5. Future directions
Research into inducible defenses in field populations is informative; however, recent studies were often based on laboratory experiments. In the laboratory, predatory kairomones are prepared based on a ”kairomone recipe” that is generally established at a much higher concentration than that in nature. It is believed that Daphnia will react sufficiently in the presence of appropriate stimuli; therefore, preparation of a ”kairomone recipe” does not assume the same response in any population of any species. Additionally, the expression and degree of inducible defenses differ among populations of the same species owing to local adaptation (Boersma et al. 1999; Boeing et al. 2006a; Reger et al. 2018). Therefore, experiments might overestimate or underestimate intraspecific variations. It is necessary to investigate dose-response curves based on initial changes in predator density, because the ”kairomone recipe” already sufficiently induces defensive traits. Inducible defense experiments can be constructed using chemical substances based on a given predator, because the chemical compositions of the Chaoborus (Weiss et al. 2018) and fish kairomones have been identified. And experimental individuals are maintained in a simpler environment than that which occurs in natural habitats.Daphnia may be used to analyze the genetic background of clones in order to elucidate how plasticity expression during a lifetime varies among factors. The relationship between traits and genetic analysis of the clones should be validated with laboratory experiments, long-term field studies, and multivariate statistics.
There remain other unresolved issues. For example, one phenomenon not yet elucidated is extraordinary inducible defenses reported by field observations (Laforsch and Tollrian 2004; Sakamoto et al. 2007; Tollrian and Laforsh 2006). Such defenses developed by Daphnia have not been successfully reproduced in the laboratory, likely because plasticity is expressed by a plurality of secondary factors. The degree of plasticity in Daphnia according to field observation is highest during predator emergence rather than during high predator density (Nagano and Doi 2018). We will attempt to elucidate the reasons for the discrepancy between experimental and field specimens in terms of their comparative degrees of inducible defense expression.
A major goal of evolutionary biology is to understand the mechanisms involved in creating biodiversity. Recent data concerning variations in phenotypic plasticity have promoted ecological speciation but with little empirical evidence (Pfennig et al. 2010). Although speciation involves several processes (Pfennig et al. 2010), phenotypic plasticity is thought to be helpful in the early stages of speciation (Pfennig et al. 2010; Thibert-Plante and Hendry 2011; Snell-Rood 2013; Forsman 2015). As the most famous example, tadpoles of Spea multiplicatemay facilitate speciation based on resource-induced plasticity in omnivorous or carnivorous morphology depending on resource availability (Pfennig and McGee 2010). In this case, both morphologies eventually separate by intraspecific variations in plasticity. This example shows that during the onset of speciation for resource utilization, spatiotemporal distribution remains the same, whereas there is variation in morphology. Similar to resource-induced plasticity, phenotypic plasticity against predation (inducible defense) creates morphological variance Unfortunately, high-quality empirical data does not yet exist for speciation of Daphnia . However, a variety of factors can cause intraspecific variation in Daphnia plasticity of inducible defense, and few experimental studies discuss how this intraspecific variation is maintained or how it is linked (or not linked) to speciation. We believe that these factors and variations will provide information regarding their effect on the early stages of speciation. Fortunately, Daphnia is useful for these kinds of experiments owing to its short generation time, ease of breeding, and the capability of using dormant eggs from previous generations. Future studies should focus on tracking both traits and genotypes through long-term evolution experiments in order to reveal how various traits that appear disadvantageous are conserved.
Because water temperature is a major secondary factor, Research into the phenotypic plasticity of living organisms in response to climate change will become increasingly significant in the future (Crispo et al. 2010; Weiss et al. 2018). Future studies should still consider not only the response of physiological activity against climate change, but the effect on predator–prey dynamics. In particular, , thereby altering the degree of defense expression in Daphnia according to the status of their predator(s).
Animal personality remains constant, regardless of environmental variation (Sih et al. 2004; Dingemanse et al. 2009; Wolf and Weissing 2012), and inducible defenses can vary because of personality differences (e.g. bold and shy) regardless of the presence of predators (crucian carp; Hulthén et al. 2014). This study showed that bold individuals undergo more substantial morphological changes than shy individuals. In contrast, shy individuals vary considerably in terms of evasion behavior. Therefore, personality-induced variation in inducible defense may be seen as both an adaptive and a maladaptive response. Under various environments and situations within the same species, bold individuals will have wide activity ranges, whereas shy individuals will have a narrow range. As Daphnia seem to have a personality (Heuschele et al. 2017), this species merits further investigation of personality as a factor contributing to variations in inducible defense. Depending on personality, the degree of expression in plasticity is expected to vary, as in the case of the crucian carp.