Formation of morphological defences in response to YOY perch and invertebrate predation in two Daphnia species coexisting in a mesotrophic lake
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This study examined the formation of morphological defences by two coexisting Daphnia species, the large-sized D. pulicaria (2 mm) and the small-sized D. mendotae (1.4 mm), in response to the presence of young-of-the-year (YOY) yellow perch (Perca flavescens) and invertebrate predators (Chaoborus, Leptodora) during summer in a mesotrophic lake. We hypothesized that due to differential size-selective predation risk by YOY fish and invertebrates, the large-sized and the small-sized Daphnia species would show different morphological responses to predation threats. We followed changes in two morphological traits (relative length of the tail spine in D. pulicaria and of the helmet in D. mendotae) among different periods during summer according to YOY fish and invertebrate predation. We defined four YOY fish predation periods based on the presence of YOY perch in the pelagic zone of the lake and the relative abundance of Daphnia preys in their gut contents, and two invertebrate predation periods based on exclusive or mutual occurrence of the invertebrate predators. The large-sized (D. pulicaria) and the small-sized (D. mendotae) species showed different morphological responses to YOY fish and invertebrate predators, respectively. The tail spine ratio of the juveniles and adults of D. pulicaria did not change in response to YOY fish predation or to invertebrate predation. A gradual increase in the helmet ratio was observed in the small-sized D. mendotae over the summer period. This change was related to the co-occurrence of the invertebrate predators (Chaoborus and Leptodora) and to YOY fish predation. The warmer temperature cannot be accounted for helmet elongation since it was constant across depths, and not related with the co-occurrence of D. mendotae and YOY perch.
KeywordsTail spine ratio D. pulicaria Helmet ratio D. mendotae YOY yellow perch Chaoborus Leptodora
This study was financed through grants from the Natural Sciences and Engineering Research Council of Canada (Discovery Grant) and from the Québec Ministry of Education (FQRNT: Team Grant) to B.P.A. and by a post-doctoral fellowship to M.S and a scholarship grant to M.G. from the GRIL (Groupe de Recherche Interuniversitaire en Limnologie et en Environnement Aquatique, Université de Montréal). We thank Alexandra Rutherford, Karène Gélinas and G. Méthot for field assistance and Louise Cloutier for the taxonomic identification of the chaoborids. We thank two anonymous reviewers for valuable comments on an earlier version of the manuscript. We also thank John Chételat who improved the language of the manuscript. The study was presented as an oral communication to the VIIth international symposium on Cladocera, 3–9 September 2005, Herzberg, Switzerland.
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