Instars
Although it is unclear how Daphnia itself perceives own body
size, the body size is an important factor in determining the extent to
which inducible defense should be expressed (Tollrian 1995; Hart and
Bychek 2010). This is because predation sensitivity changes with
age/instar changes in body size. It is important to be able to identify
the type of predator, i.e. gape-limited or visual predator, by primary
factors at first. Chaoborus larvae prefer a narrow range of
small-sized prey (Swift and Fedorenko 1975; Pastrok 1981), whereas fish
prefer larger-sized prey, because they are readily visible (Brooks and
Dodson 1965; Nunn et al. 2012). Hence inducible defense varies among
instars. For example, neckteeth induction is stronger at the 2nd and 3rd
instars of Daphnia than at other stages (Tollrian 1993; Tollrian
1995a,b; Imai et al. 2009), because the former are the most vulnerable
to Chaoborus larva predation. Therefore, depending on the trait,
the degree of expression plasticity can be vary large within instar. The
presence of fish chemicals decreases Daphnia body size (Brett
1992; Weber and Declerck 1997; Boersma et al. 1998; Fisk et al. 2007;
Carter et al. 2013). Daphnia expresses inducible defense
throughout its entire lifespan in the presence of predators capable of
ingesting prey of any size (Laforsch and Tollrian 2004; Rabus et al.
2011).