Abstract
Predation is one of the most important drivers of natural selection. In consequence a huge variety of anti-predator defenses have evolved in prey species. Under unpredictable and temporally variable predation pressure, the evolution of phenotypically plastic defensive traits is favored. These “inducible defenses”, range from changes in behavior, life history, physiology to morphology and can be found in almost all taxa from bacteria to vertebrates. An important group of model organisms in ecological, evolutionary and environmental research, water fleas of the genus Daphnia (Crustacea: Cladocera), are well known for their ability to respond to predators with an enormous variety of inducible morphological defenses. Here we report on the “twist”, a body torsion, as a so far unrecognized inducible morphological defense in Daphnia, expressed by Daphnia barbata exposed to the predatory tadpole shrimp Triops cancriformis. This defense is realized by a twisted carapace with the helmet and the tail spine deviating from the body axis into opposing directions, resulting in a complete abolishment of bilateral symmetry. The twisted morphotype should considerably interfere with the feeding apparatus of the predator, contributing to the effectiveness of the array of defensive traits in D. barbata. As such this study does not only describe a completely novel inducible defense in the genus Daphnia but also presents the first report of a free living Bilateria to flexibly respond to predation risk by abandoning bilateral symmetry.
Highlights
Plastic defensive traits in prey organisms typically evolve in environments characterized by strong variation in predation risk
Inducible defenses can be expressed on the level of behavior, life history, physiology and morphology and are found in almost all taxa ranging from bacteria and unicellular organisms to vertebrates (e.g. [4,5])
It has been shown that the African species D. barbata, which coexists with Triops in temporary freshwater ponds and lakes, responds to T. cancriformis and the backswimmer Notonecta glauca with specialized defenses which are based on the same structures, but built in a different shape [11]
Summary
Plastic defensive traits in prey organisms typically evolve in environments characterized by strong variation in predation risk. It has been shown that the African species D. barbata, which coexists with Triops in temporary freshwater ponds and lakes, responds to T. cancriformis and the backswimmer Notonecta glauca with specialized defenses which are based on the same structures (e.g. helmet, tail spine, dorsal ridge), but built in a different shape [11].
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