Predator-Induced Defense: Variation for Inducibility in an Intertidal Barnacle

Abstract

Phenotypic plasticity is a widespread and often adaptive feature of organisms living in heterogeneous environments. The advantages of plasticity seem particularly clear in organisms that show environmentally cued switches between alternative morphs. Information concerning the presence and nature of variation underlying the induction of these morphs, especially under field conditions, would be valuable. Here we examined the basis for variation underlying a predator‐induced defense in an intertidal barnacle (Chthamalus anisopoma). In a previous experiment, juvenile barnacles were exposed to a predatory gastropod (Acanthina angelica). Some of these individuals were induced to develop as a predation‐resistant form, but other individuals developed as the default, undefended morph. Here we tested two alternative explanations for this observation. One, the “continuous‐sensitivity” model, holds that there is normally distributed genetic variation for sensitivity to the cue. This model predicts that, given sufficient exposure to the predator, all individuals would develop as the induced form; it suggests that the previous findings resulted from an insufficient dose of the cue. The second model, the “discontinuous‐sensitivity” model, asserts that there is a genetic polymorphism for inducibility such that some individuals are not able to respond to the cue. This model suggests that, with repeated exposures to the predator, the resulting dose–response curve would reach an asymptote at <100%. We conducted a dose–response experiment in order to contrast these two alternatives, and to examine an expectation generated by life‐history theory, namely, that repeated exposure to the predator would induce maturity at a younger age. With respect to the life‐history prediction, we found no evidence to suggest that repeated exposure of juvenile barnacles to Acanthina affected the age at maturity, even though we found strong evidence for size‐selective attack by this predator. With respect to variation underlying induction to the defended morph, we obtained a dose–response curve showing a significant asymptote at about 22% induction, which is inconsistent with the continuous‐sensitivity model. Hence the results indicate the possibility of a developmental polymorphism in the barnacle, but no indication of life‐history shifts in response to the predator.

Investigating the genetic architecture of conditional strategies using the environmental threshold model

“…Similarly, the idea that thresholds can be present but lie beyond the cue distribution of the entire population is supported by our diet manipulation and serves to explain populations of earwigs, where fewer than 1 or 2% of males are majors [40]. The coexistence of conditional and canalized strategists in the same population has previously been supported by evidence from the defence dimorphisms of barnacles [2] and daphnids [13], and for alternative male phenotypes in bulb mites [16].…”

Evolutionary ecology of inducible morphological plasticity in predator–prey interaction: toward the practical links with population ecology

“…This approach has been instrumental in documenting the taxonomic prevalence of inducible defenses (Torllian and Harvell 1999;DeWitt and Scheiner 2004) and has shed considerable light on the adaptive value of trait plasticity (Lively 1986a,b;Kopp and Tollrian 2003a;Kishida and Nishimura 2005;Benard 2006). These insights have been further enhanced by studies of the role of inducible defenses in driving the evolution of geographic variation in prey phenotypes (Lively et al 2000;Trussell 2000a,b;Trussell and Smith 2000;Relyea 2002a;Trussell and Nicklin 2002;Laurila et al 2006;Kishida et al 2007) and the genetic basis of morphological plasticity via quantitative genetic (Relyea 2005a) and molecular genetic (Mori et al 2005(Mori et al , 2009) approaches. For example, Rana pirica frog tadpoles of the populations in the predator-common mainland have higher expression ability of defensive morph when exposed to predation risk from the salamander larvae (Hynobius retardatus) than those of a predator-free island population ).…”

Geographic Variation in a Predator-Induced Defense and Its Genetic Basis

“…One issue that has received comparatively less attention is the degree to which prey taxa display geographic variation in their capacity to express inducible morphological defenses and whether such variation is correlated with local predation pressure (but see Lively et al 2000, Trussell 2000b, Trussell and Smith 2000, Relyea 2002b, Trussell and Nicklin 2002, Laurila et al 2006). This issue is important because the intensity of interactions between predators and their prey can vary substantially across broad spatial scales, as is the case when nonnative predators invade novel habitats (e.g., Smith 2000, Freeman andByers 2006).…”