The evolution of phenotypic plasticity.--My dissertation work was on the induction and maintenance of a curious shell-shape dimorphism in an intertidal barnacle (Chthamalus anisopoma) from the northern Gulf of California. Field experiments showed that juvenile barnacles are induced by the presence of a specialized gastropod predator (Acanthina angelica) to develop as an attack-resistant form (top) in which the aperture is oriented in a plane that is perpendicular, rather than horizontal, to the rock surface (Lively 1986a). This orientation makes the barnacle more resistant to attack, because it precludes insertion of the labial spine on the margin of the predator's shell (photos of A. angelica), which is used to push through the opercular valves of the barnacle.

The predator-resistant morph is also less fecund and grows slower than the non-induced, default morph (bottom) (Lively 1986b) (show me the data). The trade-off between predation resistance and reproductive potential fulfills a necessary condition for the maintenance of conditional strategies. Theoretical work showed that, in general, the conditions for environmentally cued polymorphisms can be much broader than for genetically determined polymorphisms (Lively 1986c, 1999; Hazel et al. 2004).

The photo on the left shows a top view of the two barnacle morphs. The eggs are brooded on either side of the rostral carinal axis, which runs from left to right in the photo. The bend in the bent morph reduces the area for brooding embryos, which incurs the reproductive cost. The slower growth rate in the bent morph is most likely due to the fact that growth occurs only at the base of the shell, and is thus restricted to one side (the bottom in this photo).

Recently Wade Hazel and I have extended the experimental work by conducting dose-response experiments (more on W. Hazel). The results suggest that there is a mixture of inducible and non-inducible genotypes in the Gulf of California (Lively et al. 2000). Such a mixture is possible under theory, if there are competitive interactions between the two morphs (Lively 1986c; Hazel et al. 2004.)

Cited papers:

Hazel, W, R. Smock, and C. M. Lively. 2004. The ecological genetics of conditional strategies. American Naturalist 163:888-900.

Lively, C.M. 1986a. Predator-induced shell dimorphism in the acorn barnacle Chthamalus anisopoma. Evolution 67:858-864

Lively, C.M. 1986b. Competition, comparative life histories, and maintenance of shell dimorphism in a barnacle. Ecology 67:858-864

Lively, C.M. 1986c. Canalization versus developmental conversion in a spatially variable environment. American Naturalist 128:561-572

Lively, C.M. 1999. Developmental strategies in spatially variable environments: barnacle shell dimorphism and strategic models of selection. Pages 245-258 in R. Tollrian and C.D. Harvell (eds.), The Ecology and Evolution of Inducible Defenses. Princeton University Press, Princeton.

Lively, C.M., W.N. Hazel, M.J. Schellenberger, and K.S. Michelson. 2000. Predator-induced defense: variation for inducibility in an intertidal barnacle. Ecology 81:1240-1247.

Lively, C.M., P.T. Raimondi and L.F. Delph. 1993. Intertidal community structure: space-time interactions in the northern Gulf of California. Ecology 74:162-173.

Raimondi, P. T., S. E. Forde, L. F. Delph, and C. M. Lively. 2000. Processes structuring communities: evidence for trait-mediated indirect effects through induced polymorphisms. Oikos 91:353-361.

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