Krill: Life Under Ice
Krill are some of the most important prey species on the planet, but what are they and why are they so important?
What are Krill?
Krill are shrimp-like invertebrates that live in the ocean. Despite only growing around 2.4 inches long, these crustaceans contribute massively to ocean food webs. They are the main component in the diets of species across a whole range of taxa, including cephalopods, birds, fish and of course mammals, such as whales.
Krill feed on phytoplankton and sea ice-algae. During summer, plankton are combed out of the water column with their forelegs which are then consumed. During Winter, when plankton stocks rapidly decline, Antarctic krill feed on sea-ice algae. Krill use their specialized seatae to rake off algae from the underside of pack ice to then eat. In fact, one krill can graze a square foot clear of algae in just 10 minutes!
A krill’s purpose
Krill are one of the most fundamental prey species in the food web, see figure 3, particularly in the extremes of Antarctica. Antarctic krill provide an important link between the primary producers and higher trophic levels within the ocean. Krill are larger than most phytoplankton-feeders, such as copepods which are around 0.04-0.08 inches. Their size, along with their numerous abundance, makes them an ideal prey source, particularly to larger predators. Annually, half of all krill is eaten by whales, sea birds, seals, penguins, squid and fish. As such, krill have very efficient feeding abilities and reproduction strategies, such as multiple spawnings each of which can have as many as 10,000 eggs, allows them to stave off extinction by predation alone.
Antarctic krill survival into adulthood is correlated with sea ice cover. Survival increases when ice develops earlier in the year and remains for longer periods, allowing krill more time under the ice in a refuge from predators with a constant food source.
Being such a sought after prey species, krill have adapted defensive strategies against predation. Most obviously, krill aggregate in ‘swarms’; dense congregations which make it more difficult for predators to hunt individuals and decrease attacks from smaller predators which would only attack singular krill. However, living in swarms has disadvantages too, allowing large predators, like whales, to hunt the entire swarm for food.
Krill also diurnally migrate, meaning they stay in cold deep water (~320 feet) during the day, and rise to the surface for feeding during the night. The nocturnal movement of krill allows them to avoid the heavier predation risks of feeding in the day, especially from more sight-dependent predators, such as birds.
Physiologically, they have developed a hard exoskeleton, which reduces the risk of injury from smaller predators. This can also be used in ‘decoy moulting’; where krill shed their exoskeleton when being chased, in order to confuse the predator long enough for the krill to escape.
Additionally, krill’s bioluminescence, which remains unknown in its exact purpose, may be a self-defense tactic. Krill emit yellow-green light from photophores periodically, which is believed to counteract the light of their shadow and allow them to remain unseen, thus serving as an anti-predator device.
Declining ice mass, due to global warming, resulting in less Antarctic pack ice is the chief driver of the changes in krill abundance. Antarctic krill, particularly juveniles, rely on pack ice for both food source and refuge from predators. Losses of this needed ice results in krill decline. Additionally, during years of low krill stocks, salps (free-floating filter-feeders) proliferate. Salps have a very limited contribution to the wider food web, unlike krill, therefore their replacement when krill is low can be disastrous for hundreds of species, far beyond krill themselves.
Ocean acidification, due to rising carbon dioxide in the water, may also be having physiological affects on krill. with increasingly lower pH, their carbonate exoskeletons become more vulnerable to dissolution. This is a particular problem at the poles, where under-saturation of carbonate will be seen sooner, due to the colder waters there.
However, all is not lost for the food-web staple that is krill. Because of their known importance in the marine environment and as a human resource, many organisations are ensuring their sustainable management and conservation, including The Covention on the Conservation of Antarctic Marine Living Resources and Antarctic Krill Conservation Project.
Overall, Antarctic krill have a huge impact on the surrounding ocean for such a small invertebrate. Ultimately, helping sustain healthy krill populations may be just as important, if not more so, as protecting top predators, in regards to the health of our oceans.