In aquatic ecosystems fishes are, in general, the most important predators of crustaceans. This must not have been so at the beginning of the Mesozoic. Evolution of aquatic vertebrates led from heavy, benthic, microphagous animals to buoyant, agile, omnivorous ones. Key events of this evolution took place in fresh water, while most crustacean groups probably evolved in marine habitats. It is pointed out that until early Jurassic times the caridoid escape reaction obviously ensured the survival of eucarid species, while the later radiation suggests that selection favoured well-calcified and short-tailed species. It is suspected that the radiation of the marine Teleostei is a main cause of this evolution. This hypothesis is in accordance with the occurrence of the most archaic crustacean relict species in fish-free habitats.