The discovery completely changes scientists' understanding of the major food source for fish, squid, penguins, seals and whales. Professor Andrew Clarke of the British Antarctic Survey said, "While most krill make their living in the ocean's surface waters, the new findings revise significantly our understanding of the depth distribution and ecology of Antarctic krill.
It was a surprise to observe actively-feeding adult krill, including females that were apparently ready to spawn, close to the seabed in deep water. Scientists have been studying krill since the 'Discovery' expeditions of the early 20th century.
Krill Discovered Living In The Antarctic Abyss
Oceanographic expeditions, using a combination of echo-sound techniques and collection samples in nets, indicated that the bulk of the population of adult krill is typically confined to the top metres of the water column. He says,"Having the ability to use a deep-water ROV in Antarctica gave us a unique opportunity to observe the krill and also to observe the diversity of animals living at the deep-sea floor from depths of m down to m.
The importance of such observations is that, not only do we have the ability to identify species, but we can see the relations among individual species and their relationship to the ambient environment. The discovery holds some important lessons, Clarke continued. There is still a great deal to learn about the deep sea and an important role for exploration in our attempts to understand the world we live in.
Antarctic krill Euphausia superba , feed on phytoplankton and are in turn eaten by a wide range of animals including fish, penguins, seals and whales. Phytoplankon are the starting point for the marine food chain and use photosynthesis to extract carbon from carbon dioxide. Krill live in the open ocean, mainly in large swarms and reach particularly high numbers in Antarctica. The traditional view of krill as mainly diatom feeders has changed in light of research showing that all life history stages can adapt to locally available food.
Larvae are probably highly dependent on ice algae and on the under-ice microbial community Quetin and Ross Adults may consume detritus and heterotrophic material in winter Peris-sonotto et al. All stages utilize pelagic food sources throughout the year; adults are probably less discriminating than the smaller stages, but adults are probably inefficient at consuming items smaller than 5 micrometers Quetin and Ross Krill swarms can extend over kilometers and can contain many billions of individuals, each with a considerable requirement for food Nicol Individual krill may be selective feeders, but it is difficult to see how an individual embedded within a large swarm can afford to be anything but an indiscriminate consumer of whatever particulate matter it encounters.
Other pelagic organisms are often absent in the surface layer when krill are dense Atkinson et al. Most experiments on krill feeding have been conducted on individuals or on small groups, and so have been unable to simulate the effect of high densities and large numbers of krill. The powerful effect of krill in the pelagic zone comes about because they have a very large biomass concentrated in a limited area, and they have a high requirement for food in a restricted time period Perissonotto et al.
Within their range, which has been estimated to be only one-quarter of the Southern Ocean Nicol et al. Although there is evidence that krill are most abundant in those areas within their range where production is highest Atkinson et al. The coastal zone is highly productive and yet supports populations of another species of krill Euphausia crystallorophias , so factors other than food concentration must combine to determine the extent of krill habitat.
Krill reproduction is an energetically demanding process, but under the right environmental conditions female krill can produce continuous batches of eggs throughout the four-month summer period Ross and Quetin This is far in excess of earlier estimates, which assumed only a single spawning in a 2-year life span Clarke Female krill have been observed to lay up to eggs in a single batch and may be capable of producing nine batches in a season Ross and Quetin There is now considerable information on the importance of where krill would have to lay their eggs for the greatest hatching success Hofmann and Husrevoglu , Hofmann and Murphy and on the conditions that must be present for the success of the larvae once they have hatched Ross et al.
Generally, gravid krill are found offshore of the rest of the adult population, in deeper water Trathan et al. Antarctic krill eggs sink, which is probably a key feature in their life history. The eggs of some other euphausiid species float, and some species brood eggs Ross and Quetin , so sinking eggs are an adaptation that allows this species of krill to produce successful offspring in its natural environment.
The general assumption, though not empirically verified, is that if krill embryos reach the sea floor, their survival will be impaired. Thus eggs laid on the continental shelf are unlikely to develop, and those laid in deep water are more likely to survive Hofmann and Murphy Female krill seem to undertake spawning migrations to deeper waters, which would enhance the survival chances of their larvae Siegel , Trathan et al.
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The requirement for access to deep water for spawning may separate Antarctic krill, with a deep-sinking egg, from the coastal species of krill, E. If krill have evolved a sinking egg and a spawning migration, then what advantage accrues to the population from having the eggs and developing larvae laid offshore from the main body of the krill population?
