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

Abstract

JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org.. Ecological Society of America is collaborating with JSTOR to digitize, preserve and extend access to Ecology.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 "continuoussensitivity" 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 sizeselective 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.

The Ancient Chemistry of Avoiding Risks of Predation and Disease

“…Amphipods and isopods reduce their activity in habitats harbouring fish (Andersson et al 1986;Short and Holomuzki 1992;Holomuzki and Hatchett 1994). Intertidal barnacles deploy predator-resistant morphology against predaceous snails (Lively et al 2000) and shrimp alter their growth and antennal morphology in response to crushed conspecifics (Nga et al 2006).…”

Detecting small environmental differences: risk-response curves for predator-induced behavior and morphology

“…Organisms that possess adaptive plastic traits have the ability to alter their traits in response to environment cues to produce phenotypes that perform better under the new environmental conditions (Schlichting and a diVerence once a threshold level is encountered, resulting in a discrete polymorphism (Moczek 1998;Lively et al 2000). If there is genetic variability among individuals in the point of this threshold induction, then diVerent points along the environmental gradient will produce diVerent proportions of induced and uninduced individuals (RoV 1996;Lively et al 2000;Hazel et al 2004). However, if sensitivity is high, organisms may have the ability to detect and respond to an environmental gradient with graded phenotypic responses, where increased cue intensity increases the magnitude of the induction and not just the proportion of induced individuals (Harvell 1990;.…”

The Ecological Genetics of Conditional Strategies

“…A second theoretical approach is the environmental threshold (ET) model, a quantitative genetic model that treats conditional strategies as environmentally cued threshold traits (Hazel et al 1990;Hazel and Smock 1993;Roff 1996;Ostrowski et al 2000). The ET model postulates genetic variation underlying the response to the environmental cue, a common feature of conditional strategies (Hazel 1977;Hazel and West 1982;Hairston and Walton 1986;Knülle 1995Knülle , 2003Emlen 1996;Pulido et al 1996;Roff 1996;Harvell 1998;Doums et al 1998;Lively et al 2000;Moczek et al 2002). However, the ET model has not examined the effects of cue reliability or competitive interactions on the evolution of conditional strategies, and neither the ET nor the strategic models have considered discrete variation in the strength of the environmental cue or the possibility of genes with a large effect on the response to such cues, both of which may be important features of conditional strategies, particularly the conditional strategy of predator-induced defense (Harvell 1998;Lively et al 2000).…”