The Questions of Developmental Biology

*The shell gland is an ectodermal organ
formed through induction by mesodermal cells. Without the mesoderm,
no cells are present to induce the competent ectoderm. Here we see an example of limited
induction within a mosaic embryo. For more information on the formation of snail embryos, see Collier 1997.
Adaptation by Modifying Embryonic Cleavage
Evolution is caused by the hereditary alteration of embryonic development. Sometimes
we are able to identify a modification of embryogenesis that has enabled the organism to survive
in an otherwise inhospitable environment. One such modification, discovered by Frank Lillie in
1898, is brought about by altering the typical pattern of spiral cleavage in the unionid family of
clams.
Unlike most clams, Unio and its relatives live in swift-flowing streams. Streams create a
problem for the dispersal of larvae: because the adults are sedentary, free-swimming larvae would
always be carried downstream by the current. These clams, however, have adapted to this
environment by effecting two changes in their development. The first alters embryonic cleavage.
In the typical cleavage of molluscs, either all the macromeres are equal in size or the 2D
blastomere is the largest cell at that embryonic stage. However, the division of Unio is such that
the 2d blastomere gets the largest amount of cytoplasm (Figure 8.29).
This cell divides to produce most of the larval structures, including a gland
capable of producing a large shell.
The resulting larvae (called glochidia) resemble tiny bear traps; they have sensitive hairs
that cause the valves of the shell to snap shut when they are touched by the gills or fins of a
wandering fish. They attach themselves to the fish and "hitchhike" with it until they are ready to
drop off and metamorphose into adult clams. In this manner, they can spread upstream.
In some species, glochidia are released from the female's brood pouch and merely wait
for a fish to come wandering by. Some other species, such as Lampsilis ventricosa, have
increased the chances of their larvae finding a fish by yet another modification of their
development (Welsh 1969). Many clams develop a thin mantle that flaps around the shell and
surrounds the brood pouch. In some unionids, the shape of the brood pouch (marsupium) and the
undulations of the mantle mimic the shape and swimming behavior of a minnow. To make the
deception all the better, they develop a black "eyespot" on one end and a flaring "tail" on the
other. The "fish" seen in Figure 8.30 is not a fish at all, but the brood pouch and mantle of the
clam beneath it. When a predatory fish is lured within range, the clam discharges the glochidia
from the brood pouch. Thus, the modification of existing developmental patterns has permitted
unionid clams to survive in challenging environments