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Krill swarms are the dominant consumers of suspended particulate material within their habitat, so separating the small, food-sized eggs and larvae from the juveniles and adults must enhance their chances of survival Siegel , Quetin and Ross The embryos hatch into free-swimming larvae at depths of to meters. The larvae swim upward, developing as they swim, and reach the surface some 30 days after the eggs were laid. The first feeding stage the first calyptopis is reached after 30 days, and it is critical for survival that the larvae find food within 6 days. Temperature and pressure affect larval growth and development, and the location of spawning and the temperature of the underlying water masses and their movements play a large part in determining the rate of development Ross et al.
In late winter through early spring, the larvae metamorphose into juveniles, and they grow through the summer and the following autumn and winter to emerge the following spring as adults. Eggs laid offshore in deep water result in the embryos and larvae developing in the eastward-flowing waters of the ACC and in the warmer water layers of the UCDW Hofmann and Husrevoglu In winter, the larval habitat becomes covered with pack ice, and the larvae, which have risen to the surface, drift with the movement of the pack.
As the pack ice drifts in a series of cyclonic gyres Heil and Allison , larvae associated with the pack ice can be transported from the offshore waters, where they are spawned to the inshore waters where the juveniles are found. Krill have evolved a life history that is obviously highly successful at exploiting their seasonally variable environment.
In viewing the life history of krill, various elements, such as feeding behavior, distribution, and reproductive activity, have often been treated separately. In the model presented below, the various life history traits outlined in the previous sections are part of an overall system that allows Antarctic krill to survive in a highly seasonal fluid environment.
Biology and Ecology of Antarctic Krill - Semantic Scholar
For the purposes of this model, krill are divided into developmental stages: larvae, juveniles, and adults. A secondary division separates the gravid females from the rest of the adult population. These groupings are thought to remain geographically separate Siegel A simple seasonal representation of these life history stages is shown in figures 4 and 5.
In summer, the adult krill population is centered close to the shelf break, which allows access to pelagic food supplies but also places the populations within a counter-current circulation system with the ACC to the north and the Coastal Current to the south. Small movements within this region can have major effects on the horizontal distribution of individuals and populations. Krill females that move offshore to spawn place their young into the eastward-flowing waters of the ACC, but the developing young are not lost to the system, because of the gyral circulation patterns that link the ACC to the Coastal Current and because of the vertical circulation patterns near the shelf break figure 4a.
These currents will also bring the larvae back inshore to the shelf regions where juveniles are encountered the following summer. Sea ice communities are essential for larval krill in winter, and their movement is linked to that of the ice figure 4b. Adult krill are robust and can utilize food resources other than the under-ice community, or they can starve and shrink; in either case, they would not need to compete with the larvae that must feed, develop, and grow over the winter months.
Adult krill thus can be found deeper in winter, and may also move inshore in deep troughs or canyons. In spring, the adults feed on the phytoplankton bloom at the ice edge Brierley et al. In summer, the juvenile krill are found inshore of the adults, which again make their ontogenetic migration to the shelf break.
The effect of these differential distribution patterns is that the krill in younger stages are separated from the adults. This reduces competition for food resources and also reduces the risk of the adults' predation on the younger stages Siegel Around the Antarctic, krill populations are likely to persist where the females can find enough food to reproduce and then can reach deep water for spawning. These conditions may not occur uniformly.
Krill: Biology, Ecology and Fisheries (Fish and Aquatic Resources Series 6)
To grow and survive, the larvae resulting from this spawning must be able to find sufficient food when they reach the surface, and they must be where a sea ice community is available for their use as an overwinter food source. The circulation pattern of the area must also be such that, by spring, the newly developed juveniles are in their preferred habitat, inshore of the shelf break figure 5. Other descriptions of krill life history have included some of the elements described here. Siegel laid out the concept of ontogenetic migrations of krill to explain the observed distribution of life history stages, which was confirmed in subsequent studies Trathan et al.
The role of sea ice in the life cycle of krill has been explored Smetacek et al. Quetin and colleagues introduced the concept of a krill population that is much more reactive to its physical and biological environment than had previously been assumed, and brought together year-round observations to assemble an overall picture of the krill life cycle.
The relationship between local krill populations and gyres has been suggested before Mackintosh , Amos , Pakhomov The conceptual model outlined here has incorporated many diverse findings from the published literature and has developed a framework built around the concept of krill being more than passive players in their environment. Krill are a species with a long and complex life cycle that has evolved to exploit a highly seasonal environment; hence the complete life history of krill and its distribution cannot be explained without invoking known behaviors and observed adaptations to its environment.
