Antarctic krill

setae (bristles) on the inner side of the thoracopods.

Krill are thought to undergo between one and three vertical migrations from mixed surface waters to depths of 100 m daily.

If the phytoplankton is consumed by other components of the pelagic ecosystem, most of the carbon remains in the upper layers of the ocean. There is speculation that this process is one of the largest biofeedback mechanisms of the planet, maybe the most sizable of all, driven by a gigantic biomass. Still more research is needed to quantify the Southern Ocean ecosystem.

Biology

Bioluminescence

Krill are often referred to as light-shrimp because they emit light through

The krill's bioluminescent organs contain several fluorescent substances. The major component has a maximum fluorescence at an excitation of 355 nm and emission of 510 nm.^[16]

Escape reaction

Krill use an

The genome of E. superba spans about 48 GB and is thus one of the largest in the animal kingdom and the largest that has been assembled to date. Its content of repetitive DNA is about 70% and may reach up to 92.45% after additional repeat annotation, which is also the largest fraction known of any genome. There is no evidence of polyploidy. Shao et al. annotated 28,834 protein-coding genes in the Antarctic krill genome, which is similar to other animal genomes. The gene and intron lengths of Antarctic krill are notably shorter than those of lungfishes and Mexican axolotl, two other animals with giant genomes.^[18]

Geographic distribution

Antarctic krill has a circumpolar distribution, being found throughout the Southern Ocean, and as far north as the Antarctic Convergence.^[19] At the Antarctic Convergence, the cold Antarctic surface water submerges below the warmer subantarctic waters. This front runs roughly at 55° south; from there to the continent, the Southern Ocean covers 32 million square kilometres. This is 65 times the size of the North Sea. In the winter season, more than three-quarters of this area become covered by ice, whereas 24,000,000 square kilometres (9,300,000 sq mi) become ice free in summer. The water temperature fluctuates at −1.3–3 °C (29.7–37.4 °F).

The waters of the Southern Ocean form a system of currents. Whenever there is a West Wind Drift, the surface strata travels around Antarctica in an easterly direction. Near the continent, the East Wind Drift runs counterclockwise. At the front between both, large eddies develop, for example, in the Weddell Sea. The krill swarms swim with these water masses, to establish one single stock all around Antarctica, with gene exchange over the whole area. Currently, there is little knowledge of the precise migration patterns since individual krill cannot yet be tagged to track their movements. The largest shoals are visible from space and can be tracked by satellite.^[20] One swarm covered an area of 450 square kilometers (170 square miles) of ocean, to a depth of 200 meters (660 feet) and was estimated to contain over 2 million tons of krill.^[21] Recent research suggests that krill do not simply drift passively in these currents but actually modify them.^[21] By moving vertically through the ocean on a 12-hour cycle, the swarms play a major part in mixing deeper, nutrient-rich water with nutrient-poor water at the surface.^[21]

Ecology

Antarctic krill is the

The biomass of Antarctic krill was estimated in 2009 to be 0.05 gigatons of carbon (Gt C), similar to the total biomass of humans (0.06 Gt C).^[25] The reason Antarctic krill are able to build up such a high biomass and production is that the waters around the icy Antarctic continent harbour one of the largest plankton assemblages in the world, possibly the largest. The ocean is filled with phytoplankton; as the water rises from the depths to the light-flooded surface, it brings nutrients from all of the world's oceans back into the photic zone where they are once again available to living organisms.

Thus primary production—the conversion of sunlight into organic biomass, the foundation of the food chain—has an annual carbon fixation of 1–2 g/m² in the open ocean. Close to the ice it can reach 30–50 g/m². These values are not outstandingly high, compared to very productive areas like the North Sea or upwelling regions, but the area over which it takes place is enormous, even compared to other large primary producers such as rainforests. In addition, during the Austral summer there are many hours of daylight to fuel the process. All of these factors make the plankton and the krill a critical part of the planet's ecocycle.

Decline with shrinking pack ice

A possible decline in Antarctic krill biomass may have been caused by the reduction of the

Another challenge for Antarctic krill, as well as many calcifying organisms (corals, bivalve mussels, snails etc.), is the acidification of the oceans caused by increasing levels of carbon dioxide.^[29] Krill exoskeleton contains carbonate, which is susceptible to dissolution under low pH conditions. It has already been shown that increased carbon dioxide can disrupt the development of krill eggs and even prevent the juvenile krill from hatching, leading to future geographically widespread decreases in krill hatching success.^[30][31] The further effects of ocean acidification on the krill life cycle however remains unclear but scientists fear that it could significantly impact on its distribution, abundance and survival.^[32][33]

Fisheries

The fishery of Antarctic krill is on the order of 100,000 tonnes per year. The major catching nations are South Korea, Norway, Japan and Poland.^[35] The products are used as animal food and fish bait. Krill fisheries are difficult to operate in two important respects. First, a krill net needs to have very fine meshes, producing a very high drag, which generates a bow wave that deflects the krill to the sides. Second, fine meshes tend to clog very fast.

