Ascophyllum nodosum is a large, common cold water seaweed or brown alga (Phaeophyceae) in the family Fucaceae, being the only species in the genus Ascophyllum. It is a seaweed that grows only in the northern Atlantic Ocean, also known in localities as feamainn bhuí, rockweed, Norwegian kelp, knotted kelp, knotted wrack or egg wrack. It is common on the north-western coast of Europe (from the White Sea to Portugal) including east Greenland and the north-eastern coast of North America, its range further south of these latitudes being limited by warmer ocean waters. Description Ascophyllum nodosum has long tough and leathery fronds, irregularly dichotomously branched fronds with large, egg-shaped air bladders set in series at regular intervals along the fronds and not stalked. The fronds can reach 2 m in length and are attached by a holdfast to rocks and boulders. The fronds are olive-green, olive-brown in color and somewhat compressed, but without a midrib. Reproduction Its life history is of one diploid plant and gametes. Each individual plant is either male or female. The gametes are produced in the spring in conceptacles embedded in yellowish receptacles on short branches. Varieties and forms Several different varieties and forms of this species have been described. Ascophyllum nodosum var. minor has been described from Larne Lough in Northern Ireland. Free-floating forms of this species are found in, for example, A. n. mackaii Cotton, which is found at very sheltered locations, such as at the heads of sea lochs in Scotland and Ireland. Ecology Ascophyllum nodosum is found mostly on sheltered sites on shores in the midlittoral, where it can become the dominant species in the littoral zone. The species is found in a range of coastal habitats from sheltered estuaries to moderately exposed coasts, and often it dominates the intertidal zone (although subtidal populations are known to exist in very clear waters). However, it is rarely found on exposed shores, and if it is found, the fronds are usually small and badly scratched. This seaweed grows quite slowly, 0.5% per day, carrying capacity is about 40 kg wet weight per square meter, and it may live for 10–15 years. It may typically overlap in distribution with Fucus vesiculosus and Fucus serratus. Its distribution is also limited by salinity, wave exposure, temperature, desiccation, and general stress. It may take approximately five years before becoming fertile. Phlorotannins in A. nodosum act as chemical defenses against