Spiny butterfly rays are ovoviviparous and give birth to live young. They have an annual reproductive cycle with a gestation period of 4 to 9 months. The embryos initially subsist on a yolk sac; later in development long villi develop from the uterine wall into the embryos' spiracles, which direct uterine milk into the oral cavity.[11] Litter size is up to 8, depending on geographical location: 4 per litter in the Gulf of Mexico, 1–6 in the Mediterranean, up to 5 off Brazil, and up to 8 in the northwest Atlantic.