Kelp is a common brown alga that predominantly grows on limestone reefs and other hard surfaces. It can also form large beds that become home to numerous invertebrates and is an important food source for sea urchins, buffalo bream and other animals. Often washed ashore after rough seas, it also forms a large component of sea wrack. Kelp and other brown algae contain a gelatinous substance called algin that is widely used as a thickening or stabilising agent in products such as ice cream, salad dressing, pet food and toothpaste.