Male guppies have a modified anal fin called a gonopodium, which is used to transfer sperm to the female during mating. The gonopodium is inserted into the female's genital opening, and the sperm is released. The female guppy then stores the sperm in her body until she is ready to release eggs, known as ovulation.
Fertilization occurs when the male's sperm fertilizes the female's eggs inside her body. The fertilized eggs develop inside the female and are eventually released as live young, a process known as livebearing.