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Mollusk Defenses

An image of various fossil mollusks at the Peabody Museum of Natural History at Yale University provides an example of an ancient and continuing evolutionary arms race. A shell can offer protection through many different forms. How might each of these shells defend against a predator?

Credits: Courtesy of the Peabody Museum of Natural History, Yale University, New Haven, CT

Mollusk Defenses

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Resource Type:
Image

Format:
Graphic

Topics Covered:
Adaptation and Natural Selection

Backgrounder

Mollusk Defenses:

Pick up a marine snail shell, hold it in your hand. Feel the smooth, curved surface and the spiral ridges at its pointed end. Most humans appreciate shells for their beauty. We often use them to decorate our living spaces.

To evolutionary scientists, snail shells are mute witnesses to eons of predator-prey conflict. Over millions of years, the shells that best protect their inhabitants from crushing, breaking, or drilling by determined predators have survived. The smooth surface makes it harder for a crab or other sea creature to get a grip. The sheer thickness of the calcium carbonate shell defeats many attacks. The small, narrow, toothed opening into the shell's interior doubtless sends many would-be diners swimming away in frustration.

Paleobiologist Geerat Vermeij has studied extensively the ongoing evolutionary "arms race" between predator and prey.

The shells of the more modern snails, he noticed, were thicker than those of ancient ones. Their fortresses, so to speak, had thicker walls, and to Vermeij it suggested that they had evolved in an escalating arms race in response to more powerful crushing attacks.

Indeed, crabs with sharp teeth lining their claws and certain fish with strong jaws can smash many snail shells. Vermeij has found, however, that even with stronger defenses, more and more snails are found with shells that have suffered cracks or cuts but survived.

The tit-for-tat race goes on and on. But, says Vermeij, there are long periods where neither side develops better adaptations. Predator and prey reach an equilibrium. Regarding the coevolution of predator and prey, Vermeij comments: "Why it stops and why it starts are the two most interesting questions."

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