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Bladder wrack

Fucus vesiculosus

Probably the first organism you see at West Runton rockpools, Fucus vesiculosus is instantly recognisable by the air-filled 'bladders' on the surface of this seaweed. These bladders are filled with oxygen gas, a product of the seaweed's photosynthesis, and they help keep it towards the surface of the water when the tide is in. It is hardy and well-suited to life in the mid-shore, providing shelter for other organisms such as crabs and fish, and food for some of the mollusc species found here.

F. vesiculosus has male and female plants, and during the breeding season (Spring and Summer), the female releases eggs into the water, which in turn release a chemical signal to attract the male sperm. The zygote (the fertilised egg) that is produced is sticky and will attach to a hard substrate quickly. If it's lucky, it will grow at about a rate of 0.48 cm a week into an adult plant and live for 4 to 5 years.

There are two other very similar-looking species of Fucus at West Runton. Fucus spiralis lives high-up on the shore, mainly growing on the groynes at the back of the beach. Fucus serratus can be found further down the shore towards the sea and it has a zig-zag margin and no air-bladders.

Source 1: http://bioweb.uwlax.edu/bio203/2011/harris_benj/#
Source 2: https://www.marlin.ac.uk/species/detail/1330