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Red seaweed can be seen in rock pools, on the shore, and in the sea throughout the year. However, their beautiful underwater color, varying from deep purple to pale pink, fades rapidly on exposure to sunlight.

There are over 6,000 species of red seaweed worldwide, making up the phylum Rhodophyta. Their pigment allows them to live in completely underwater areas. The red pigments can absorb low-intensity blue-green light, the dim light penetrating deep water and crevices where red seaweed live.

Seaweeds have traditionally been classified into three major groups, red, green, and brown. Most of the familiar, large seashore seaweeds are brown algae. If you look closely into rock crevices, pools, or under the cover of brown seaweeds, many examples of red algae are to be found.

If you want to find out more, please read on.

Paul Scott Flickr S-A 2.0

Their red color is due to pigments of phycoerythrin and phycocyanin in their cells. These reddish pigments can absorb low-intensity blue-green light, which is the dim light that penetrates deep water, crevices, and among the brown algae, where the red algae live.

Without the special pigments, the algae would not be able to live in such shaded places or in the deeper waters of the sublittoral zone (the area of the shore that is always underwater), where they are also common. The energy absorbed as light is trapped by chlorophyll, which is present in the cells and used in photosynthesis.

How Does Temperature Affect Seaweed?

On the beach, one of the greatest threats to the algae is desiccation, a problem that rarely affects the red algae, as they always live in the water. They may dry out in shallow water, especially on a hot day.

Scientists have demonstrated that as the body of the seaweed dries out, the metabolic rate also falls. The algae become dormant, resembling desert animals that sleep through the day’s heat, conserving energy and water.

Temperature changes can also present problems for the algae. A sheltered cove may produce high temperatures on a hot summer’s day, but a cold winter’s night in the same place can produce freezing conditions.

The algae seem able to adapt to a specific temperature range, but a degree or two outside this range can kill them. Varieties of one species adapt to different temperature ranges, and seaweed that occurs in Britain would not be able to survive in the Mediterranean.

Red algae are common throughout both mid-shore and low tide zones, and many of them extend into the sublittoral zone. However, few are found on the higher levels of the beach since they cannot survive direct sunlight.

Living in these areas, they can avoid desiccation and high-temperature changes. Species of Corallina are so common in mid-shore or low-tide level rock pools that they are sometimes known as coralline pools

However, even though a pool may not dry out, a large proportion of the seawater may evaporate in hot sunshine. In such a case, the salinity of the water increases dramatically, and the algae employ complicated metabolic processes to maintain the correct concentration of salt (osmotic) balance within their cells, reversing the process in heavy rain when the salinity decreases.

Living In Pools and Reefs

Coralline pools are very easy to spot, as they are a distinctive species. Their thalli, impregnated with calcium and magnesium salts, make them hard and brittle to the touch and pink rather than red.

Corallina officinalis is common in rock pools or on rocks near the low water mark around many coastal areas of the British Isles, sharing the southern and Irish coastlines with another species Corallina squamata, the two species being very difficult to tell apart.

Corallina is tufted in growth, but two more related species, Lithophyllum incrustans, and Phymatolithon lenormandii can grow in the crevices of mid-shore pools.

In many parts of the world, including the sublittoral fringe of the western coasts of Britain, Lithothcvnnion forms large lumps and reefs resembling those formed by the corals.

It was not until the middle of the last century that scientists realized that these reefs were formed by algae and not coral. Algal reefs are so extensive that they extend over the sea bed for several miles at a time in many areas, from the Arctic to the tropics.

Floating Seaweed

Not all the algal occupants of coralline pools are encrusted. Many are much more typical red algae with soft, many-branched thalli, tufted in growth and attached to a rock or seaweed by a holdfast.

Ceramium rubrum, common in mid-shore pools on rocky coastlines, is an example. This is one of the most common British species of Ceramium, but the differences between them are only identifiable under a microscope.

Lomentaria clavellosa is another common rock pool alga of the low tide level, along with Chondrus crispus (Irish moss) and Gigartina stellata, which covers large areas of rocks with its branched fronds.

Gracilaria verrucosa, an agar-producing seaweed, is a mid-shore species growing on rocks or gravelly beaches where its holdfasts are buried in the sand.

Gelidium corneum is found around the edges of pools throughout tidal zones on British beaches and is one of the few red algae that can withstand high-intensity light.

James St. John Flickr S-A 2.0

Red Seaweed As Food

Before World War Il, a Japanese species of red algae, Gelidium amansii, provided most of the world’s agar, a substance used as a gelling agent and thickener in foods such as jams and jellies and the cosmetic industry in lotions and ointments.

During the war, Japanese agar was unavailable, and alternatives had to be found. Gigartina stellata and Chondrus crispus were found to contain another gelling agent, carrageenan, in commercial quantities.

Porphyra umbilicalis is another of the few reds found throughout the tide zones. Although very little direct use today is made of algae as a food by western countries, this seaweed is considered a delicacy in Japan and China, where demand has exceeded supply and has to be cultivated.

Palmaria palmata has also been used for human consumption in the past. Sheep can be seen eating it in the wild in many parts of the world.

It grows on the fronds of Laminaria in the low tide areas and on rocks in the same zone. The stalk is almost absent, and it is attached by a disc-like holdfast.

One of the most common red seaweeds, it can be recognized by its thick-lobed and branched fronds which vary in color from dark red to purple.