Fucus is a genus of brown algae found in the intertidal zones of rocky seashores almost throughout the world. Description and life cycle The thallus is perennial with an irregular or disc-shaped holdfast or with haptera. The erect portion of the thallus is dichotomous or subpinnately branched, flattened and with a distinct midrib. Gas-filled pneumatocysts (air-vesicles) are present in pairs in some species, one on either side of the midrib. The erect portion of the thallus bears cryptostomata and caecostomata (sterile surface cavities). The base of the thallus is stipe-like due to abrasion of the tissue lateral to the midrib and it is attached to the rock by a holdfast. The gametangia develop in conceptacles embedded in receptacles in the apices of the final branches. They may be monoecious or dioecious. These algae have a relatively simple life cycle and produce only one type of thallus which grows to a maximum size of 2 m. Fertile cavities, the conceptacles, containing the reproductive cells are immersed in the receptacles near the ends of the branches. After meiosis oogonia and antheridia, the female and male reproductive organs, produce egg cells and sperm respectively that are released into the sea where fertilisation takes place. The resulting zygote develops directly into the diploid plant. This contrasts with the life cycle of the flowering plant, where the egg cells and sperm are produced by a haploid multicellular generation, albeit very strongly reduced, and the egg cells are fertilised within the ovules of the parent plant and then released as seeds. Distribution and ecology Species of Fucus are recorded almost worldwide. They are dominant on the shores of the British Isles, the northeastern coast of North America and California. In the British Isles these larger brown algae occur on sheltered shores in fairly well defined zones along the shore from high-water mark to below low water mark. On the more exposed shores not all of these species can be found and on very exposed shores few, if any, occur. Pelvetia canaliculata forms a zone at the top of the shore. Just below this Fucus spiralis, Fucus vesiculosus, Fucus serratus and Laminaria form clear zones, one below the other, along the shore down to low water mark. On sheltered shores Ascophyllum nodosum usually forms a broad and dominating zone along the shore at the mid-littoral. Other brown algae can be found at the low-littoral such as Himanthalia,