Elusion
In contrast to predators, prey would prefer to avoid capture and elude their pursuer. The objective here is essentially the opposite of the intercept scenario: elude capture until a safe haven can be reached or the threat is otherwise eliminated. Many of the same propulsion features important for pursuit (fast acceleration and/or high top speed) are important for elusion as well. Aside from a capable propulsion system, an extra bag of tricks contributes to the success of elusive prey, as illustrated by the following examples.
Squid are one of nature's most well known examples of jet propulsion. Most of the body of a squid is a special structure (called a mantle) that contains a sack into which the squid can draw fluid and later eject it from a nozzle (also called a funnel). When threatened, a squid quickly contracts its mantle forcing the water out of the funnel at a high rate of speed. The jet thrusts the squid in the opposite direction of the jet and out of the dangerous situation. This process is known as “escape jetting” and is illustrated in the movie below. The sleek, streamlined shape of squid contributes to low drag and high acceleration. In extreme situations, squid will eject a thick, black cloud of ink with the jet of water to disguise their escape. Multiple escape tactics makes squid skilled escape artists. |
 Squid. Photographed by the NOAA Ocean Explorer |
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Escape jet of a juvenile Oval Squid (Sepioteuthis lessoniana). The PVC pipe on the left is used to startle the squid and induce its escape jetting behavior. The mantle length of the squid is 12 mm and the movie was filmed at 125 frames per second (fps). The black blob in the squid is its ink sack. Oval squid are native to the Western Pacific.
Video footage courtesy of Joseph T. Thompson (Franklin and Marshall College) http://www.fandm.edu/x18304.xml |
 Barracuda with prey. Photographed by the National Oceanic and Atmospheric Administration. |
Barracudas have long, sleek bodies. Their streamlined shape allows them to accelerate quickly when necessary. Barracudas are skilled at using this trait to capture prey. As illustrated in this photo, the barracuda was easily able to outrun and capture its prey. Speed and acceleration are therefore key components of the barracuda’s method of capture. |
| Anchovies have small, sleek bodies. The low mass and streamlined shape allows them to accelerate quickly when necessary. Their petit figure also gives them excellent maneuverability, allowing them to turn sharply and dart away from predators. Anchovies are skilled at using these traits as long as necessary to elude predators. Endurance is therefore a key component of the anchovies’ method of elusion as well. | 
Californian anchovy. Photographed by the National Oceanic and Atmospheric Administration. |
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Example Problem: Escape Acceleration |
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