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Rocky Shores | Rocky Shore Creatures

Bladderwrack (Fucus vesiculosus)


Bladder Wrack (Fucus vesiculosus) has pairs of air bladders which float its fronds towards the light. Like most algae many variants (ecads) develop in response to changes in environmental factors, on wave exposed shores a bladderless form occurs. It grows rapidly after establishment though individuals at the edge of the range may remain small. It may have a breaking strain 45.5 kg/cm2. It is tolerant of freshwater, but not very tolerant of drying out. It has thinner cell walls than Pelvetia canaliculata.

Brown algae all lose water at about 80% of the rate at which an open water surface would evaporate. They all lose water at about the same rate. Fucoid algae survive where they do because they tolerate water loss (rather than avoid it). High humidity makes heat damage worse. Overlapping fronds of fucoid seaweeds lose water at only 20% of the rate of isolated plants. Flopping over into a moist heap during emersion therefore reduces water loss. Pelvetia canaliculata and Fucus spiralis can tolerate losing 95% of their water. Fucus serratus can tolerate losing about 40% and Laminaria digitata about 20%.

Sheltered BladderwrackSheltered Bladderwrack, floating at the surface.

The number of bladders present varies with wave action. The images above are sheltered specimens and there are many bladders present. With increasing wave action the number decline but so too will the length and general size of the fronds. The reproductive structures at the ends of the frond (the receptacles) become elongated and narrow. These changes are essentially to reduce the drag on the alga. Wave action is more likely to dislodge the plant. Reducing the surface area in contact with the waves’ energy will increase the chance of survival. People have studied the size of the bladders as well as their density and this too seems to vary with wave action. In extreme wave action the wrack has no bladders and is only 20 centimetres in length. This variety is known as Fucus vesiculosus var. linearis

The Fucoid algae do not have alternation of generations. They are dioecious having the reproductive structures at the end of the frond. These swellings (receptacles) contain the gonads. The male release tiny sperm-like gametes and with the aid of a flagellum move through the water to fertilise the oogonium in the female gonad.

These wracks live for around three years, typically in the middle shore. This is also the habitat of Ascophyllum nodosum. Competitive exclusion may result in the latter pushing out the Bladderwrack as Ascophyllum can live much longer, up to twenty years. So if the Bladderwrack dies after three years the Ascophyllum spreads to occupy the space. However, although longer lived it cannot survive in wave action and so will only really compete with Bladderwrack under sheltered conditions.


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