Saw or Serrated Wrack (Fucus serratus)

This is a very flattened wrack which grows in heavy bunches of fronds, up to 50 - 80 cm in length. It has distinct dark brown fronds with a strongly serrated edge and no bladders. With close inspection clusters of small white hairs can be seen on the frond. In very sheltered conditions the fronds become curled and the entire plant becomes more bulky.

This abundant Fucus alga can form dense layers on the lower shore of rocky seashores. The commonest species of the lower shore. Widespread from northern Scandinavia down to the Bay of Biscay

Like most wracks this lives for 3 years and grows very well on slow draining shores where it may occupy a third of the area of the entire seashore. It is intolerant of desiccation, with a cell wall thickness 0.42 micrometers. With the flattened fronds only the top surface of the frond dries at low tide. The growth, form and breeding is variable being related to climate and temperature, e.g. the fronds are longer with fewer branches in the north Scotland compared with shorter, much-branched individuals in the English Channel. Also, with increase latitude, it is found higher up the seashore.
It intolerant of heavy wave action and at such times it is replaced by Himanthalia elongata. Like many other algae of the lower shore it is adapted to low light intensity by developing additional pigments for absorbing what little light is penetrating the water. One of the reasons that lower shore examples are almost black, absorbing all the light.
Under sheltered conditions a wide range of epizooites, such as tube worms, bryozoans and hydroids, colonise the fronds. The most common of these is Spirorbis, a small polychaete worm which secretes a calcareous, spiralled tube (diameter 3mm). Many epizoites are filter feeders and use the algal frond as an attachment only, some protection from dehydration may be an added benefit. The sexes are separate unlike the Fucoids of the upper shore and the receptacles containing these reproductive parts are more streamlined at the tips. Like the bladder wrack, female gametes produce chemical attractants (pheromones)to attract the male. This is called fucoserratin.