Abstract Resource polyphenisms result in the production of environmentally induced morphs with differential niche use. These striking examples of phenotypic plasticity are taxonomically widespread and ecologically important. However, cannibalism is a frequent repercussion of resource polyphenisms. Despite some benefits, cannibalism is always costly to victims and sometimes costly to the cannibal. Therefore, we here evaluate how morphological and behavioral plastic strategies to minimize these costs may evolve. To better understand the evolution and consequences of cannibalistic polyphenisms, we tested whether rotifers Asplanchna brightwellii possess effective morphological defenses against cannibalism and the ability to discriminate by genetic relationship. We found that small humps produced by vulnerable A. brightwellii limit cannibalism. We also found that cannibals were less likely to attack clonemates than non-kin. The rotifer genus Asplanchna comprises species with varying degrees of resource polyphenism, cannibalism, induced morphological defenses, and behavioral kin discrimination. The observed induced defense and kin discrimination in Asplanchna brightwellii likely represent evolutionary intermediates facilitating the evolution of a unique trimorphic resource polyphenism in congeners.