Plankton are small water-dwelling organisms. Most of them cannot be seen without a microscope. Plankton sometimes form large, concentrated distributions that can color the water green or red.

Plankton flow in the water. When they do this, they do not actively swim or float on the surface of the water. Instead, they remain close to the surface by using structures such as gas vacuoles or small drops of oil.

Unicellular plankton organisms reproduce asexually by division. The asexually produced daughter cells are genetically identical with their mother cell. Asexual reproduction can be extremely rapid when the conditions are right.

Algae belong to the group of phytoplankton, and other microscopically small organisms belong to the group of animal plankton.

Animal plankton can reproduce sexually or asexually.