Antarctic krill

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Antarctic krill
Antarctic krill (Euphausia superba)

Antarctic krill ( Euphausia superba )

Systematics
Sub-stem : Crustaceans (Crustacea)
Class : Higher crabs (Malacostraca)
Order : Light shrimp (Euphausiacea)
Family : Euphausiidae
Genre : Euphausia
Type : Antarctic krill
Scientific name
Euphausia superba
Dana , 1850

The Antarctic krill ( Euphausia superba ) is a species of crustacean from the family Euphausiidae that lives in the Southern Ocean in the waters around Antarctica . Like other species known as krill , Euphausia superba is one of the shrimp-like invertebrates that live in large schools . Such a swarm can contain 10,000 to 30,000 individuals per cubic meter of water. The crustaceans feed on phytoplankton , which builds up its body substance with the help of photosynthesis from carbon dioxide and nutrients; they use the primary production of phytoplankton in order to be able to lead their pelagic way of life in the open ocean . As a food source for all larger animals in Antarctica, the Antarctic krill is a key species in the Antarctic ecosystem. Based on its biomass of around 500 million tons , the species is probably the most successful animal species in the world.

The Antarctic krill reaches a body length of a maximum of six centimeters. The animals weigh up to two grams and can reach a maximum age of six years.

Systematics

All members of the order Euphausiacea are shrimp-like crustaceans from the parentage of the Eucarida . In these, the breast plates or thoracomers are connected to the carapace . Due to the shortness of these thoracomers, the gills are visible from the outside. The foremost leg pairs ( thoracopods ) forms with them no mouth parts in the form of Maxillipoden as other higher crustaceans such as the decapods is the case (Decapoda).

development

The eggs are laid near the surface and begin to sink. In the open ocean they sink for about ten days. The nauplius hatches at a depth of around 3000 meters.

The main spawning season of Antarctic krill is from January to March, with the eggs being laid both on the continental shelf and in the surface waters of the deep-sea ocean areas. As with all krill species, the male attaches a sperm package to the female's genital opening. For this purpose, the first legs of the abdomen, the pleopods , are transformed into mating organs. The females lay 6,000-10,000 eggs, each 0.6 mm in size, at a time, which are fertilized as the sperm packet passes .

According to the hypothesis of the British researcher Marr, which he put forward based on the results of the research trip of the well-known Discovery , the development of the eggs takes place in the following steps: Embryonic development , especially gastrulation , takes place during the sinking of the eggs to the Antarctic seabed in 2,000– 3,000 meters depth. As soon as the larva, a typical nauplius, hatches from the egg, it begins to rise to the surface of the water ( developmental ascent ).

Like the first nauplius stage, the next two larval stages, called the second nauplius and metanauplius , feed exclusively on their yolk reserves and do not take in any other food. After about three weeks, the krill will have reached the surface waters again and the larva will grow through further larval stages. These different stages are characterized primarily by the increasing number of legs, as well as the development of the compound eyes and the bristling. With a body length of around 15 millimeters, the young crabs have the same habitus as their parents, but continue to grow and reach sexual maturity after two or three years. With each growth spurt there is a molt , which takes place approximately every 13 to 20 days and during which the entire chitin shell is renewed.

nutrition

The head of the Antarctic krill. Recognizable are the luminous organ on eye stem and the nerve in the sensors , the gizzard and the filter net of thoracopods with the hooks at their tips.

The gut of the krill can often be recognized as a green translucent structure through the transparent skin. This will reveal that the Krill primarily of green, photosynthetic operated diatoms fed, picked up by a filter apparatus (see below). The shells of the diatoms are crushed in the gizzard and then the algae are digested in the hepatopancreas .

In addition to these algae, krill catches and zooplankton as hop Linge (copepods) and amphipods (amphipods). The intestine forms a straight tube and digestion is relatively inefficient, which means that the excretions still contain a large amount of undigested food. In the aquarium it was also observed that krill crabs also eat their conspecifics when they lack food ( cannibalism ).

