Adaptación en segmentación

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"Unlike most clams, Unio and it's 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 affecting two chages 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 on Unio is such that the 2d blastomere gest the largest amount of cytoplasm. 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 the 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."