Channelled Wrack - Pelvetia canaliculata

This small and brown bushy seaweed derives it's name from the channel-like gutters along its fronds and is attached to the rock surface by a small holdfast.

It is a common alga found at the highest level on the rocky shore and its position is a good indicator of the highwater mark.  However its position on the shore is inhospitable due to the short time covered by the tide and as a result the  growth is quite small.  It has a number of remarkable adaptations that give it the chance of survival high up the seashore,  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; a fatty (oily) layer over the cell slows the desiccation by stopping water evaporating and  a thick cell wall which shrinks with drying.  The seaweed can lose 90% of its water and rehydrate very rapidly in about 20 mins of being covered again by the tide.  When mature, the alga produces swollen tips to the fronds. It is these receptacles which contain the reproductive conceptacles.  Being hermaphroditic each plant has 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 and is improved by having both male and female sexes in close proximity on the same plant as  immersion by the tide is brief.

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