The Cracitds were a crustacean species which originated on the planet Helheim.
The Cractids were crustaceans with an exoskeleton, two claws, six legs, and mandibles. They were the only organism in the entire galaxy which had an immunity to Richter's Syndrome.
A swarm of of Cractids could consume a human being - flesh and bone - in under a minute.
The only way to stop them was a sonic burst at the resonant frequency of the Cractids' outer shells it knocked them out for a few minutes.