The American toad (Anaxyrus americanus, formerly Bufo americanus) is a common species of toad found throughout the eastern United States and Canada. It is divided into three subspecies—the eastern American toad (A. a. americanus), the dwarf American toad (A. a. charlesmithi), and the rare Hudson Bay toad (A. a. copei). Recent taxonomic treatments place this species in the genus Anaxyrus instead of Bufo. he eggs of the American toad are laid in two strings and can hatch in 2–14 days. When hatched the tadpoles are recognizable by their skinny tails in relation to the size of their black bodies. They may advance to adulthood in 50–65 days. When metamorphosis is completed, the "toadlets" may stay in the water for a short period of time before they become mostly land based. Often entire groups of tadpoles reach the toadlet stage at once and a mass migration to higher ground takes place usually to shaded areas of mid range and upland forests bordering the marshes from where they bred. Toadlets can be observed eating microscopic bugs as fast as they can in the ground area they roam between various vegetation; they are also known to eat ants, spiders, slugs and worms. Studies have shown that they have a mutualistic relationship with Chlorogonium alga, which makes tadpoles develop faster than normal. Tadpoles have several mechanisms to reduce predation. They avoid predators by swimming in very shallow water often with thick grass vegetation, and by swimming close together in schools during the day. Tadpoles also produce toxic chemicals in their skin that discourage some potential predators. Fish have been reported to die after consuming one tadpole; however, most fish quickly learn to avoid eating American toad tadpoles. The tadpoles are also very small and they are a solid black color. Based on DNA sequence comparisons, Anaxyrus americanus and other North American species of Anaxyrus are thought to be descended from an invasion of toads from South America prior to the formation of the Isthmus of Panama land bridge, presumably by means of rafting.