Unexpected Predator: Jellyfish Shown to Hunt Polychaete worms

Most polychaete species spend their lives in burrows in the seabed. However, adult individuals of two species, Alitta succinea and Platynereis dumerilii, leave their burrows to spawn during warm summer nights around full moon.

The worms swarm with thousands of individuals swimming simultaneously in the open water. During this journey their bodies rupture while releasing eggs and sperm before they die. They are a luxury diet for many carnivores and are snapped up by trout and other predatory fish. And now SDU biologists have shown that certain jellyfish also capture and eat the swarming polychaetes.

“Although jellyfish are known to be omnivorous and generally consume whatever they encounter, it is the first time, this behaviour is documented,” says one of the researchers behind the discovery, biologist and postdoc Hannah Yeo from SDU’s Marine Biology Research Centre in Kerteminde, Denmark.

The team behind the discovery also includes biologists Laura Ferreira (SDU and University of Copenhagen), Erik Kristensen (SDU), Anders Garm (UCPH) and Jamileh Javidpour (SDU). Their findings have been published in the scientific journal Hydrobiologia.

Over the course of a year, the researchers recorded 56 cases where a jellyfish contained  at least one polychaetes in their gut. The worms were found in two jellyfish species: the common moon jelly Aurelia aurita and the invasive comb jelly Mnemiopsis leidyi. The most common polychaete species recorded was Platynereis dumerilii.

All observations were made in Kerteminde Fjord and the nearby Kertinge Nor in Denmark. Among the 166 collected Aurelia aurita, 45 individuals contained at least one polychaete. Among the 71 Mnemiopsis leidyi, three contained at least one Polychaete.

Digested in a few hours

The actual number is likely higher, the researchers say.

“It takes only a couple of hours for the jellyfish to digest these worms, so there were probably more cases than the ones we observed,” says co-author, Professor Erik Kristensen from Department of Biology, University of Southern Denmark.

To confirm that the worms served as actual nutrition – and weren’t ingested by accident – the researchers performed isotope analyses on the jellyfish tissue. These showed that nutrients from the worms were absorbed by the jellyfish rather than simply passing through.

Night time feeding

In the bigger picture, polychaetes are probably not a major food source for jellyfish, but they do represent a concentrated seasonal burst of energy in summer that may help jellyfish thrive.

For the invasive Mnemiopsis leidyi, this is more concerning, the researchers note – precisely because its invasive nature makes any newly documented feeding behaviour a potential ecological concern.

“We suspect that Mnemiopsis consumed more worms than we could detect. They also hunt at night, which is exactly when the worms swarm. By the time we sampled them in daylight, the worms would have been fully digested and impossible for us to see,” says co-author Jamileh Javidpour, associate professor at Department of Biology, SDU, “Mnemiopsis are invasive and unwanted in Danish waters because they can outcompete native species. They are highly opportunistic feeders, and this appears to be yet another food source they are able to exploit.”

Previously overlooked exchange of energy

The discovery not only sheds new light on jellyfish feeding behaviour but also reveals a previously overlooked exchange of energy between the seafloor and the water column. Typically, this flow is seen as one‑way: organic material sinks down to bottom‑dwelling animals – such as dead polychaetes after spawning.

“But here we see energy moving upward instead: bottom‑dwelling animals rising into the water column and being hunted there. While several predator species are known to exploit these spawning events, it has not been documented before in jellyfish or Mnemiopsis. This means gelatinous zooplankton are tapping into a benthic energy source that has been largely overlooked”, said Jamileh Javidpour, concluding:

“We need to consider this upward transfer when modelling the resilience of coastal ecosystems. Even sporadic access to benthic resources may influence how both native and invasive gelatinous species compete and persist.”