When it is time, the eggs and sperm are released into the water. When the sperm and egg meet in the water, they combine and from that union, a new plant can develop. The brown and red algae are of tremendous commercial importance. Most likely you have used seaweed today. It is in your toothpaste and your ice cream.
Japan has a very strong seaweed market.
Most Japanese people eat seaweed daily. Seaweed farms are common. L. littorea feeds on sea lettuce. Sea lettuce is a good indicator of pollution. If there is a lot of sea lettuce then there must be a good source of nitrogen near by.