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Biology LibreTexts

12.1: Further Examples of Predation

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  • Amazing mechanisms for both capturing prey and avoiding predators have been discovered through evolution. Fulmar chicks, for example, can direct “projectile vomit” at predators approaching the nest too closely (Figure \(\PageIndex{1}\)). This is not just icky for the predator. By damaging the waterproofing of an avian predator’s feathers, this ultimately can kill the predator. The chick’s projectile vomit is thus a lethal weapon.

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    Figure \(\PageIndex{1}\). Fulmar chick with projectile vomit as predator defense.

    One of the most remarkable predatory weapons is that of pistol shrimp. These shrimp have one special claw adapted to cavitation, and are capable of shooting bullets at their prey; colonies of these shrimp are loud from the sound of these bullets. But where does an underwater crustacean get bullets? Actually, it creates them from nothing at all—from cavitation. If you’ve ever piloted a powerful motorboat and pushed the throttle too hard, or watched a pilot do so, you’ve seen the propellers start kicking out bubbles, which look like air bubbles. But the propellers are well below the water line, where there is no air. The propellers are in fact creating bubbles of vacuum—separating the water so instantly that there is nothing left in between, except perhaps very low density water vapor. Such bubbles collapse back with numerous blasts, each so powerful that it rips off pieces of bronze off the propeller itself, leaving a rough surface that is the telltale sign of cavitation.


    Figure \(\PageIndex{2}\). Pistol-packing shrimp shoot cavitation bullets.

    A pistol crab snap its pistol claw together so quickly that it creates a vacuum where water used to be. With the right circulation of water around the vacuum bubble, the bubble can move, and a crab can actually project its bullet of vacuum toward its prey. When the bubble collapses, the effect is like thunder attending a lightning bolt, when air snaps together after the lightning has created a column of near-vacuum. But the consequences are quite different in water. While a loud sound might hurt the ears of a terrestrial animal, the sound does not rip apart the fabric of the animal’s body. This is, however, what intense sounds in water can do, traveling through water and through the water-filled bodies of animals. In effect, pistol shrimp shoot bullets that explode near their prey and numb them into immobility. Somehow evolution discovered and perfected this amazing mechanism!

    Figure \(\PageIndex{3}\). Rifle-carrying humans shoot lead bullets—bison bones and remnant herd.

    The ultimate weapons of predation, however, are those of our own species. Figure \(\PageIndex{3}\) (right) shows a remnant bison herd, a few hundred of the hundreds of millions of bison that migrated the plains not many generations ago. No matter how vast their numbers, they were no match for gunpowder and lead bullets, and they dropped to near extinction by the beginning of the twentieth century. The image at the left illustrates the epic efficiency of lead bullets by showing a nineteenth-century pile of bison bones, with members of the predator species positioned atop and aside.

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