A colony of garden eels often looks at first like sea grass swaying in the current—until you get closer. As you approach, you might get a glimpse of the big googly-like eyes on either side of their pencil-thin bodies before they disappear into the safety of their burrow. Though social animals, each eel remains rooted to the burrow its muscular tail carved out of the sand. Glands in their skin secrete a mucus that fortifies the sand around their burrow and seals the entrance when they dart in to avoid danger. And when the coast is clear, they straighten up and out, using their relatively large mouths to catch zooplankton floating past.
While this homebody strategy is great for avoiding predators, scientists wondered how garden eels cope in stronger currents which require more energy to hold steady in, and carry zooplankton past more quickly. To find out, scientists placed a garden eel in a tank with a sandy bottom, and three cameras. Then they added zooplankton and filmed the eel’s behavior at four different flow rates. A computer learning program analyzed the footage and revealed the eels’ trick for staying fed.
As the current speeds up, the eels extend less from their burrows and curve their bodies more. This reduces the drag on their bodies and the amount of energy needed to resist the current. They also limit their catches to the zooplankton that are closest to their burrow. But they still catch plenty of prey: a shorter striking distance means greater success in snatching their meals, and the faster current carries more critters past them. If the current gets too strong—0.25 meters per second or greater—they take the day off from feeding because they would spend more energy feeding than they would get from the food. But most of the time, staying home to eat suits these eels just fine.