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The Seashore

Knotted or Egg Wrack (Ascophylum nodosum)(Ascophylum nodosum)

Close-up of young fronds showing single mid-frond bladder

The Knotted or Egg Wrack grows up to 2 metres or more in length. The long, fronds have a single, large air bladder in the centre of the frond. Along the edge of the frond grow numerous "lollipops". These are stalked receptacles containing the reproductive organs and may be found in a varying state of development. When ripe they are yellow. When the tide goes out they can form huge piles of seaweed. On very sheltered, muddy rocky seashores, especially where a dilution of seawater occurs, Ascophyllum can develop into a floating (unattached) form shaped like a ball. Where the form of an organism is affected by local conditions the variant is referred to as an ecad, in this case known as e. Mackaii.

A very sheltered shore dominated by Knotted Wrack

A very sheltered shore dominated by Knotted Wrack

The Knotted or Egg Wrack is the dominant alga in the middle shore on sheltered rocky seashores. It is a good indicator of such conditions. The large air bladders take it towards the light for maximum photosynthesis. It has the capability to survive low temperatures and has a breaking strain of 37.6 kg/cm2. It has a longevity of 15 years or more whilst other wracks are barely 3 years old when they die off. Therefore it competes favourably with other algae by forming a dominant blanket over the rocks, preventing colonisation by new competitors. Little can grow below it as it produces so much shade. Due to the large surface area it is easily cut by wave action (see example below). It is unpalatable for most animals to eat and there is a general lack of grazers on the seaweed. Although this is one of the longest lived wracks it is the least adaptable of the brown algae. The sexes are found on separate plants and the reproductive structures fall off after the release of spores. This produces small holes which allows the colonisation by a red alga, Polysiphonia lanosa. The growth of this epiphyte may cause the death of the wrack by increasing the surface area and weight still further so that wave action will rip it off the rocks. Ascophyllum produces one air bladders per frond per year and it might be fun to estimate the age of any plants you find.

Severely damaged frond

Severely damaged frond, cut by wave action on a moderately exposed shore. The red alga, Polysiphonia, is growing all around it.

This is an alga of sheltered rocky seashores. It shows no adaptations to strong water movement and the more wave action experienced the more the plant material becomes eroded and the smaller the plant. Living in the middle zone it competes for space with Bladderwrack and the two may be found together. In extreme shelter, where the water is diluted with freshwater they develop the ecad form. This is most abundant in north-west Scotland on the edges of sea lochs. The normal form is found widespread around Europe from north Iceland to the Bay of Biscay.


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