The serrated wrack is easily identified by the 'toothed edge' to its fronds. Each plant is anchored to the rocks by a holdfast. The fronds end in pitted areas called receptacles - each pit is a tiny opening into a special cup called a conceptacle. At low tide, as the wracks begin to dry out, the eggs and sperm are squeezed out of the conceptacles on to the plant's surface. The photo shows receptacles of a male plant, with orange pustules of sperm being squeezed out of the conceptacles.
Female receptacles are similar but the egg pustules are dark brown.
As the tide comes in, the eggs and sperm get mixed together.