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.