Computational Fluid Dynamics Reveal Hydromechanical Adaptations of Ceriodaphnia dubia as an Inducible Defence Against the Carnivorous Plant Utricularia × neglecta.

Water fleas of the family Daphniidae are strong ecological interactors in many lentic ecosystems. As the most abundant filter feeders in these systems, they form a crucial link between primary production and higher trophic levels. Due to this central role in freshwater food webs and their capacity to react to a range of environmental changes with phenotypic plasticity, Daphniidae are widely regarded as model organisms in ecological research. They form inducible defences, representing a form of phenotypic plasticity, which reduces an organism's vulnerability to many animal predators. Just recently, the first inducible defence against a carnivorous water plant, the Southern Bladderwort, Utricularia × neglecta, was observed in Ceriodaphnia dubia, coinciding with a significant reduction in predation rate. This species' defensive traits are not yet fully understood. C. dubia was observed to swim more slowly, reducing contacts. Furthermore, its body is more slender. This is counterintuitive, as the plant is gape-limited, rendering outgrowing that gape intuitive. Using computational fluid dynamics (CFD), we followed a new hypothesis on the inducible defence's function. We simulated the drag forces acting on daphniids in the typical and the U. × neglecta-exposed morph. Our results indicate a decreased drag on the plant-exposed morph, likely acting in concert with a reduced probability of contact with the plant's suction-eliciting trigger hairs due to proportionally smaller second antennae. We therefore propose a protective inducible defence mechanism based on altered morphology in combination with modified hydromechanical properties. This newly discovered defensive strategy of hydromechanical adjustments adds another example to Daphnia's reputation as an ecoresponsive organism.