Electron microscope image of a complex eye - the eyes of living animals are deep black

If there is not enough food, the crabs shrink and continue to shed their skin regularly. This response is unique among animals the size of krill and is viewed as an adaptation to seasonal food shortages in the Antarctic winter, when there is no light for photosynthesis. However, the complex eyes remain unchanged, which is why the ratio of the diameter of the eyes to the height of the crabs is a good measure of the extent of the diet.

Filter

Krill when filtering in high concentration of plankton. See also slow motion film (300 frames / sec; 490 kB).

The Antarctic krill is able to use the small plankton cells of the Antarctic waters, which no other, higher organism can use as food. This happens through a filter mechanism, for which the front, specially redesigned legs are used: The six thoracopods form a collecting basket with which plankton is picked up from the water. This basket closes so tightly that there are gaps of no more than one micrometer between the legs and the bristles attached to them. When the feed concentration is low, the collecting basket is opened, pushed over half a meter through the water and the algae that have got stuck are transported to the mouth via a special device made of comb bristles on the inside of the legs.

Ice willows

Antarctic krill eating ice algae . The ice surface on the left is colored green. This picture was taken with an ROV .

Antarctic krill can graze on the green ice-algae lawn that grows on the underside of the pack ice . The picture opposite shows such a grazing swarm. The animals have specialized bristles at the ends of the thoracopods that can scrape off algae from the ice like a rake . A krill crab can graze an area of ​​one square meter in just ten minutes. The knowledge that the algae lawn is formed over large areas below the ice is still relatively new. This lawn often contains more usable food than the entire open water area underneath. This is an important source of nutrition for krill, especially in spring.

"Biological pump" and carbon fixation

In-situ image recorded with an ecoSCOPE . A green spit ball is visible at the bottom right, a green thread of feces at the bottom left.

When eating, the krill occasionally spits agglomerations of thousands of algae in the form of a “spit ball”, and its excretions also contain a large proportion of undigested algae within the shells of the ingested diatoms. Both are relatively heavy and accordingly sink to greater depths. This is known as sea ​​snow or also as a “biological pump”, by means of which large amounts of carbon sink to depths of 2000 to 4000 meters and can be stored there for over 1000 years , bound as a carbon reservoir .

Some of the carbon is captured and absorbed by other organisms in the upper water layers, so that it remains here. It is believed that this is one of the largest biological feedback processes on earth, since the crabs represent a gigantic biomass and accordingly produce a lot of carbon residues. The research on this has not yet progressed very far.

Biological properties

Bioluminescence

Water discoloration by bioluminescent krill crabs

Krill crabs are often referred to as luminous shrimp because they are able to produce light with the help of special organs as bioluminescence . These organs are found in different parts of the body. There is a pair of pits of light on the eye stalks, further pairs on the hip joints ( coxae ) of the second and seventh thoracopods, and individual organs on the four sternites of the abdomen ( pleon ). The luminous organs produce blue light (approx. 490 nm), possibly in the form of periodic flashes of light.

The structure of the light organs is comparable to that of a flashlight . They have a concave reflector in the light pit and a lens that closes the pit. With the help of muscles , the entire organ can be moved. The function of light has not yet been fully clarified. There is a hypothesis according to which the glow should compensate for the shadow of the animals so that they cannot be easily recognized by predators. Another assumption is that the luminous organs play an important role in finding a partner and forming swarms at night.

The luminous organs contain several photoactive substances, the main substance having a maximum fluorescence with an excitation of 355 nanometers and an emission of 510 nanometers.

Escape reaction

Escape reaction

Krill crabs have a very specific form of flight response to escape predators. In this case, you swim backwards very quickly and obtain the necessary drive by striking movements with the telson . This form of swimming is often referred to as "lobstering" because other cancers also use it. In this way, krill crabs can reach speeds of 60 centimeters per second. The reaction time to the optical stimulus is 55 milliseconds and is a very quick reaction, especially for the cold waters.

