Behavior Biological oceanography

The physics of tiny jellyfish hunting

Sutherland, K. R., Gemmell, B. J., Colin, S. P. and Costello, J. H. (2016), Prey capture by the cosmopolitan hydromedusae, Obelia spp., in the viscous regime. Limnol. Oceanogr. doi: 10.1002/lno.10390

Figure 1 – Obelia spp. (courtesy of Scripps Plankton Camera System – SIO/UCSD)

When most people think of jellyfish, they envision pests that can cause a nasty rash. While cnidarians, the Latin name for jellies, do sting, they are also important top predators in many regions. In fact, jellies are often the dominant consumer and can exert a lot of pressure on the biological community. To understand and predict how jellies might influence a certain ecosystem, scientists must learn how they go about capturing their food.

Cnidarian predation behavior is generally classified as either ambush or filter feeding. Ambush predators will sit around and pounce on food that drifts by. In contrast, filter feeders are constantly swimming trying to encounter prey. These terms succinctly describe what the jelly is doing. The actual physics of how these organisms get their food is much hazier.

To cast some light on the issue, Dr. Kelly Sutherland of the University of Oregon studied the physical factors that influence how the small, cosmopolitan cnidarian Obelia spp eats (Fig 1). Dr. Sutherland hypothesized that since these jellies are so small, typically less than a centimeter in size, they experience the ocean as a thick soup. To get an idea of what this is like, imagine swimming through a giant vat of honey. In this viscous regime, the movement of a tentacle might actually push prey away from the jelly.

To figure out how the jellies get enough food to survive, Dr. Sutherland and her team hand collected a bunch of jellies and fed them a variety of organisms in the laboratory. The group filmed the organisms swimming and eating to analyze their behavior. They also used a high tech method called particle image velocimetry, or PIV, to visualize the fluid motion close to Obelias’ bodies.

Figure 2 – Stills from a video of a jelly capturing some food. The prey is highlighted by the red circle. Notice that the jelly first encounters the food on its bell, but uses its tentacles to eat it. (Adapted from Sutherland et al., 2016)

The feeding videos revealed that Obelia preferentially ate organisms about 70 microns in size. Dr. Sutherland noticed that no matter how the jelly found the prey, it always transported the food along its tentacles to the mouth (Fig. 2). She also found that the jellies spend most of their time swimming, consistent with their classification as filter feeders.

Dr. Sutherland’s team used the swimming footage to measure how fast the Obelia were expanding and contracting their bells to move. They quantified both the speed of the body and individual tentacles. The measurements confirmed that the jellies operated in a viscous regime. The water is so syrupy to the Obelia they are actually pushed slightly backward each time they move forward.

As the jellies swam, a region of fluid called a boundary layer, moved with them. The size of the boundary layer depends on the size of the moving object, what the object’s velocity is, and what the fluid around it is like. Using the PIV measurements, Dr. Sutherland was able to measure the size of the boundary layer around the Obelia (Fig. 3). Her analysis revealed that the boundary layer was the thinnest at the tip of the tentacle when it was at its peak velocity.

Figure 3 – Illustration of the velocity field computed from the particle image velocimetry measurements. The arrows represent how the water is moving around the jelly. The color scale is an indication of the vorticity, or how much the fluid is spinning. Hot colors show counterclockwise motion and cool colors signify clockwise movement. (Adapted from Sutherland et al., 2016)

Dr. Sutherland and her colleagues realized that the minimum boundary layer thickness around the tentacle was the same size as the prey the Obelia were preferentially eating. Furthermore, the maximum speed of the tentacle and the fluid around it were greater than the escape velocity of its prey. The implication, says Dr. Sutherland, is that the jellies are using the fluid properties to increase their chance of capturing food.

This all may seem purely academic, but consider that Obelia can reach densities of nearly 2000 individuals per cubic meter of ocean water. Taken together, these jellies can eat a lot of plankton and thereby impact an entire ecosystem. Understanding the mechanics of how such predators eat is important when considering how a given area will respond to change.

Dr. Sutherland offers an example: suppose the seawater temperature in some region goes up. As the temperature rises, the viscosity of the water goes down. As the viscosity drops, the boundary layer around a jelly will become thinner, allowing it to eat smaller organisms and, possibly, out competing other organisms. These sorts of changes can have big impacts on a community.

This study of Obelia has quite a narrow focus, but underscores an important point. Changes to an ecological system are a function of physics and biology. Scientists must connect those two disciplines to understand how a community will respond to a shift.

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