@stux They're not incredibly rare as you think actually a lot of people have had these as pets without knowing it.
Notostraca Triopsidae or Triops are known by the common name Tadpole shrimp but even better known as Sea Monkeys. This is what they look like when they're allowed to continuously grow. Problem is the places they would best survive, Stagnant pools with lots of food, dry up. Or if they're too big to dry up, they're overpopulated with other fish who would prevent them from getting too large.
IIRC they are technically immortal like lobsters and will only die when they can no longer shed their shells (or killed by external means).