Hydrodynamics of diverse inducible morphological defences in the model organism Daphnia: effects, costs and benefits

• Applicant: Dr. Martin Horstmann
• Subject Area: Ecology and Biodiversity of Animals and Ecosystems, Organismic Interactions ; Systematics and Morphology (Zoology)
• Term: since 2024
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 545843880

Predation is a major selective agent. To thwart this predation, many organisms have evolved inducible defences, a special form of phenotypic plasticity. Well-known for this ability is the genus Daphnia, freshwater crustaceans, which form different morphological, behavioural and life-history responses in the presence of predators. A fundamental condition for defences to “stay” inducible and not become permanently expressed, is costs. Inducible defences are formed when needed, and these costs are saved if the predatory threat is not present. However, determining these costs of inducible defences is difficult. While costs for morphological defences can at least be estimated based on the gain of body surface area, operational costs for altered behavioural patterns are complex to measure. Additionally, the effects of defensive shapes on drag forces have rarely been investigated. As morphological defences often comprise elongated appendages or the formation of helmets, they potentially increase locomotion costs severely. With this proposal, I aim at simulations of the locomotion process in different Daphnia species and instars, allowing to estimate the operational lifetime costs of defences. Therefore, the shape of defended and undefended animals in different instars will be determined three-dimensionally. Using a tracking setup for a three-dimensional examination of swimming paths will allow me to determine swimming velocity. Furthermore, potential stealth effects due to altered turbulence through body shape or antenna (beat) alterations will be investigated. Finally, the effect of water viscosity, which changes with temperature, and the efficiency of shape alterations against the drag elicited by suction-feeding predators, even carnivorous plants, will be examined. With the results of this proposal, we will better understand the trade-offs concerning the costs of inducible defences. Furthermore, mechanisms behind the protective effect of morphological and behavioural defences will be better understood, especially regarding potential hydrodynamic stealth effects masking the animals’ (real) position to predators. However, apart from these hydrodynamic effects, an analysis of the mode of action of defences will due to the extent not be covered in this proposal. From Daphnia as a model organism, the insights generated on the operational costs of inducible defences will be transferable to a generally deepened understanding of the ecological and evolutional framework.

• DFG Programme: Research Grants