Stretching more than two vertical miles from the seafloor to the ocean’s surface, the water column is Earth’s biggest habitat by volume. For researchers trying to untangle its complex, multi-tentacled food web—the way energy flows from one ocean denizen to the next—it’s a vast and challenging realm in which to accomplish this task.
Recent work by scientists at the Monterey Bay Aquarium Research Institute (MBARI) has revealed whole new layers of predator-prey interactions in the water column, particularly in the often overlooked roles played by jellies and other soft-bodied animals—many of which, researchers discovered, feed on their own kind.
This research is promising, says Anela Choy, the biological oceanographer who led the study, but much more remains to be discovered about deep-sea food webs.
“I wish I knew just how much there was that we didn’t know,” she says. “That’s what keeps us all going.”
New appreciation for jellies
Many feeding interactions in the deep sea are difficult to observe because they take place in total darkness, thousands of feet below the surface, in cold, crushing conditions that test even the capacities of MBARI’s advanced robots. Before the advent of robotic exploration technology, much of what scientists gleaned about food webs was gathered from animals hauled to the surface in nets—or discovered in a predator’s guts.
One problem with that approach, Anela says, is that squishy animals like jellyfish and other gelata, while among the most prevalent life forms in this ecosystem, almost never make it to the surface intact.
“They’re really hard to capture—that’s the traditional way of studying diet, is to capture those animals and look in their stomachs,” she says. “With a net, they often immediately break apart. “If they are the predator of interest, we cannot ascertain their gut contents this way because they are very damaged.”
Obstacles to overcome
There are other obstacles to understanding food webs. The traditional way of studying diet is to capture an animal and look into its stomach to see what prey have been eaten. Anela notes that gelata digest very quickly and thus are often missed with diet work.
The high-definition cameras on MBARI’s diving robots have recorded thousands of deep-sea animal observations since 1989. All of the video has been rigorously archived to reflect its subject, location, time, depth and even water temperature and other physical parameters. From this footage, Anela and her colleagues gleaned a wealth of information: 743 documented instances of undersea creatures eating, being eaten, or having just fed.
(Anela singled out two video technicians at MBARI, Susan von Thun and Kyra Schlining, who “watched every single hour of videotape from every midwater dive” to build an unprecedented underwater feeding dataset.)
Hundreds of feeding observations
From the video, the team tallied 242 unique kinds of predator-prey relationships. Many involved jellyfish and other soft-bodied animals, which don’t seem to particularly mind having a robot watch them eat, and which are often transparent, meaning the researchers could easily peer inside their bodies to view their most recent meal.
In their published study, they documented the complexity of predator-prey relationships they uncovered from this treasure trove of data.
A key illustration from the study draws lines showing predator-prey interactions between 20 different functional groups seen feeding on each other in the footage, from fish to crustaceans to jellies to cephalopods like squid. Fittingly, the resulting tangle of colorful who-eats-whom lines resembles a jellyfish.
“Jellyfish get kind of a bad rap,” Anela says, noting that some biologists cast them as nuisances—trophic dead ends that don’t feed back into the food web.
“This shows something totally different,” she says.” It shows they’re central parts of deep sea ecosystems, with really diverse diets and serving as both predators and prey.”
One species of jellyfish was observed eating 22 different kinds of prey.
(In the figure, many of predator-prey nodes loop back on themselves. “That,” says Anela, “is cannibalism—species within those broad animal groups feeding on one another.”)
There’s more to come
“Our method gives you a totally different view of the interactions going on in the food web,” Steve Haddock says.
It’s a bit like going from a map with only train tracks to one that includes highways, he says: “You feel like things are connected in only a certain way, but suddenly you see these other connections. This study really complements and expands our view of what’s going on in the ocean.”
Still, Steve says there’s much left to learn.
“Even though this method has revealed a large diversity of interactions, there’s still a whole other universe of interactions we haven’t discovered,” he says.
The next layer of discovery may not come from video observations. Steve sees great promise in techniques like analyzing predators’ gut DNA for hints about their recent meals. Another avenue that is already widely utilized is compound-specific stable isotope analysis, which looks for chemical signatures that might accumulate in a creature’s tissue from eating certain prey.
(That’s the approach used in a recent study by Aquarium researchers to document changes in North Pacific seabird diets over the past 130 years.)
“There will continue to be a lot more revelations about food web connections,” Steve says.
Anela agrees: “You hear that the deep sea is like outer space—it’s so poorly known and so poorly explored, every time we go down there we learn new things. All of that is true. But really, understanding that food webs tie everything in the ocean together is the reason I study them.”
Our ever-growing understanding of those connections, she says, will be critical to stewarding the ocean in the future.
Choy, C.A., Haddock, S.H.D., Robison, B.H. (2017). Deep pelagic food web structure as revealed by in situ feeding observations. Proceedings of the Royal Society B. 284: 20172116, doi: