack is a very slow grower. Its age can be estimated by counting the number of bladders on a frond as one air bladder is produced at the tip of a frond each year. The age may be underestimated, however, as this wrack may not produce its first bladder for several years. Also fronds may be broken off and replaced by new ones. This wrack may live for about twelve years. In the past, children made the bladders into whistles.

Growths of the red seaweed Polysiphonia lanosa may be found on the knotted wrack’s fronds.

  • Habitat

    • Found on the middle reaches of sheltered shores.

  • Key Identification Features

    • Olive-green in colour.

    • Narrow strap-like frond.

    • Large swollen air bladders.

    • Branches continuously split in two.

    • Up to 2 m or more in length.

Thong weed Himanthalia elongata (Linnaeus)

Thong weed may be found on exposed shores just above the kelp zone. The fronds are small and button-shaped. More distinctive are the long strap-like reproductive bodies that grow from the centre of each ‘button’. This seaweed only lives for two or three years. It dies as soon as it has released its gametes.

The button-shaped fronds may have other seaweeds growing on the under surface but the inner surface is always free from growth.

  • Habitat

    • Found on the lower shore forming a distinct zone above the kelp zone

  • Key Identification Features

    • Olive-green in colour.

    • Frond button like (3 cm across).

    • Long strap-like reproductive bodies grow from the centre of the button dividing into pared branches.

    • Up to 2 m in length.

Serrated wrack Fucus serratus (Linnaeus)

One of the most common seaweeds of British shores, the serrated wrack often forms a zone on the lower shore just above the kelp zone.

When the tide is out, the dense flat fronds of this wrack provide shelter for many marine animals. The flat periwinkle (Littorina littoralis) may be found amongst its fronds. Encrusting animals such as sponges, sea squirts, sea oak (Dynamena pumila), sea mat (Membranipora membranacea) and the small white spiral shells of the worm Spirobis spirobis may cover the fronds of the seaweed. The serrated wrack has a distinct concave and convex surface. The abundance of encrusting animals tends to be higher on the concave surface.

In autumn, plants develop swellings at the tips of their branches. These are the reproductive receptacles. After the plants shed their eggs they lose a large amount of foliage and are very much reduced in size. The serrated wrack lives for four to five years.

In some regions of the world the serrated wrack has been collected by humans to be used as animal fodder and manure. Slime from these plants has also been used to treat glandular diseases and as a remedy for bone diseases.

  • Habitat

    • Found in rock pools and on the lower zone of fairly sheltered shores. Often in a distinct zone below Ascophyllum nodosum and Fucus vesiculosus and above the kelp zone.

  • Key Identification Features

    • Olive-brown colour.

    • Flat and strap-like.

    • Distinct midrib.

    • Frond with a serrated edge.

    • No air bladders.

    • Branches divide continuously into two.

    • Up to 1 m in length.

Bladder wrack Fucus vesiculosus (Linnaeus)

This wrack is able to tolerate violent wave action. Plants on exposed shores differ in that they have fewer air bladders than those found on sheltered shores. In sheltered waters, the air bladders buoy the plants to help keep them towards the light. In exposed waters, the buoyancy of the air bladders would put the plants at risk of being torn away and here a bladderless form of Fucus vesiculosus (F. linearis) occurs.

The bladder wrack is a fast growing species that will rapidly colonize areas cleared of seaweed. The plants are either male or female. In January, the tips of some of the seaweed’s side branches swell to form reproductive receptacles. Up to a million eggs are released from March to August.

Parts of this plant were once used to treat bone diseases, glandular diseases and obesity.

  • Habitat

    • Found on the middle shore with Ascophyllum nodosum, above the Fucus serratus zone.

  • Key Identification Features

    • Olive-brown in colour.

    • Air bladders usually in pairs but may be absent in small plants.

    • Branches continuously divide into two.

    • Margin of frond smooth.

    • Up to 1m or more in length.

