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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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