Sinking balls of krill food could be good news for the planet
Paul Arnold
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Gaby Clark
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Robert Egan
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Antarctic krill, tiny shrimp-like creatures, are an important species in the Southern Ocean ecosystem and global carbon cycle, in part because of their poop. Their dense and rich fecal pellets sink rapidly, transporting carbon from surface waters to the deep ocean. In a new study in the journal Biology Letters, scientists have discovered another way these crustaceans keep carbon locked away from the atmosphere—by ejecting carbon-rich masses of rejected food, called food boluses, that also plummet to the ocean depths.
When krill eat, they use a mesh-like structure called a feeding basket to filter microscopic phytoplankton from the water. Sometimes, a food bolus, a compact ball of partially processed food, is formed within this basket and subsequently rejected. Ecologist Anita Butterly of the University of Tasmania in Australia, and colleagues studied this behavior in captive krill in a laboratory setting to learn more about the conditions that cause it.
They found that krill start making and rejecting food boluses when the amount of phytoplankton in the water is so high that it exceeds their ability to filter and swallow it. The higher the concentration of phytoplankton, the more boluses were rejected, up to 17 boluses an hour.
One of the key findings was that the boluses sink faster than their fecal pellets, at an average speed of 367 meters per day. The fastest bolus sank at 1,403 meters per day. Fecal pellets typically sink at a speed of 269 meters per day. This strongly suggests that boluses are another route for sequestering carbon, which comes from the phytoplankton within them. These single-celled organisms are primarily composed of organic carbon.
"Our findings suggest this behavior may also occur in situ and could contribute to organic carbon export, with bolus sinking rates comparable to or exceeding those of Antarctic krill fecal pellets," wrote Butterly in the paper's abstract.
Ecological threat
The researchers also made an incidental discovery with potentially serious real-world implications. When krill in their experiment were accidentally exposed to microplastics, bolus production increased dramatically, most likely because the foreign material was acting as a trap within the feeding basket.
If this were to happen in the wild, it could reduce the amount of food they are able to ingest, which in turn could affect their survival and the Antarctic food web (krill is a key food source). This could also reduce the amount of carbon krill are able to send to the deep ocean.
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More information: Anita Butterley et al, The production of 'food boluses' by Antarctic krill and implications for organic matter transport, Biology Letters (2025).
Journal information: Biology Letters
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