Sea anemones sometimes eat… ants. But why? And how?

New research shows how little we know about the diets of some underwater scavengers—and the intricate connections between land and marine food webs.

Published 5 Jul 2021, 09:57 BST
giant plumose anemone 2
When researcher Christopher Wells discovered that these giant plumose anemones eat ants, he was surprised. “I was not expecting that at all.”
Photograph by Gina Kelly / Alamy Stock Photo

Take a dip along the coast of the Pacific Northwest, and you’re likely to see underwater forests of ghostly white, carnivorous pom-poms. Scientists call these creatures giant plumose anemones, and at upwards of three feet, they’re the tallest sea anemones on earth.

But though giant plumose anemones (Metridium farcimen) are large, easy to observe, and downright dominant in the ecosystems they inhabit, much remains unknown about them—such as what, exactly, they eat.

Part of the difficulty in studying their diet is that while most sea anemones have long, thick tentacles which they use to capture and subdue prey, these predators sport a dense bouquet of tiny, thin feelers—an “indication that they’re eating really small prey,” says Christopher Wells, a marine ecologist at the University of Buffalo in New York. And these small animals turn into even smaller bits of stomach mush upon ingestion.  

Instead of trying to sort through this material visually under a microscope, Wells took the gut contents of 16 individuals from Friday Harbour in Washington State and utilised a technology known as DNA barcoding. This tool isolates snippets of DNA within a sample and then searches existing databases for matches of known species.

After all the analyses were done, Wells stared at what he saw. There were hits for all the usual suspects of the surf—copepods, barnacles, crab larvae, and the like—tiny creatures that wouldn’t be able to escape the giant plumose anemones’ feathery, venomous clutches. There was also a curious amount of insect DNA, including three flies, one bee, and one beetle. But the most unusual matches came from a species known as the yellow meadow ant, or Lasius pallitarsis, which made up 98 percent of the insect DNA found in the sea anemones’ guts.

“It was a total surprise,” says Wells, the lead author of a study announcing the first usage of DNA metabarcoding on sea anemone gut contents in the journal Environmental DNA. “I was not expecting that at all.”

Ant enigma

Plumose anemones are found from Alaska to California, and survive by filtering small creatures out of the water column. Their diet mostly consists of animals as small as a few cells to as big as, well, an ant. Unlike their cousins, which use long tentacles to place pieces of food into their mouths, these sea anemones catch small prey items and then funnel them to their stomach using a series of interlocking grooves.

Most of the organisms whose DNA turned up in the study do indeed spend the earliest parts of their life cycle as free-floating eggs or microscopic larvae skittering through the water, so it makes sense that the sea anemones would gorge upon them. As for the ants, though, scientists don’t know exactly how they wound up in the maw of an underwater predator. But they do have a pretty good theory.

When ants of this species are ready to reproduce, they take to the skies in huge numbers to find mates. Afterwards, the females land and attempt to start colonies of their own. Their stories are just beginning. But for the males, there’s nothing left to do but die.

While Wells doesn’t remember a particularly high number of flying ants when he was conducting his sea anemone surveys, he says there are always a lot of insects flying around Friday Harbour. Other scientists have documented that yellow meadow ants perform mating or nuptial flights in August, which is when the sea anemone samples were taken. Presumably, all the insects would have to do is fall into the water and drift down until they hit a grove of carnivorous dandelions. It’s probably also no coincidence that the other insects found in the DNA analysis are also fliers.

“Many animals take advantage of the boom of sexual ants flying around in very large numbers during their mating flights,” says Corrie Moreau, an ant expert and director and curator of the Cornell University Insect Collection. “I guess it isn’t unreasonable for some number of these reproductive ants to be blown over nearby ocean waters and become prey for marine organisms.”

Giant plumose anemone (Metridium giganteum or Metridium farcimen) is a large sea anemone. How DNA from ants arrives in these creatures is being investigated. It remains to be discovered if the anemones are consuming the ants directly, or if the ants are in the diet of something else the anemone eats.

Photograph by agefotostock / Alamy Stock Photo

As for the aquatic side of things, sea anemone expert Michela Mitchell agreed that it was possible, if unusual, for marine predators to make a meal out of terrestrial insects.

In fact, thick-tentacled sea anemones—which are more widespread, tend to forage more widely, and capable of eating larger prey—have been found feeding on all sorts of things, from sandwich crusts to whole rabbits.

“There is still so little work completed on sea anemone feeding ecology,” says Mitchell, who is an honorary researcher at the Museum of Tropical Queensland in Australia.

“Groundtruthing” required

Even if it seems likely the sea anemones gobbled up some ants, until someone sees the behaviour in action—what scientists call “groundtruthing”—Mitchell cautions that other explanations have to be considered. For instance, it’s possible the sea anemones ingested ant predators, instead of the ants themselves.

“You could be looking in a hall of mirrors where you are reflecting back the gut contents of something else and heading down the chain of the food web at the same time as trying to establish what the food is,” she says.

Study coauthor Gustav Paulay says this type of misdirection is possible, given the nature of DNA metabarcoding. But in this case, he finds it unlikely.

“Most of the [sea anemone’s diet] is ant-sized, so a predator large enough to eat enough ants to show up at this level in the sequences should not be on the menu,” says Paulay, curator of Invertebrate Zoology at the Florida Museum of Natural History. “Most of the other foods are tiny, planktonic animals that could not handle an ant.”

The researchers also found one arachnid within the sea anemones’ stomach contents: a type of oribatid mite, tiny creatures that are mostly terrestrial but can also live in the ocean.

The question reveals one of the drawbacks of the DNA metabarcoding method—you know a species is present, but you can’t necessarily say how it got there. Still, the study stands as another exciting example of how this technology can be used to reveal unseen interactions between creatures. And as scientists continue to add more genomes of new species to the databases, the techique will only become more robust, useful, and awe-inducing.

All you need is a cell somewhere in that sample that contains DNA, says Wells. With that, “you can identify the freckle of a copepod.”

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