Universität zu Köln

Ahlers, Johanna ORCID: 0009-0009-6545-9668 (2026). The ecological role of the fish-derived kairomone 5α-cyprinol sulfate in shaping Daphnia vertical migration and risk perception. PhD thesis, Universität zu Köln.

PDF Dissertation_JohannaAhlers_200926._Veroeffentlichung.pdf - Published Version Download (3MB)

Abstract

In freshwater ecosystems, predator-prey interactions shape trophic relationships, nutrient dynamics, and species distributions. As key herbivorous zooplankton and while also serving as prey for planktivorous fish, Daphnia link primary production to higher trophic levels. In response to predation, Daphnia exhibit a range of inducible defenses triggered by chemical cues, so-called kairomones. One of the most prominent behavioral defenses is diel vertical migration (DVM), a daily movement through the water column that helps reduce predation risk. During this cycle, Daphnia descend to cooler, darker depths at daybreak and return to surface waters after dusk. While DVM has long been recognized as a predator-avoidance strategy, the identity of the chemical signal driving this response remained unknown until recently. The bile salt 5α-cyprinol sulfate (CPS), excreted by cyprinid fish, was identified as the kairomone responsible for inducing DVM in Daphnia magna. However, its ecological relevance under natural conditions and its role in shaping zooplankton behavior in the field had yet to be confirmed. The aim of this dissertation was to investigate the ecological role, distribution, and dynamics of CPS in natural lake ecosystems. Specifically, I addressed three questions: (1) Can CPS alone induce DVM in Daphnia under natural conditions? (2) How do spatial and temporal variations in CPS concentration define a chemical landscape of fear? (3) What determines the production and degradation of CPS in aquatic environments? In the first study, a mesocosm experiment was conducted in a natural lake to test whether purified CPS can trigger DVM in a field-derived Daphnia community. Exposure to nanomolar concentrations of CPS caused a significant downward shift in daytime vertical distribution. The response was dose-dependent and reversible, confirming that CPS is a behaviorally active infochemical in natural conditions. Chemical analysis using high-performance liquid chromatography–mass spectrometry (HPLC-MS) verified that these concentrations are within the range observed in the lake, validating the ecological relevance of CPS as a kairomone. The second study explored diel and seasonal CPS dynamics in Lake Klostersee and investigated how this chemical cue contributes to a spatially and temporally variable landscape of fear. CPS concentrations varied markedly with depth and time, likely peaking during periods of increased fish activity. Bioassays with Daphnia longispina clones revealed genotypic variation in sensitivity to CPS, with individual thresholds differing by several orders of magnitude. These results show that even within a single population, prey interpret the same chemical landscape differently, resulting in diverse behavioral outcomes. The third study examined the production and degradation of CPS under controlled and natural conditions. Laboratory feeding experiments demonstrated that CPS excretion was closely linked to fish feeding intensity and not to fish size. In parallel, degradation experiments revealed rapid microbial breakdown of CPS in lake water, with temperature strongly accelerating the process. A temperature-dependent model predicted that more than 80% of CPS can be degraded within 24 hours under summer conditions. These findings indicate that CPS is a short-lived signal, reflecting real-time predator activity rather than long-term predator presence. Taken together, the three studies demonstrate that CPS functions as a powerful, transient, and ecologically relevant kairomone in freshwater systems. CPS not only induces DVM in Daphnia but also creates a dynamic chemical landscape of fear that varies in time and space and is interpreted differently by genetically diverse prey. The results establish CPS as a key link between predator behavior and ecosystem function, offering new insights into chemically mediated interactions and their role in structuring freshwater food webs.

Item Type:	Thesis (PhD thesis)
Creators:	Creators Email ORCID ORCID Put Code Ahlers, Johanna johanna.ahlers@outlook.de https://orcid.org/0009-0009-6545-9668 UNSPECIFIED
URN:	urn:nbn:de:hbz:38-813830
Date:	2026
Language:	English
Faculty:	Faculty of Mathematics and Natural Sciences
Divisions:	Faculty of Mathematics and Natural Sciences > Department of Biology > Zoologisches Institut
Subjects:	Natural sciences and mathematics Life sciences
Uncontrolled Keywords:	Keywords Language Chemical ecology English Diel vertical migration English Predator–prey interactions English Kairomones English Daphnia English
Date of oral exam:	31 October 2025
Referee:	Name Academic Title Von Elert, Eric Prof. Dr.
Refereed:	Yes
URI:	http://kups.ub.uni-koeln.de/id/eprint/81383