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The Seashore

Channel Wrack (Pelvetia canaliculata)

Channel Wrack (Pelvetia canaliculata)

This small brown alga grows up to l6 cm long. It has a bushy growth with channel-like gutters along the fronds. When mature they produce swollen tips to the fronds, the receptacles which contain the reproductive conceptacles. They have a small holdfast.

Their position on the seashore (top of the upper shore) is very inhospitable due to the very short period covered by sea water. As a consequence the growth after three years is quite small. It has a number of adaptations that give it the chance of survival here not found in the algae lower down. Special adaptations include: rolled fronds to reduce the water lost by evaporation; trapping water with the channels in the frond; fatty (oily) layer over the cell slows the desiccation by stopping water evaporating; a thick cell wall which shrinks with drying; it can survive lower nutrient level than other large alga; growth is slow but may continue at low tide; it has a rapid recovery of metabolism e.g. respiration and photosynthesis, when the tide does return. In fact they can lose almost 90% of its water and yet will hydrate very rapidly in 25 minutes. It is therefore, a very stress-tolerant alga, well adapted to the niche at the top of shore.

These seaweeds, living on the upper shore area, are hermaphrodite, having both male and female structures on the same frond. The conceptacles ripen in summer with the release of gametes in September to coincide with the high spring tides. Fertilisation is therefore external in the water. The settlement of the tiny sporelings occur then but little development occurs until late winter. They take twelve months to mature. Having both sexes on the same plant greatly increases the chances of fertilisation on the upper shore where the tide does not give a long period of immersion in water. Light is an important factor in allowing good settlement of the spores, stimulating the growth of rhizoids which anchor the young plant to the rock.

This is a common brown alga and is found at the highest level on the rocky seashore, in fact making a good indicator of the high water mark. Due to its position it can be found on quite wave exposed shores, being one of the last Fucoid wracks on the shore. Found around the Atlantic shores, in the English Channel and up into the North Sea.

Note: In the same zone, Fucus spiralis, is found but does not quite have the same ability to survive the desiccation. As a consequence it is found just below the Pelvetia. The lower limit of Pelvetia is proably determined by competition with the F. spiralis.


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