Article: Ugland KI, Aksnes DL, Klevjer TA, Titelman J, Kaartvedt S (2014) Lévy night flights by the jellyfish Periphylla periphylla. Mar Ecol Prog Ser 513:121-130
The jellyfish population is exponentially increasing in response to human induced stresses, such as fishing, increased boat traffic, and pollution. Not only have more jellyfish been seen in the open ocean, but thousands of jellyfish have been seen camping out in the northern Norwegian fjords. Unlike fishes that primarily use sight to feed (hunt), jellyfish use tactile strategies to feed, meaning jellyfish have to be very close to their zooplankton prey in order to consume it. Therefore, fjords are perfect places for jellyfish to live as they can harvest the high density of zooplankton brought in by the currents. These “jellyfish fjords” provide an ideal place to observe and study jellyfish behavior, specifically how jellyfish use vertical movement to improve feeding behavior.
Researchers from Norway set out to understand the movement pattern of jellyfish (Periphylla periphylla) and how they can efficiently find their zooplankton prey. Is it purely coincidence that they bump into their food, or do they actively try to find their prey using the Lévy-like pattern (use long glides mixed with random floating)?
Just as fishermen use echosounders to locate fishes, the researchers in this study used echosounders to detect the vertical movement of jellyfish. The echosounder was put on the seafloor (280 meters deep) and would send sound waves up to the surface every second. The waves would then either bounce off obstacles (jellyfish) or straight to the surface, allowing the researchers to see exactly where the jellyfish were in the water column, how big they were and how far and fast the jellyfish were drifting. The data was then automatically uploaded to computers ashore. A specifically designed software allowed the researchers to track the movements of the jellyfish detected from the echosounders. The echosounders were able to reveal if jellyfish were moving vertically upward, vertically downward, or not moving vertically at all. For three weeks, the movement of jellyfish in the Swedish fjords was observed and analyzed. A specific model was set up, allowing the observers to tell how many “steps” (i.e. how many times did they jellyfish propel themselves) the jellyfish “walked” (i.e. how far did each burst of speed move them) per day and per night and in what direction.
Significance of Results
Turns out, jellyfish are night owls! The jellyfish moved farther during the night than during the day, some exceeding 50 meters (m). They also seemed to have a longer step pattern at night, with 3% of the step lengths exceeding a vertical distance of 16m (Fig. 1)! During the day, the echosounders showed that the jellyfish preferred a more Brownian type swimming method, meaning they randomly floated with the currents and just happened to run into their zooplankton prey. During the night, however, the jellyfish seem to take on more of the Lévy-like foraging pattern, meaning they used long glides to actively bump into their zooplankton prey. Lévy-type movements have previously been shown to maximize the probability that a predator will find its prey in sparsely distributed areas. However, since the Norwegian fjords are booming with zooplankton, it seems like a waste of energy for the jellyfish to use the Lévy-type movement compared to a more random form of swimming behavior. It is also curious that this behavior was only witnessed at night, while a random downward swimming pattern was witnessed during the day. Since jellyfish use tactile cues to hunt their prey, light shouldn’t be a factor. These results were indeed a surprise to the researchers, but it leaves us with very interesting hypotheses and future studies to work on! It could be that the jellyfish in these fjords have evolved to prey on food sources other than zooplankton, being that there are very little other predators in the fjords to compete with. It is also possible that since there has been such an increase in the population of jellyfish in the Norwegian fjords, the jellyfish species as a whole can risk using a less energy efficient method of foraging and therefore have evolved to use the Lévy-like search pattern over the Brownian method.