Instead of swimming freely in the ocean, garden eels anchor themselves into burrows in the sandy seabed, staying in one place for most of their lives. Their heads face against the flow of the current as they strike at zooplankton drifting by. Until recently, scientists knew little about how these fascinating creatures feed.
Now, a team of researchers led by the Okinawa Institute of Science and Technology Graduate University (OIST) has investigated for the first time the feeding behavior of garden eels in a laboratory setting, and revealed how these animals make use of their burrows, and change their posture and movements in response to strong currents, allowing them to feed in a wider range of flow speeds than many free-swimming fish.
“Free-swimming fish can shelter from currents by hiding in cracks and crevices in the reef,” said study lead author Kota Ishikawa, a doctoral student in Marine Biophysics at OIST. “But garden eels are stuck in a more exposed area, with only their own burrows for shelter, so they’ve had to develop their own unique strategies for dealing with strong currents.”
In their laboratory, the scientists recreated typical conditions by making a flume with a sandy bottom, containing several portable burrows, each of them housing a single spotted garden eel, Heteroconger hassi, a species commonly found in Okinawa. After adding zooplankton to the water, they used cameras to capture the motion of the garden eels as they fed at four different flow rates (0.1, 0.15, 0.2, and 0.25 m/s).
The experiment revealed that, as the current increased, the eels retreated farther into their burrows and focused their strikes on zooplankton which passed closer by. “This is a really important adaptation, as faster currents require more energy to move in,” Ishikawa explained.
However, as the current increased in speed up to 0.2 m/s, the retreat into burrows did not stop the eels from feeding at a rapid pace, due to the fact that more zooplankton drifted by in the given amount of time, thus offsetting the eels’ behavioral changes. Moreover, due to the shorter strike distance, the eels were more successful in catching their prey. At higher flows, the eels adopted a more curved posture, enabling them to reduce the amount of drag on their bodies by around 57 percent, which helped them conserve energy.
The feeding rate of the eels hit its peak at just under 0.2 m/s, proving that these animals are adapted to feed at a wider range of flow speeds than free-swimming reef fish (which have a peak rate of about 0.15 m/s). “We can see that their unique strategy of retreating into the burrows and reducing their strike distance really pays off when dealing with strong currents,” Ishikawa concluded.
The study is published in the Journal of Experimental Biology.