Scientists have shown for the first time that Antarctic krill show a stereotypical reaction in the presence of guano from Adélie penguins: they swim faster and make more turns over greater angles. It is unknown to what kind of water-borne chemical cues they respond, but the authors speculate that this behavior might be a universal escape response to the excreta of predators, irrespective of species.

Imagine looking at the world through the stalked compound eyes of krill in the Southern Ocean. All of a sudden, a penguin appears like a voracious giant, streamlined like a torpedo, chasing and consuming thousands of krill at rapid speed.

Now, researchers have shown that the water-borne smell of the poo of these flightless birds is enough to cause the krill to show escape behaviors.

“Here we show for the first time that a small amount of penguin guano causes a sudden change in the feeding and swimming behaviors of Antarctic krill,” said Dr Nicole Hellessey, a postdoctoral researcher at the Bigelow Laboratory for Ocean Sciences in Maine, and the corresponding author of a new study in Frontiers in Marine Science.

As food for a plethora of species and massive sequesterers of carbon, krill are a keystone species in the Antarctic. It is estimated that there are approximately 700 trillion adult Antarctic krill in the Southern Ocean today, but their population is moving further south due to climate change, the loss of sea ice and ocean acidification.

There is something in the water

Zooplankton like krill are sensitive to chemical cues about food, mates, and pollution and adapt their behavior accordingly, and Hellessey et al. wanted to know if krill also do this in response to odors from predators. The authors focused on Adélies, the most southernly breeding species of penguin, as 99.6% of their diet consists of Antarctic krill. An adult Adélie eats up to 1.6 kg of it per day, and the yearly consumption of Antarctic krill by the world’s population of Adélies is approximately 1.5 million tons.

In late 2022, researchers on board the research vessels ‘Laurence M Gould’ and ‘Nathaniel B Palmer’ in Antarctica’s Bransfield Strait trawled for Antarctic krill to take to Palmer Station’s research aquarium. The catch was kept alive in holding tanks on a diet of algal slurry. Bird experts had previously collected 78g of Adélie guano (“Smells like rotten shellfish. Not pleasant to handle,” said Hellessey) from a colony on Torgersen Island off the Antarctic Peninsula near Palmer Station.

To test the krill’s response to guano, the scientists placed six to eight krill per five-minute trial in a flume filled with seawater with a temperature of 1.5°C. The light was dimmed to mimic the intensity typical of a depth of 40 meters, the most productive layer within the Southern Ocean. Water flowed through the flume at a velocity of either 3cm or 5.9cm per second. This flow was fed with one of three kinds of seawater: containing either algae, Adélie poo, or both. In total, they tested each combination of flow velocity and water composition four times.

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They recorded the behavior of four individual krill per trial with two automated cameras, and processed these videos to calculate each krill’s 3D position and its speed and direction of swimming throughout the trial.

The results showed that krill typically swam straight upstream, a behavior called rheotaxis. But once in the presence of poo, they changed their behavior dramatically: they varied their swimming speed more, while swimming between 1.2 and 1.5 times faster. They also made three times more turns, over an angle that was on average 1.4 times greater.

In a second set of experiments, the researchers showed that krill reduced their rate of ingesting algae for food by 64% when penguin poo was added to the water, dropping from 12.7 microgram of carbon per hour per krill (or 13% of their body mass) to 4.6 microgram per hour per krill. These results indicate that krill foraged less efficiently in the presence of guano, due to their frequent change of direction.

The authors concluded that this ‘zigzagging’ is an avoidance or escape reaction.

“Such behavior to escape from nearby penguins would greatly increase the krill’s odds of survival. And these odds would increase exponentially in a swarm, if their neighbors could detect the same cues and communicate the danger to each other,” said Hellessey.

Smells like krill kin spirit

But which chemicals within the poo do the krill respond to? This remains to be elucidated.

“We hypothesize that Antarctic krill are avoiding the odor of ground-up krill and fish in the penguins’ guano. We thus expect krill to show similar swimming behaviors and suppressed feeding around seals, whales, and other types of krill predators in Antarctica,” said Hellessey.

“We don't yet know how the ability of krill to sense these chemical cues and their escape behavior towards them might vary when diluted in open waters, or under global warming or ocean acidification conditions. Any changes to krill’s behavior could have major impacts on the future Southern Ocean, as Antarctic krill are a keystone species in this ecosystem.”

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