Zooplankton are tiny, often microscopic animals that drift in oceans, lakes, and rivers. They play a crucial role in aquatic ecosystems as they serve as a primary food source for larger animals, such as fish and whales. Zooplankton can be divided into two main groups: holoplankton, which spend their entire lives as plankton, and meroplankton, which are only planktonic during their early life stages. These organisms come in various shapes and sizes, ranging from single-celled protozoa to larger forms like krill. Zooplankton feed on phytoplankton, which are microscopic plants, and help recycle nutrients in the water. Their presence is vital for maintaining the balance of aquatic food webs.