Spiral wrack Fucus spiralis

Spiral wrack is able to tolerate long periods out of water. However, when plants are transplanted up to the channelled wrack zone they die within a few weeks. This is likely to be due to desiccation as spiral wrack plants at the top of their zone die when exposed to periods of calm, dry weather. However, spiral wrack is a faster growing seaweed than channelled wrack and out-competes the latter wrack lower down the shore.

Individual plants have both male and female reproductive organs and self-fertilization occurs. The seaweed lives for up to four years.

Spiral wrack was once believed to cure corns. People soaked their feet in boiling salt water which contained the reproductive receptacles of the seaweed.

  • Habitat

    • Forms a zone below the channelled wrack (Pelvetia canaliculata) and above knotted wrack (Ascophyllum nodosum) and bladder wrack (Fucus vesiculosus) on sheltered shores.

  • Key Identification Features

    • Fronds twisted or spiralled.

    • Branches are split continuously into two.

    • No air bladders.

    • Olive-brown in colour.

    • Up to 40 cm in length.

Channelled wrack Pelvetia canaliculata (Linnaeus)

The channelled frond of this wrack enables it to retain moisture when it lies flat on rocks, with the concave side of the fronds downward. This adaptation helps it to live at the extreme high shore, sometimes growing in places which are only ever washed by the spray. In fact, it has been found that if these plants are immersed under water for long periods they will die. Despite their tolerance to exposure out of water the upper limit of the channelled wrack zone is set by physical factors such as desiccation. For example, a few days of continuous exposure in sunny weather can kill the upper most plants in the zone.

The channelled wrack reproduces in the summer. Individual plants possess female and male reproductive organs and self fertilization occurs. The seaweed lives for up to five years.

  • Habitat

    • Forms a distinct zone on the upper shore above all the other wrack species.

  • Key Identification Features

    • Fronds curled to form a marked channel.

    • Fronds continuously divide into two.

    • High shore species are almost black, while species just above the spiral wrack are a yellowish-olive.

    • Up to 15 cm in length.

    • No air bladders.

Rainbow bladder weed Cystoseira tamariscifolia (Hudson)

This distinctive species is noted for its blue-green iridescent colour when immersed under water and for its bulging fruiting bodies. It is only found in the south-west of Britain. A low shore zone of the weed occurs at Trevone and Trebetherick, near Padstow, North Cornwall. It is also abundant at Whitesand Bay, Looe, Falmouth and Penzance and between Salcombe and Wembury.

  • Habitat

    • Rock pools on the lower shore.

  • Key Identification Features

    • Olive-green colour, blue-green iridescence underwater.

    • Prickly-looking, bushy plant.

    • Mass of branches end in short spiny-like branchlets.

    • Round, spiny fruiting bodies on the end of branches.

    • Air bladders may occur at the base of the fruiting bodies.

    • Up to 45 cm in length.

Purple laver  Porphyra umbilicalis (Linnaeus)

This red seaweed is widespread and common on exposed rocky shores. Purple larver is able to survive lengthy periods out of the water, moulding its slimy fronds against the rock.

Species of Porphyra are found all over the world and in many countries they are collected for food. In Wales, Porphyra is used to make ‘Laver bread’. The plants are washed in freshwater and boiled to produce a brownish-black jelly. The jelly is then usually coated with oatmeal and fried with bacon. In Japan, Porphyra has been cultivated for centuries. The seaweed is dried and the thin sheets are used to wrap up balls of dry boiled rice. Purple larver is high in protein and rich in vitamin B and C.

  • Habitat

    • Abundant on all types of rocky shores in exposed areas. Found mainly on the upper shore. On steep rock surfaces Porphyra often forms a zone just below lichens. They are one of the few species to survive on exposed boulder shores. It is also found in pools and in association with mussels.

  • Key Identification Features

    • Dark purple or red in colour.

    • Delicate, membranous frond.

    • Up to 20 cm in length.

