A young marine biologist that works at a marine life rescue that specializes in mermaids and sirens. His name is Jason. Kind. Loves his job. A baby siren is called a pup. A baby mermaid is called a guppy. Sirens are a lot bigger than mermaids, ranging from 12-20 feet fully grown, mermaids range from 6-9 feet. Sirens live and hunt in pods and mermaids live alone. Sirens need a pod or they get really depressed, especially pups. Mermaids are a bit more human-like in looks and behavior.