Darwin’s theory of evolution stated that biodiversity arose through descent with modification. This explanation united and explained the commonalities of form (such as the same types of bones in the arms of humans and the flippers of seals) as having evolved from a common ancestor. It also explained how natural selection results in changes that can enable an organism to better survive in its particular environment. We see both these principles in snail development. E. B. Wilson demonstrated that snails, annelids, and flatworms have spiral cleavage and that the similar roles played by the embryonic cells could best be explained by all these animal groups having evolved from a common ancestor.

The same year, the embryologist Frank R. Lillie showed that a new structure can evolve by changing the pattern of development. He thus showed that evolution can be the result of hereditary alterations in embryonic development—in other words, that selection on a larval structure acts on mutations that affect embryonic development. One such modification, discovered by Lillie in 1898, is brought about by an alteration of the typical pattern of molluscan spiral cleavage in a family of bivalve mollusks, the unionid 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. Unio clams have adapted to this environment via two modifications of their development. The first is an alteration in embryonic cleavage. In typical molluscan cleavage, either all the macromeres are equal in size or the 2D macromere is the largest cell at that embryonic stage. However, cell division in Unio is such that the 2d “micromere” gets the largest amount of cytoplasm (Figure 1). This cell then divides to produce most of the larval structures, including a gland capable of producing a large shell. The resulting larva is called a glochidium and resembles a tiny bear trap. Glochidia 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. The larvae can thus attach themselves to the fish and “hitchhike” until they are ready to drop off and metamorphose into adult clams. In this manner, they can spread upstream as well as downstream.

Figure 1  Formation of a glochidium larva by modification of spiral cleavage. After the 8-cell embryo is formed (A), the placement of the mitotic spindle causes most of the D cytoplasm to enter the 2d blastomere (B). This large 2d blastomere divides (C), eventually giving rise to the large “bear trap” shell of the larva (D).

In some unionid species, glochidia are released from the female’s brood pouch (marsupium) and then wait passively for a fish to swim by. Some other species, such as Lampsilis altilis, have increased the chances of their larvae finding a fish by yet another developmental modification. 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 and the undulations of the mantle mimic the shape and swimming behavior of a minnow (Welsh 1969). To make the deception even better, the clams develop a black “eyespot” on one end and a flaring “tail” on the other (Figure 2). When a predatory fish is lured within range of this “prey,” the clam discharges the glochidia from the brood pouch and the larvae attach to the fish’s gills. Thus, the modification of existing developmental patterns has permitted unionid clams to survive in challenging environments.

Figure 2  Phony fish atop the unionid clam Lampsilis altilis. The “fish” is actually the brood pouch and mantle of the clam. The “eyes” and flaring “tail” attract predatory fish, and the glochidium larvae attach to the fish’s gills.