Fucus, better known as kelp or seaweed, is a genus of exclusively-marine brown macroalgae in the family Phaeophyceae. The phytopigment fucoxanthin masks the natural green of the chlorophylls in these multicellular intertidal and subtidal plants.

Known also as wracks, the Fucus brown seaweeds are always diploid and meiosis takes place before the gametes are formed. The reproductive organs of the macroalgae consist of receptacles and conceptacles. Gamete production takes place in specialized crypt-like structures known as conceptacles. The conceptacles are carried in receptacles, which are swollen areas found at the tips of the plants. While some species of wracks are dioecious, others feature both types of sexual organs on one plant and are classified as monoecious. To add to the diversity of reproductive strategy featured by this genus of brown algae, some of the monoecious species have both sexes in a single conceptacle while others contain them in separate organs. The conceptacles for the male reproductive organs are lined with antheridiophores, which are branched and feature inflated, terminal antheridia on their branches. The antheridia are the sites of meiosis, forming 64 to 128 biflagellated male gametes that are referred to as antherozoids. The male and female gametes are usually released into the water during a rising tide with the female eggs emitting a potent volatile hydrocarbon compound (fucoserraten), which acts as an attractant for the mobile male antherozoids. The fertilized egg settles onto the sediment and germinates as a new diploid thallus.

Known also as the medicinal seaweed or Fucus, species in this genus have the highest antioxidant activity of the edible seaweeds, most likely because of the fucoxanthin. As an extract, it is available as a dietary supplement and supports a thriving industry along Ireland's coasts. Brown algae, particularly members of this genus, are especially rich in iodine and including it in the diet is thought to possibly remedy some thyroid problems. In Japan, F. vesiculosus, easily recognized by its paired air bladders, is also used as a dietary preventative of breast cancer and reportedly reduces cholesterol levels and enhances the immune system.

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