you're reading...


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.


No comments yet.

Post a Comment


  • by oceanbites 2 months ago
    Happy Earth Day! Take some time today to do something for the planet and appreciate the ocean, which covers 71% of the Earth’s surface.  #EarthDay   #OceanAppreciation   #Oceanbites   #CoastalVibes   #CoastalRI 
  • by oceanbites 3 months ago
    Not all outdoor science is fieldwork. Some of the best days in the lab can be setting up experiments, especially when you get to do it outdoors. It’s an exciting mix of problem solving, precision, preparation, and teamwork. Here is
  • by oceanbites 3 months ago
    Being on a research cruise is a unique experience with the open water, 12-hour working shifts, and close quarters, but there are some familiar practices too. Here Diana is filtering seawater to gather chlorophyll for analysis, the same process on
  • by oceanbites 5 months ago
    This week for  #WriterWednesday  on  #oceanbites  we are featuring Hannah Collins  @hannahh_irene  Hannah works with marine suspension feeding bivalves and microplastics, investigating whether ingesting microplastics causes changes to the gut microbial community or gut tissues. She hopes to keep working
  • by oceanbites 5 months ago
    Leveling up - did you know that crabs have a larval phase? These are both porcelain crabs, but the one on the right is the earlier stage. It’s massive spine makes it both difficult to eat and quite conspicuous in
  • by oceanbites 5 months ago
    This week for  #WriterWednesday  on  #Oceanbites  we are featuring Cierra Braga. Cierra works ultraviolet c (UVC) to discover how this light can be used to combat biofouling, or the growth of living things, on the hulls of ships. Here, you
  • by oceanbites 5 months ago
    This week for  #WriterWednesday  at  #Oceanbites  we are featuring Elena Gadoutsis  @haysailor  These photos feature her “favorite marine research so far: From surveying tropical coral reefs, photographing dolphins and whales, and growing my own algae to expose it to different
  • by oceanbites 6 months ago
    This week for  #WriterWednesday  on Oceanbites we are featuring Eliza Oldach. According to Ellie, “I study coastal communities, and try to understand the policies and decisions and interactions and adaptations that communities use to navigate an ever-changing world. Most of
  • by oceanbites 6 months ago
    This week for  #WriterWednesday  at  #Oceanbites  we are featuring Jiwoon Park with a little photographic help from Ryan Tabata at the University of Hawaii. When asked about her research, Jiwoon wrote “Just like we need vitamins and minerals to stay
  • by oceanbites 7 months ago
    This week for  #WriterWednesday  on  #Oceanbites  we are featuring  @riley_henning  According to Riley, ”I am interested in studying small things that make a big impact in the ocean. Right now for my master's research at the University of San Diego,
  • by oceanbites 7 months ago
    This week for  #WriterWednesday  at  #Oceanbites  we are featuring Gabby Stedman. Gabby is interested in interested in understanding how many species of small-bodied animals there are in the deep-sea and where they live so we can better protect them from
  • by oceanbites 7 months ago
    This week for  #WriterWednesday  at  #Oceanbites  we are featuring Shawn Wang! Shawn is “an oceanographer that studies ocean conditions of the past. I use everything from microfossils to complex computer models to understand how climate has changed in the past
  • by oceanbites 7 months ago
    Today we are highlighting some of our awesome new authors for  #WriterWednesday  Today we have Daniel Speer! He says, “I am driven to investigate the interface of biology, chemistry, and physics, asking questions about how organisms or biological systems respond
  • by oceanbites 8 months ago
    Here at Oceanbites we love long-term datasets. So much happens in the ocean that sometimes it can be hard to tell if a trend is a part of a natural cycle or actually an anomaly, but as we gather more
  • by oceanbites 8 months ago
    Have you ever seen a lobster molt? Because lobsters have exoskeletons, every time they grow they have to climb out of their old shell, leaving them soft and vulnerable for a few days until their new shell hardens. Young, small
  • by oceanbites 9 months ago
    A lot of zooplankton are translucent, making it much easier to hide from predators. This juvenile mantis shrimp was almost impossible to spot floating in the water, but under a dissecting scope it’s features really come into view. See the
  • by oceanbites 9 months ago
    This is a clump of Dead Man’s Fingers, scientific name Codium fragile. It’s native to the Pacific Ocean and is invasive where I found it on the east coast of the US. It’s a bit velvety, and the coolest thing
  • by oceanbites 10 months ago
    You’ve probably heard of jellyfish, but have you heard of salps? These gelatinous sea creatures band together to form long chains, but they can also fall apart and will wash up onshore like tiny gemstones that squish. Have you seen
  • by oceanbites 11 months ago
    Check out what’s happening on a cool summer research cruise! On the  #neslter  summer transect cruise, we deployed a tow sled called the In Situ Icthyoplankton Imaging System. This can take pictures of gelatinous zooplankton (like jellyfish) that would be
  • by oceanbites 11 months ago
    Did you know horseshoe crabs have more than just two eyes? In these juveniles you can see another set in the middle of the shell. Check out our website to learn about some awesome horseshoe crab research.  #oceanbites   #plankton   #horseshoecrabs 
WP2Social Auto Publish Powered By : XYZScripts.com