Aqua Primates, or A-Primes for short, are a species of brine shrimp (specifically, Artemia salina x nyos) which are frequently sold as a novelty pet.
Although the packaging they are sold in depicts the A-Primes engaging in such activities as crowning their queen, fighting robots, and designing websites; in actuality, the nearly-microscopic creatures can do only one trick: move towards a light source. While vacationing in Hawaii, Phineas and Ferb purchased their own A-Primes and grew them to a much larger size with the help of some growth elixir, which made them much more entertaining- although they still enjoyed moving towards light sources more than anything else.
Despite their name (and another aquatically simian name they are frequently sold under), it is generally believed that Aqua Primates are unrelated to monkeys or similar primates evolutionarily linked to Homo sapiens. However, in one unexplained freak occurrence, a group of them somehow evolved into humans. Whether or not this phenomenon was natural or the result of outside interference has yet to be determined by science.
They are based on Sea Monkeys, who are also suspended brine shrimp that do not do much, unlike in their advertisements.