Giant sea anemones eat ants… and also the occasional spider

Sea anemones sit on rocks, gently waving their tentacles back and forth, catching floating prey like larvae, plankton and… ants?

A team of US-based researchers used a method called ‘DNA metabarcoding’ to identify the gut contents of 12 giant plumose anemones (Metridium farcimen). They found that the anemones ate a lot of crab and barnacle larvae, small crustaceans called copepods and, surprisingly, insects.

The scientists first extracted genetic material from partially digested food in the anemones’ guts, then compared this information to DNA databases of various organisms. This method allowed them to accurately analyse what species the anemones had been eating, and they were also able to identify rare or degraded prey items.

“We’ve greatly expanded the list of things we know that they eat. They’re eating whatever they can catch, whatever isn’t too big or too small, whatever can’t swim away,” said first author Dr Christopher Wells, a postdoctoral researcher in the University at Buffalo. “One of the most surprising results is that in addition to all the usual suspects you’d find in marine plankton, we also found that a part of the diet – about 10 per cent at the time of the study – consisted of ants, which are not marine.”

As the name suggests, giant plumose anemones are huge, measuring up to a metre in height. The body consists of a tall column that’s topped with fluffy, white, cauliflower-like tentacles. The anemones use these tentacles to collect food, which they shovel into their mouth.

The anemones live in the eastern Pacific Ocean from Alaska down to California, where they sit on rocks or other surfaces. It is important for researchers to decipher exactly what animals eat, in order to fully understand marine communities and ecosystems.

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“When a plankton community floats overtop a bed of anemones, the plankton is filtered by millions of grasping tentacles,” Wells said. “This can drastically change the composition of the plankton community, which is food for many economically important animals such as bivalves and fish.”

The researchers identified the ants as being a pale-legged field ant (Lasius pallitarsis). As the ants were unexpected items on the menu, the researchers did a little digging to come up with a possible explanation for how these insects ended up in the sea.

They carried out the research in August, when the ants have mating flights. “They produce winged queens and drones, which mate and make new colonies. They’re not strong fliers and the wind pushes them around, potentially into the water,” said Wells.

Anemones found near to each other seemed to have different diets, but that’s not because one prefers snacking on ants, while another finds plankton to be tastier. “They eat what they can, and it’s very patchy what they get, depending on what’s there,” said Wells.

As well as ants, the researchers were surprised to find the occasional bit of spider DNA in the anemones’ guts. However, they also noted that some of the DNA sequences could not be matched with any known species, reiterating just how much is still waiting to be discovered in the oceans.