Food quality and exposure time affect induced defences in Daphnia: Based on key morphology, life history traits, and expression of related genes

Abstract Kairomone‐induced anti‐predation defences in zooplankton mediate trophic interactions in aquatic ecosystems. Changes in food quality greatly affect induced defences because of the potential influence on resource allocation. Understanding how zooplankton respond to predation risk under poor nutrition conditions is critical for predicting ecosystem function. The present study focused on the fish kairomone‐induced defences in Daphnia pulex fed unicellular non‐toxic Microcystis aeruginosa . Transcriptions of key genes were detected to interpret changes in morphological and life history traits. Additionally, the degree of responsiveness of traits to increasing proportions of M. aeruginosa in the diet was compared at different endpoints with varied exposure duration. Daphnia pulex decreased body size but showed an elongated tail spine as morphological adaptations to the presence of fish kairomones, supported by enhanced transcriptions of chitin deacetylase and cuticle protein genes. However, with suppressed expression of the related genes, tail spine length was linearly reduced by increasing proportions of M. aeruginosa in the diet, accompanied by decreased somatic growth and reproduction. Presence of fish kairomones interactively affected the inhibitive effects of increasing proportions of M. aeruginosa in the diet on Daphnia growth, reproduction, and tail spine defence but not body size. In addition, the magnitude of effects of M. aeruginosa on Daphnia growth and morphological responses under fish kairomone exposure was increased with the increasing length of exposure time from 3–7 days (reaching maturity) to 14 days (by the end of tests), indicating an aggravated inhibitive effect of M. aeruginosa on induced defences over time. The present study provides references for evaluating zooplankton‐fish interactions under poor nutrition conditions. Since a cyanobacterial bloom may last from a few weeks to several months, experiments involving the whole life span of zooplankton are thought to fully reveal the actual impact of cyanobacteria on interspecific interactions related to zooplankton. In addition, unifying the exposure time to cyanobacteria for comparative studies is recommended in future research.