To cope with predation, many prey species have developed inducible defenses in terms of morphology, behavior, and life history. Rotifers were the first model organisms used to evaluate the ecology and evolution of inducible defenses in aquatic ecosystems. Since the middle of last century, only visible morphological defenses, such as spine development, have been found and only in a few rotifer species. Given the development of ultrastructural defenses is taxonomically widespread in aquatic ecosystems, we hypothesize that rotifer prey, particularly small-sized species, can develop such inducible defenses. We evaluated morphological response of two common Brachionus herbivores (B. calyciflorus and B. angularis) to predatory rotifer Asplanchna brightwellii. Results confirmed existence of predator-induced ultrastructural defenses, which are expressed as increased lorica thickness and enhanced lorica hardness. Such inducible defenses are more evident and effective in the smaller sized B. angularis, leading to higher fitness of B. angularis in predator-prey interactions. As anticipated, development of defenses has inevitable fitness costs manifested as decreased reproduction or reduced sex investment. Our results not only extend understanding of inducible ultrastructural defense to other planktonic taxa that were previously observed only in cladocerans, but also verify effective mechanical protection of such hidden defensive morphology.

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