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