Hybrid African clawed frog (Silurana sp.) These frogs, hybridized from various species of Silurana (African dwarf clawed frogs) are specifically bred for educational uses and are smaller than the African clawed frog (Xenopus laevis), which were originally imported for use in human pregnancy tests, and were later sold widely as pets. Feral populations of Xenopus laevis are now established in many countries, including the United States, where they are a threat to native amphibians and fish. Especially large populations of Xenopus laevis exist in California (where sale of Xenopus sp. frogs is now prohibited), preying on endangered California Red-legged Frogs (Rana aurora draytonii), and endangered Unarmored Threespine Sticklebacks (Gasterosteus aculeatus williamson). Recent evidence suggests the chytrid fungus that is currently decimating frog populations around the world originated in Africa and was spread by Xenopus sp. frogs.