Irish moss Chondrus crispus

Irish moss tolerates some reduction in salinity and is sometimes found in estuaries. Its appearance may vary depending on the exposure of the shore and the level at which it is growing on the shore. In the past, Irish moss was collected to be used in drinks as a medicine, particularly for lung complaints. The seaweed is now harvested for the seaweed extracts, carrageenans. Carrageenans are essential ingredients of many pharmaceutical and cosmetic and food products. They are used as a stabilizer in blancmange, as an agent that causes the suspension of cocoa powder in milk, and in the production of sun creams, shampoos, shaving foams and tooth paste.

  • Habitat

    • Found on rocks and in rock pools on the middle and lower shore.

  • Key Identification Features

    • Red colour. Violet iridescence when under water.

    • Flat fronds divide continuously into two.

    • Up to 20 cm in length.

Gut Weed Enteromorpha intestinalis (Linnaeus)

Luxuriant growths of this seaweed may be found in high shore rock pools. These plants are fast growing and are rapid colonisers of any bare rock. For example, when dispersants were sprayed on Cornish shores after the 1967 Torrey Canyon oil spill most of the rocky shore life in the vicinity was killed. However, within a few months, Enteromorpha and another colonizer, the sea lettuce (Ulva lactuca), were growing on the surface of these rocks. This rapid colonization is due in part to their prolific reproduction. In tidal pools the water may turn green due to the density of spores and gametes that are produced. Sometimes the fronds of Enteromorpha detach from their substrate but still they manage to survive as a floating mass. Where they are very abundant the decaying plants can cause pollution problems.

In China and Japan, Enteromorpha species are sold as food.

  • Habitat

    • Pools on the upper shore and where freshwater drainage runs across the beach.

  • Key Identification Features

    • Green to bright-green.

    • Frond elongate and tubular.

    • Up to 7 cm in length.

Sea-lettuce Ulva lactuca (Linnaeus

Sea-lettuce is widely distributed over rocky shores. It is also found on mud-flats attached to stones and shells. It is a rapid colonizer and like the green seaweed, Enteromorpha, it is the first to occupy any bare rock. In sheltered conditions the plants may grow to a large size until they detach from the rock. They are able to survive as a floating mass. In Wales the plant is sometimes used to make ‘larver bread’ in place of the red seaweed, purple larver. It is, therefore, sometimes referred to as green larver.

  • Habitat

    • All over the shore except high shore.

  • Key Identification Features

    • Very thin membranous frond.

    • Disc-like holdfast.

    • Green to dark green.

    • Normally about 10 cm in length.

Eel grass Zostera marina (Linnaeus)

Eel grass is one of the few flowering plants that live in the sea. The plants spread along the sea bed when pieces of the root of the parent plant break off and are carried away on currents. However, these plants do produce flowers and the pollen and seeds are dispersed by the water.

At the beginning of the 20th century, eel grass was widespread around British coasts, especially in estuaries. Unfortunately, in the 1930’s a wasting disease, caused by a slime mould, drastically reduced its abundance. The plant recovered slowly but now the plant only occurs sporadically around Britain. In the south-west, beds of eel grass are found in the Isles of Scilly, the Helford River, the Yealm Estuary, Salcombe harbour, and between Hannafore Point and Looe Island. In 1991, however, monitoring work in the Isles of Scilly recorded the reappearance of the Zostera ‘wasting disease’. Surveys in the Helford Voluntary Marine Conservation Area between 1986 and 1988 also reported the loss of eelgrass beds at Treath and Helford Passage. It is important that as much as possible is done to conserve this plant as it provides an important habitat for many animals. Worms, anemones, crustaceans and molluscs are found living on or amongst its leaves and roots. It is also one of the main food plants of Brent geese overwintering in Britain.

In the last century, Zostera was used as packing material for mattresses and cushions and to build sea dykes in the Netherlands. Since its decline in the 1930’s it does not appear to have been used commercially.

  • Habitat

    • Sheltered bays or estuaries in sand, mud or gravel from the low water to a depth of 4 m.

  • Key Identification Features

    • Grass like.

    • Long narrow flattened leaves with rounded tips.

    • Dark green.

    • Up to 1 m high. Leaves 5-10 mm wide.


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