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The Malacostra were a species of Neptunian aliens that Casey Jones encountered in the story North by Downeast. Their natural form were as giant green lobsters, but they were transformed by their leader, Big Snapper, into blue humanoids. After the Malacostra found the Royal Roe, the last of Big Snapper's surviving spawn, he lifted the curse and turned them back into lobsters.
Malacostraca is the class of crustaceans in which lobsters are a part of.