The Questions of Developmental Biology

Predator-Induced Defenses
One survival strategy for coping with a harsh environment is for an animal to evolve the
ability to develop a new structure when confronted by a particular predator. In such cases, the
development of the animal is changed by chemicals released by the predator, enabling the
embryos or juveniles to better escape those same predators. This is sometimes called predatorinduced
defense, or predator-induced polyphenism.
To demonstrate predator-induced polyphenism, one has to show that the phenotypic
change is caused by the predator (usually by soluble chemicals released by the predator) and that
the phenotypic modification increases the fitness of its bearers when the predator is present
(Adler and Harvell 1990; Tollrian and Harvell 1999). For instance, several rotifer species will
alter their morphology when they develop in pond water in which their predators were cultured
(Figure 21.13; Dodson 1989; Adler and Harvell 1990). The predatory rotifer Asplanchna releases
into its water a soluble compound that induces the eggs of a prey rotifer species, Keratella slacki,
to develop into individuals with slightly larger bodies, but with anterior spines 130 percent longer
than they would otherwise be. These changes make them more difficult to eat. The snail Thais
lamellosa develops a thickened shell and a "tooth" in its aperture when exposed to the effluent of
the crab species that preys on it. In a mixed population, crabs will not attack the thicker snails
until more than 50 percent of the normal snails are devoured (Palmer 1985). Figure 21.13 shows
the typical and predator-induced morphs for several species. In each case, soluble filtrate from
water surrounding the predator is able to induce these changes, and the induced morph is more
successful at surviving the predator.
The predator-induced polyphenism of the parthenogenetic water flea Daphnia is
beneficial not only to itself, but also to its offspring. When Daphnia cucullata encounter the
predatory larvae of the fly Chaeoborus, their "helmets" grows to twice their normal size.
This inhibits their being eaten by the fly larvae. This same helmet induction occurs if the Daphnia
are exposed to extracts of water in which the fly larvae had been swimming (Figure 21.14).
Chemicals that are released by a predator and can induce defenses in the prey are called
kairomones. Agrawal and colleagues (1999) have shown that the offspring of such an induced
Daphnia will be born with this same altered head morphology. It is possible that the kairomone
regulates gene expression both in the adult and in the developing embryos. We still do not know
how Daphnia evolved the ability to make receptors that bind the kairomone or to utilize the
kairomone to generate an adaptive morphological change.