Yet another problem is bringing the krill catch on board. When the full net is hauled out of the water, the organisms compress each other, resulting in great loss of the krill's liquids. Experiments have been carried out to pump krill, while still in water, through a large tube on board. Special krill nets also are currently under development. The processing of the krill must be very rapid since the catch deteriorates within several hours. Its high protein and vitamin content makes krill quite suitable for both direct human consumption and the animal-feed industry.^[36]

Fishing and potentially overfishing krill is an issue of increasing concern. ^{[37] [38]}

References

Further reading

• Clarke, A.; D. J. Morris (1983). "Towards an energy budget for krill: the physiology and biochemistry of Euphausia superba Dana".
  S2CID 23804588
  .
• Hempel, G. (1985). "Antarctic marine food webs". In W. R. Siegfried; P. R. Condy; R. M. Laws (eds.). Antarctic nutrient cycles and food webs. Berlin:
  ISBN 978-0-387-13417-8
  .
• Hempel, G. (1987). Martin V. Angel; W.
  doi:10.1016/0160-4120(87)90041-9. Scientific Requirements for Antarctic Conservation. Archived from the original
  on 2020-07-26. Retrieved 2019-12-12.
• Hempel, Gotthilf (1991). "Life in the Antarctic sea ice zone".
  S2CID 128710441
  .
• Hempel, Gotthilf; Kenneth Sherman (2003). Large Marine Ecosystems of the World: Trends in Exploitation, Protection, and Research. Vol. 12. Amsterdam:
  ISBN 978-0-444-51027-3
  .
• Hill, Simeon (2013). "Prospects for a Sustainable Increase in the Availability of Long Chain Omega 3s: Lessons from the Antarctic Krill Fishery" (PDF). Omega-6/3 Fatty Acids. New York: Springer Business + Media. pp. 267–299.
  ISBN 978-1-62703-215-5. Archived
  (PDF) from the original on 2018-07-20.
• Ikeda, T. & P. Dixon (1984). "The influence of feeding on the metabolic activity of Antarctic krill (Euphausia superba Dana)".
  S2CID 41999027
  .
• Ishii, H.; M. Omori; M. Maeda & Y. Watanabe (1987). "Metabolic rates and elemental composition of the Antarctic krill, Euphausia superba Dana".
  S2CID 24088774
  .
• Kils, U. (2006). "So frisst der Krill" [
  ISBN 978-3-89757-310-9
  .
• Mauchline, J. & L. R. Fisher (1969). The Biology of Euphausiids. Advances in Marine Biology. Vol. 7. Academic Press.
• Nicol, S.; W. K. de la Mare (1993). "Ecosystem management and the Antarctic krill".
  Bibcode:1993AmSci..81...36N
  .
• Nicol, Stephen & Jacqueline Foster (2003). "Recent trends in the fishery for Antarctic krill" (PDF).
  PDF
  ) on 2006-01-18. Retrieved 2005-06-17.
• Quetin, L. B.; R. M. Ross & A. Clarke (1994). "Krill energetics: seasonal and environmental aspects of the physiology of Euphausia superba". In Sayed Zakaria El-Sayed (ed.). Southern Ocean Ecology: the BIOMASS Perspective. Cambridge:
  ISBN 978-0-521-44332-6
  .
• Sahrhage, Dietrich (1989). "Antarctic krill fisheries: potential resources and ecological concerns". In John F. Caddy (ed.). Marine Invertebrate Fisheries: their Assessment and Management.
  ISBN 978-0-471-83237-9
  .

External links

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• Media related to Euphausia superba at Wikimedia Commons
• Data related to Euphausia superba at Wikispecies
• Krill Count Project
• Diary of the RRS James Clark Ross, giving a popular introduction to the Antarctic krill
• Euphausia superba from MarineBio
• A time to krill, Australian Antarctic Division
• Webcam of krill aquarium, Australian Antarctic Division
• Antarctic Krill fact sheet, Australian Antarctic Division
• "Krill fights for survival as sea ice melts". Earth Observatory. NASA. 10 January 2000.
• Mark Kinver (April 12, 2011). "Penguins suffer as Antarctic krill declines". BBC News.