Geographical distribution

Krill distribution on a NASA / SeaWIFS map - the main concentrations are found in the Scottish Sea and on the Antarctic Peninsula

Antarctic krill populate the surface waters of the Southern Ocean . It has a circumpolar distribution with a main concentration on the Atlantic sea ​​area.

The northern boundary of the Southern Ocean runs along the Antarctic Convergence , i.e. the area in which the cold water of the Antarctic mixes with the warmer water of the Atlantic, Pacific and Indian Oceans . This border is roughly along the 55th degree of latitude south latitude. Correspondingly, the Southern Ocean extends from this border to the Antarctic continent over an area of ​​water of around 32 million square kilometers. In winter around three quarters of this water surface is covered with ice, in summer, however, around 24 million square kilometers are ice-free. The water temperature ranges from -1.3 to 3 degrees Celsius.

The Southern Ocean consists of a complex current system. In westerly winds, the surface currents around the Antarctic drift to the east. Near the land mass, the wind current runs in reverse clockwise direction. In the border zone of the two rivers, large eddies known as dead water develop, for example in the Weddell Sea . The schools of krill drift with these currents and thus form a single population that includes the entire Antarctic continent. There is a continuous flow of genes throughout the area. Little is known about the exact migration routes, as individual krill crabs have so far not been able to be marked for telemetric examinations at great distances. It is therefore not yet possible to track the detailed movement patterns.

Position in the Antarctic ecosystem

The Antarctic krill is the species that plays a key role in the Antarctic ecosystem. It is the basic food source for all whales , seals , penguins and other sea birds, as well as most of the fish in the Antarctic. The seal species, known as crab-eater, has even developed special teeth to adapt to the krill's diet, which, due to their structure, enable them to sift the krill out of the water. They are the seals most specialized in a food source. 98 percent of their diet consists of Antarctic krill, of which they consume around 63 million tons per year. In summary, up to 130 million tons of krill are consumed annually by seals, 43 million tons by whales, 15 to 20 million tons by birds, up to 100 million tons by squid and up to 20 million tons by fish.

Both the size difference between krill and its food, the approximately 20 micrometer large algae, and between krill and its hunters, which include the blue whale , the largest living animal of all, is very large. These relationships are unique in the world. In the North Atlantic Meganyctiphanes norvegica is the dominant krill species, in the North Pacific Euphausia pacifica .

Biomass and production

The total biomass of Antarctic krill is estimated at 125 to 725 million tons. This distinguishes E. superba as the most successful animal species in the world. It should be noted that some biologists are debating whether ants provide the largest biomass of all animals visible to the naked eye . However, this group includes over 10,000 species . The same applies to the copepods include species (Copepoda), which also hundreds. For comparison: the annual catch of all fish and other marine animals is currently around 100 million tons, while estimates of the annual biomass production of krill range from 13 million to several billion tons.

The reason for these enormous rates of reproduction is that the waters around the Antarctic ice shelf represent one of the largest areas of concentration for plankton, if not the largest. Because the deep currents come up here due to upwelling , the area is supplied with such a huge amount of nutrients that is not found in any other marine area. The water is correspondingly saturated with phytoplankton.

Normally, primary production , i.e. the conversion of sunlight and nutrients into usable, high-energy carbon compounds, is one to two grams per square meter per year in the ocean. In the area of ​​the Antarctic ice it increases to values ​​of up to 30 grams per square meter and year. Compared to other highly productive marine regions such as the North Sea , this number is not that extremely high, but in relation to the huge area it is gigantic, even when compared with the tropical rainforests , which also have a large amount of biomass and annual primary production. Then there are the long, sunny days in the Antarctic summer.

