The stinging spine on the ray's tail is an effective weapon against predators. The edges of the spine are serrated so that once driven into a victim it is very difficult to remove without creating more damage. A thin skin covers the entire structure, and when ruptured, venom is released into the victim. Since skates do not have stinging spines, they rely on large thorns on their backs and tails to deter predators. Skates and rays differ from each other in the way they give birth.
All skates lay eggs, which are leathery cases very similar to shark egg capsules.
These cases sometimes wash up on beaches and are called mermaid's purses. Rays, on the other hand, give birth to live young. These differences determine how skates and rays are categorized and classified scientifically. Despite the obvious distinctions, classifications of these unique animals are often the subject of debate among researchers. Skates are members of completely separate order known as Rajiformes.