The life history of krill remains unresolved because of the uncertainties surrounding the forces that determine the distribution of the species at all scales. Krill have been viewed as particles in a generally eastward-flowing current field, which means that the clues to determining the success of any particular life history stage most likely lie in waters to the west, not in local events Hofmann and Murphy This makes the population dynamics of krill at a single location difficult to interpret, because the factors that are thought to affect observed processes, such as recruitment and growth, are remote from the site of the observations.
The concept of teleconnections between areas—larger-scale processes that affect population dynamics on a regionwide scale Brierley et al. The construction of models that look at whether local population processes in krill can be explained by local effects would be a useful alternative to the more mechanistic models that have been put forward to date.
So Kawaguchi and the ecology of Antarctic krill
Some of the elements presented here have been developed by studying krill population processes in the waters off East Antarctica. In this region, the biogeographic zones are widely spread, and regional differences allow the examination of key physical determinants of krill distribution Nicol et al. Much of the biology of krill, however, is known from studies off the Antarctic Peninsula, where the tightly constrained current systems, sea ice at its circumpolar minimum, and the convoluted coastline and many island groups constitute a complicated physical environment from which it is more difficult to draw generalizations.
South Georgia is a particular anomaly because large krill populations are found there in the absence of sea ice. It is supposed that krill do not reproduce successfully at South Georgia, because first-year krill are rarely found in the surrounding waters. Gravid female krill are, however, frequently encountered around South Georgia, so it is likely that spawning does occur there; it is the fate of the larvae that is in question. If the Weddell Gyre is viewed as a magnified version of the gyres that link the current systems in other areas, then spawning at South Georgia may result in larvae that overwinter far to the south in the Weddell Sea, and recruitment to the South Georgia population may occur only after the larvae have circumnavigated the gyre.
Owing to the size of the gyre, this would take twice as long as in other retention systems, and thus the krill would return as 2-year-olds, not 1-year-olds. This conjecture accords with observations on the size structure of krill populations at South Georgia and their inter-annual variability Reid et al.
Krill inhabit the Southern Ocean, which is subject to major cyclical environmental fluctuations, and has also been considerably affected by human-induced changes Smetacek and Nicol Harvesting of seals, whales, fish, and krill has had effects on the ecosystems of the Southern Ocean both at a global scale e.
The krill fishery has been the largest in the Southern Ocean for more than 25 years, although it remains small relative to the overall stock size Nicol and Foster Projected increases in harvesting could result in the krill fishery becoming a globally important supplier of aquaculture meal, which in turn could have significant ecological impacts if not managed carefully.
The changing physical environment of the Southern Ocean affects features that are known to be critical to the life history of krill: sea ice extent and concentration, water temperatures, and circulation patterns. The combined effects of all these changes on the circumpolar population of krill are difficult to predict Smetacek and Nicol , but there have been indications that changes in distribution and abundance are occurring Atkinson et al.
Detecting responses to future change will require efficient monitoring of krill stocks and the populations of their predators. This conceptual model of the life history of Antarctic krill brings together what is known about the biology of krill and their environment into a cohesive account that views their biology as part of a successful system. The concept of krill as active players in their own life cycle generates some features that can be examined experimentally or through surveys at a number of scales.
In particular, the model highlights the need for studies into the detailed relationship between the life history stages and hydrography at appropriate scales. Coupling new technology such as multibeam sonar with acoustic Doppler current profilers will allow three-dimensional imaging of krill and currents. Whether concentrations of krill constitute genetically distinct stocks will be determined by conducting appropriately designed population genetic studies.
A better understanding of the behavior of krill as individuals and in their normally aggregated state will come from both field and laboratory investigations Nicol This was used to create the depth histograms Fig 5 and niche tables Fig 6. Blank cells contain no data.
There are only larval data for the late season. These data were used to create isobaths and to derive mean water depth for each of the grid cells. SST: Climatological February mean sea surface temperature calculated as described in the main text. This information was used in the construction of Fig 6.
It is the same map as the main text Fig 1 which presents the latitude and longitudes of the map domain, allowing re-plotting and re-analyses of these data if necessary. S3 — S5 Tables link to these labelled cells, presenting the data extracted from KRILLBASE databases on krill post-larval and larval abundance and post-larval length frequency and plotted on a 1 degree latitude by 2 degree longitude grid. It thus contains the source data used for the construction of all the figures in the paper. We thank Hauke Flores, Grace Saba and two other referees for their comments which allowed us to improve a previous draft.