Once every two years the females give birth to three young (two females and one male). At birth the offspring resemble sharks, but in days of birth arms and legs develop. At 20 yeas old they have grown to resemble humans in shape. Once they develop limbs they can leave water, and once in humanoid form, may live wholly on land. This lasts for four years and then the process reverses itself and the creature becomes increasingly shark-like. Certain ones, the more intelligent and powerful, retain limbs during these later years, although they become more batrachian in form. Raandeese average lives of 700 years length. There are different "types" of shark among the Raandeese, with Hammerheads being leaders, followed by Mako sharks, then tiger sharks.
According to Les Enfants du Poisson, a book about them, they sometimes travel via a gate known as "The Threshold of Yuggoth", which lies within the Bermuda Triangle area. Their relationship to the Deep Ones, and particularly the dolphin-like Laniqua Lua'huan, is unclear, but it may be that the Raandeese are a sub-species of Deep One.