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Sunday 19 November 2017

Wracks the most common and familiar of our brown seaweeds

Bladder Wrack has paired, round gas bladders
Bladder Wrack has paired, round gas bladders

Jim Hurley - Nature trail

The most common and most familiar family of brown seaweeds on the seashore are the wracks. There are just five common members of the wrack family and they are easy to tell apart.

Bladder Wrack is well known. A visit to the seaside with children often includes the ritual of popping the gas bladders on this marine plant. On closer inspection, it will be noted that the bladders are almost spherical in shape and occur in pairs. Furthermore, each pair is separated by a midrib that runs along the centre of the seaweed's frond.

The presence or absence of gas bladders and the presence or absence of a midrib are key features in the identification of wracks. Only two of the five common members of the wrack family have gas bladders: Bladder Wrack and Egg Wrack.

Egg Wrack gets its name because its bladders are egg-shaped rather than being round as in Bladder Wrack. Egg Wrack is also unique in that its bladders are bigger and are single unlike the smaller, paired bladders of Bladder Wrack.

Both Bladder Wrack and Egg Wrack grow on the middle shore. Egg Wrack is intolerant of exposure so it occurs only on sheltered shores. Bladder Wrack is the dominant species on rocky shores that are exposed to waves, wind and weather.

The three remaining members of the group of five wracks have no gas bladders. Two of them have midribs but the third, Channelled Wrack, has none. It gets its name from the fact that the edges of the flat fronds are curled over to form a channel or gutter. It is a small seaweed and grows on the upper shore.

Flat Wrack has a midrib and while it lies flat on the shore, if it is picked and allowed to hang from the hand it often tends to coil spirally. It grows on the upper middle shore.

The last member of the group is Toothed Wrack, so called because the edges of its fronds are toothed or serrated like the edge of the blade of a handsaw. It has a midrib but no gas bladders and grows on the lower shore.

The fact that these seaweeds grow on different parts of a rocky seashore facilitates the division of the shore into three distinct zones depending on which species are present. Toothed Wrack defines the lower shore, Channelled Wrack defines the upper shore and Bladder Wrack defines the middle shore.

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