Brown seaweed. Close-up of the brown seaweed of rocky shores, Fucus platycarpus. This seaweed, an alga belonging to the Phaeophyta, is found 5-6 metres above the low water mark of the shore. It is a branching blade-like plant, attaching itself to rocks by disc-like holdfasts (not seen). The bladders or vesicles seen at the ends of the fronds are reproductive structures known as receptacles. They are covered with tiny pimples, the conceptacles. The conceptacles are the reproductive organs and contain the sperm and eggs. Photographed in North West Scotland.