The Questions of Developmental Biology

Colonial invertebrates have some spectacular developmental responses to predators. Due
to their sessile growth, they cannot escape by moving away, and predators often treat them like
plants, eating modules without killing off the entire colony. Membranipora membranacea is a
widely distributed bryozoan often seen on kelp. It is grazed upon by certain nudibranch molluscs
that suck in the modules at the periphery of the colony (Figure 21.15). When exposed to such
predation, the modules near the nudibranch develop spines. These spines interfere with the
predators establishing the suction needed to feed. Spines can also be induced within three days by
treating a colony with chemical extracts of their predator (Harvell 1986; 1999.)
Predator-induced polyphenism is not limited to invertebrates. McCollum and Van
Buskirk (1996) have shown that in the presence of its predators, the tail fin of the tadpole of the
gray treefrog Hyla chrysoscelis grows larger and becomes bright red. This allows the tadpole to
swim away faster and to deflect strikes toward the tail region. The carp Carassius carassius is
able to respond to the presence of a predatory pike only if the pike had already eaten other fish.
The carp grows into a pot-bellied, hunchbacked morph that will not fit into the pike's jaws.
However, as in most predator-induced defenses, there is a trade-off (otherwise one would expect
the induced morph to become the normal phenotype). In this case, the induced morphology puts a
drag on swimming efficiency, and the fatter fish cannot swim as well (Brönmark and Pettersson
1994).
Mammalian Immunity as a Predator-Induced Response
If predator-induced polyphenism is an adaptive response to potential threats, the
mammalian immune system is its highest achievement. The mammalian immune system is an
incredibly elaborate mechanism for sensing and destroying materials that are foreign to the body.
When we are exposed to a foreign molecule (called an antigen), we manufacture antibodies and
secrete them into our blood serum (see Chapter 4). These antibodies combine with the antigen to
inactivate or eliminate the antigen. The basis for the immune response is summarized in the five
major postulates of the clonal selection hypothesis (Burnett 1959):
1. Each B lymphocyte (B cell) can make one, and only one, type of antibody. It is specific for one
shape of antigen only
2. Each B cell places the antibodies it makes into its cell membrane with the specificity-bearing
side outward.