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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