A Journal of Integrative Biology Elaborate Regulations of the Predator-Induced Polyphenism in the Water Flea Daphnia pulex: Kairomone-Sensitive Periods and Life-History Tradeoffs MAKI IMAI 1 , YUKA NARAKI 2 , SHIN TOCHINAI 2 , AND TORU MIURA 1Ã 1 Laboratory of Ecological Genetics, Graduate School of Environmental Science, Hokkaido University, Sapporo, Hokkaido, Japan 2 Department of Natural History Sciences, Graduate School of Science, Hokkaido University, Sapporo, Hokkaido, Japan ABSTRACT Adaptive polyphenism produces alternative phenotypes depending on environ- mental stimuli. The water flea Daphnia pulex shows predator-induced polyphenism, facultatively forming neckteeth in response to kairomones released by Chaoborus larvae. This study was designed to reveal the regulatory systems producing the defensive morph during embryonic and postembryonic development. As noted previously, the crest epithelium at the site of neckteeth is shown to thicken earlier the neckteeth formation, and the neckteeth number increased until the third instar, and later disappeared. Exposure to kairomone at various time points and intervals during development showed that the signal was required even at early postembryonic stages to maintain neckteeth. Moreover, two different induction methods, i.e. embryonic and maternal exposures, enabled us to discriminate maternal and zygotic effects in response to kairomone. Direct embryonic exposure is shown to be sufficient to form neckteeth without maternal effect although their growth was diminished; namely, there is a trade-off for neckteeth production. However, maternal exposures resulted in larger progenies in smaller numbers, suggesting that the mother daphnids change their reproductive strategy depending on kairomone signals. Taken together, the developmental responses to the presence of predators are regulated elaborately at various levels. J. Exp. Zool. 311A:788–795, 2009. r 2009 Wiley-Liss, Inc. How to cite this article: Imai M, Naraki Y, Tochinai S, Miura T. 2009. Elaborate regulations of the predator-induced polyphenism in the water flea Daphnia pulex: kairomone-sensitive periods and life-history tradeoffs. J. Exp. Zool. 311A:788–795. Prey organisms have many defense strategies against their predators to increase their fitness. Recently many studies reported that several prey species develop inducible defenses in response to predators (Adler and Harvell, ’90; Tollrian and Harvell, ’99; Gilbert, 2001). Among them, the water flea Daphnia (Crustacea, Cladocera, Anom- opoda) shows the most prominent examples of phenotypic plasticity in response to chemical cues released by predators, so-called kairomones (Tollrian and Dodson, ’99). Defense strategies of Daphnia, such as defensive morphs, life-history, and behavioral changes, are well studied and their adaptive significance discussed (Tollrian and Dodson, ’99). The predator-induced polyphenism in Daphnia pulex represents the potential to greatly advance our understanding of plasticity and cryptic genetic variation, because of the accumulation of genomic, genetic, ecological, and toxicological data (Eads et al., 2007; Jenner and Wills, 2007). D. pulex generates neckteeth primarily in the younger instar (first to third instar; Fig. 1A–C) in the presence of the predatory phantom midges, i.e. Chaoborus lavae. The neckteeth effectively decreases the predation risk, because for the predators it is Published online 1 September 2009 in Wiley InterScience (www. interscience.wiley.com). DOI: 10.1002/jez.565 Received 7 April 2009; Revised 24 May 2009; Accepted 8 July 2009 Grant sponsor: Ministry of Education, Culture, Sports, Science and Technology of Japan; Grant numbers: 20033002; 18370007; Grant sponsor: JSPS Research Fellowships for Young Scientists. Ã Correspondence to: Toru Miura, Graduate School of Environ- mental Science, Hokkaido University, N10 W5, Sapporo, Hokkaido 060-0810, Japan. E-mail: miu@ees.hokudai.ac.jp r 2009 WILEY-LISS, INC. JOURNAL OF EXPERIMENTAL ZOOLOGY 311A:788–795 (2009)