They look a little bit like shrimps, though they are quite different in appearance when you take a closer look at them. Krill are about 6 centimeters long, and they carry their gills on the external skeleton, while the gills of shrimps are located within their body. Krill are a huge deal. They hold a key position in the Southern Ocean: countless animals survive on krill in very harsh polar conditions that most of us can't even imagine.
Dr. Robert King, krill biologist at the Australian Antarctic Division in Hobart Tasmania, explains that krill consists of 85 different species of the order Euphausiacea. Antarctic krill, Euphausia superba, is most famous and widely studied within Antarctic Research Programmes. This is no surprise when you consider their importance to Antarctic food chains: the majority of animals feed on krill, and the abundance of krill availability is therefore key to their survival (OU Frozen Planet S175-13D Activities, Activity 7.2, Clip 3).
Millions of penguins, seals, sea birds, fish, squid and thousands of whales depend on krill to assemble by the billions in summer. "They (krill) are the most numerous animal on earth. Their total weight far exceeds that of the total human population", according to Sir David Attenborough, as he describes in the BBC series Life in the Freezer (OU Frozen Planet S175-13D Activities, Activity 7.2, Clip 2).
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| Antarctic krill (Euphausia superba). Photograph by Uwe Kils Source: Wikimedia Commons |
Animals foraging on Antarctic krill continuously search for patches of these magnificent crustaceans. Sometimes they get lucky, the discovery of a superswarm all across the ice-cold ocean surface in one particular good spot, leading to a feeding frenzy. They, the animals foraging on krill, are in turn hunted by huge predators like leopard seals and killer whales.
You might wonder how krill survives in these harsh and cold conditions, especially considering the impact of massive exploitation by numerous animals?
The seasons
During the winter months Antarctic krill demonstrate different types of survival strategies, adapted to their niche: the extremely cold surroundings of the Antarctic. The food is scarce and they mainly feed by scraping algae off the underside of the ice, closing in the world above them. To save energy they also use a tactic to reduce their metabolism, shrinking to a smaller body size. They do so by moulting more than once, particularly in 'really bad times'.
When spring arises, little bubbles of oxygen fill the ocean waters and the algae that are on them are collected by krill. When the ice thaws completely, krill hunts in the open primarily on phytoplankton and occasionaly on zooplankton (OU Frozen Planet S175-13D Activities, Activity 7.2, Clip 1 & Clip 3).
In the spawning season from early January to mid-February female krill lay several thousands of eggs at once, which is one of the reasons why they are capable to exist in such abundancy. Even more, they survive on massive blooms of phytoplankton in summer to build up the huge biomass in which krill occurs in the Antarctic ocean (Species Fact Sheet Euphausia superba, 2013). This is why krill can 'overcome' the massive feeding frenzies by numerous predators.
However, human impacts play an important role as well in the continuation of future generations of Antarctic krill.
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To eat and be eaten
First photo EFI: krill primarily feed on phytoplankton,
in particular on diatoms, like this one from the
order Thalassiosirales.
Second photo: the blue whale (Balaenoptera musculus)
is a marine mammal that depends on krill
as their main food source during the spawning season
of Antarctic krill. An adult can take up to
4 tons of these crustaceans per day,
which is about 4 million individuals per day!
(Paccalet & Gohier 1993, p. 51)
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A huge deal
Changes in the Antarctic ecosystem need to be monitored intensively to keep track of how all animals respond to these changes. So what about human exploitation? Krill are harvested extensively by human fishery activities all around the Antarctic. It is, for that reason, crucial to understand the distribution of krill in the Southern Ocean: to safeguard sustainable use of the polar environment in relation to human exploitation and the availability of krill within this environment.
Overexploitation might be disasterous, not only to krill abundancy and distribution for the sake of krill, but also for the animals depending on Antarctic krill as their main food supply. For example, the Antarctic holds a history of intensive whaling and the whale population may not recover enough if the krill abundancy is in decline. (Brandon & Robinson 2011, p. 184, 185 & 224). And the truth is, the amount of krill in the Southern Ocean is declining rapidly and will most likely continue to decline. The dependency on krill is therefore a huge deal for Antarctic species to survive in the future and a huge deal for the Antarctic as a whole.
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This short article was written by me for the End-of-Module Assessment of S175 The frozen planet, Open University UK.
References:
Brandon, M. & Robinson, D. (2011) The frozen planet, Science Short Module, S175 The frozen planet, The Open University, Walton Hall, Milton Keynes MK 7 6AA, p. 184, 185 & 224.
Paccalet, Y. (author) & Gohier, F. (photography) (1993) Walvissen in hun element, Dutch publication by Zuid-Hollandse uitgeversmaatschappij, M & P Uitgeverij bv Weert, original title in French Baleines (1992) by Editions Atlas Paris, p. 51.
OU Frozen Planet S175-13D Activities, Activity 7.2, Clip 1 From the Blue Planet series, narrated by Sir David Attenborough, first transmitted on 12 September 2001.
OU Frozen Planet S175-13D Activities, Activity 7.2, Clip 2 From Life in the Freezer, written and presented by Sir David Attenborough, first transmitted on 18 November 1993.
OU Frozen Planet S175-13D Activities, Activity 7.2, Clip 3 From Saving Species, BBC Radio 4 series presented by interviewer Miles Barton: an audio interview with krill biologist Robert King from the Australian Antarctic Division, 8 September 2010.
Species Fact Sheet Euphausia superba, Food and Agriculture Organization. Retrieved August 24, 2013 at http://www.fao.org/fishery/species/3393/en
