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Laboratory experiments showed that A. bright-welli could outreproduce, or coexist with, A. silvestrii only when the absence of dietary tocopherol prevented the latter from transforming to the giant cruciform and campanulate morphs. When tocopherol permitted polymorphic transformations, as often occurs in nature, the giant morphs of A. silvestrii ingested and rapidly excluded the much smaller, only slightly polymorphic A. brightwelli. Such interference (or encounter) competition from trimorphic Asplanchna species is known to occur in nature and must limit the distribution and abundance of monomorphic or only slightly polymorphic species. The ability to eat congeneric competitors may have provided some selective pressure for the evolution of gigantism in the genus. [ABSTRACT FROM AUTHOR]