Temperature and pack ice area (based on data from Loeb et al. 1997). The scale for the ice (right) is inverted to clarify the correlation . The horizontal line is the freezing point . The sloping line is the mean temperature, in 1995 it reached freezing point.

fishing

Annual catch of E. superba , according to FAO data

The fishing share of Antarctic krill is over 230,000 tons per year (as of 2013, FAO ). The main catching nations are Japan and Poland . In Japan products made from krill are considered a delicacy, in other regions around the world krill is mainly used as animal feed or fish bait. The krill fishery is problematic for two main reasons:

First of all, the net has to be very close-meshed, which gives it a very high resistance in the water. This creates a wave that deflects the crabs sideways. In addition, fine nets in particular are very sensitive. The first krill nets developed were therefore torn when they were used.

The second problem is hauling in the net. When the net is full and pulled out of the water, the crustaceans crush each other due to the mass and the main part of the meat is squeezed out. In experiments, the krill was pumped through pipes on board; special networks are also being developed. The processing must be done very quickly, as the animals autolysis within a few hours . For this, the muscular tails are usually separated from the front body and freed from the chitin shell, after which they are frozen or ground into powder. The products made from krill contain high concentrations of proteins and vitamins , which make them valuable for consumption and feeding.

Global warming and acidification of the oceans

It is feared that global warming could have a devastating effect on Antarctic krill stocks. Studies show that a reduction in the Antarctic sea ice leads to lower krill populations, since the larvae and young animals in particular are dependent on the ice algae in winter. There is also concern about the effects of high levels of carbon dioxide in the Antarctic Ocean and the associated acidification of the oceans. Since the krill's chitin shell consists largely of calcium components, it is very susceptible to acid. Experiments have shown that young animals are no longer able to hatch at very high CO 2 concentrations. Even with a moderate increase in carbon dioxide, they show development problems. Since krill plays such a central role in the Antarctic ecosystem, even a moderate decline in population could have very far-reaching effects on the global ecosystem.

Future visions and "Ocean Engineering"

Despite the very little knowledge of the entire Antarctic ecosystem, several long-term studies have been started with the krill to increase carbon fixation . There are enormous amounts of nutrients in large regions of the Southern Ocean. Even so, there is not much phytoplankton growth here. These areas are known as HNLC ( high nutrient, low chlorophyll ), and the phenomenon itself is called the Antarctic Paradox . The main reason is the lack of iron ions . Relatively small amounts of iron from research vessels could lead to algal blooms in these areas . One of the future visions is that a sufficient supply of iron in these areas could result in more carbon dioxide being bound from the burning of fossil fuels . With regard to the sinking of this bound carbon to the sea floor, the krill crabs play a key role through the formation of spit balls and fecal strings .

Krill oil

Krill oil is extracted from the Antarctic krill and is used in studies in alternative medicine .

literature

Web links

Commons : Antarctic Krill  - Collection of images, videos and audio files

Individual evidence

  1. ↑ The Performance of Krill vs. Salps to withstand in a warming Southern Ocean (PEKRIS). In: uol.de . Retrieved November 21, 2019 .
  2. ↑ Corinna Dahm-Brey: How does climate change affect the Antarctic krill? In: idw-online.de . November 20, 2019, accessed November 21, 2019 .
  3. ^ Angus Atkinson, Volker Siegel, Evgeny Pakhomov & Peter Rothery: Long-term decline in krill stock and increase in salps within the Southern Ocean . In: Nature . 432, 2004, pp. 100-103. doi : 10.1038 / nature02996 .
  4. ↑ So Kawaguchi, Haruko Kurihara, Robert King, Lillian Hale, Thomas Berli, James P. Robinson, Akio Ishida, Masahide Wakita, Patti Virtue, Stephen Nicol and Atsushi Ishimatsu: Will krill fare well under Southern Ocean acidification? . In: Biology Letters . 7, No. 2, 2011, pp. 288-291. doi : 10.1098 / rsbl.2010.0777 .
This article was added to the list of excellent articles on July 29, 2005 in this version .