Antarctic krill live in the Southern and Indian Antarctic Oceans, in the Antarctic Peninsula region. Their distribution spans the Antarctic Continental Shelf break to the Antarctic Polar Frontal Zone. Areas of particularly high biomass include the Antarctic Coastal Current, near the Antarctic Continent, near Prydz Bay, at northern and western coastal regions of the Antarctic Continent, and the areas where the Antarctic Coastal Current interacts with the Antarctic Circumpolar Current. ("Species Fact Sheet: Euphausia superba (Dana, 1852)", 2013; El-Sayed, 1994; Everson, 2000)
Antarctic krill live in open marine waters. Adults are found at depths ranging from surface waters to depths of 350 m; they have occasionally been found as deep as 600 m. They are found in deeper waters during winter months. Larvae begin life near the sea floor and ascend toward the surface as development progresses. ("Species Fact Sheet: Euphausia superba (Dana, 1852)", 2013; Knox, 1994; Siegel, 2013)
The body is pink and slightly opaque, with a hard, calcified exoskeleton (also known as a carapace) divided into a cephalothorax (head and thorax fused) and an abdomen. These animals are similar in appearance to shrimp. Antarctic krill have six pairs of thoracic appendages and a pair of antennae. A tail is formed by fusion of the final appendages. There are luminous organs called photophores located near the mouthparts, at the genitals (located on the cephalothorax), and at the base of the abdominal pleopods (which are the forked limbs these animals use for swimming). These photophores produce a blue light. The gills are located ventrally, under the carapace. Adults range from 5-6.5 cm in length and weigh an average of 2 g. Females are slightly larger than males and minor differences in body shape are present between sexes (males have a more elongate body shape, slightly larger eyes, longer antennae and slightly shorter rostrums, for example). Also, mature males have modified endopods called petasmae as the first pair of pleopods. These are used during mating to transfer spermatophores. Females have a tri-lobed structure on the ventral posterior part of their body called the thelycum. In unmated females, this structure is often a bright red color. Spermatophores (small white vesicles), can occasionally be seen attached to it in mated females. ("Species Fact Sheet: Euphausia superba (Dana, 1852)", 2013; Everson, 2000; Hamner and Hamner, 2000; Kawaguchi, et al., 2011; Knox, 1994; "Antarctic krill (Euphausia superba)", 2012)
Krill undergo several larval stages, known as nauplius, metanauplius, calyptopus, and furcilia; molts occur between (and sometimes within) each stage, with each larval stage lasting from 8-15 days. Once eggs have been laid, they sink for about 10 days, as deep as a few hundred to 2,000 m deep. There, they hatch as nauplii, which have only one eye and no body segments or limb buds. Nauplii ascend and enter a metanauplius stage, where limb development begins. As larvae continue to rise, they develop into calyptopes; these reach the surface and begin to feed. After three additional molts, larvae become known as furcilia. The furcilia stage is marked by the development of movable compound eyes, which project from the edge of the carapace. Furcilia develop into juveniles, reaching lengths of 4-10 mm by early winter, with growth slowing down by late March. Juveniles begin to develop gonads during their second year (spring/summer) and begin to spawn at two years of age. ("Species Fact Sheet: Euphausia superba (Dana, 1852)", 2013; Everson, 2000; Hamner and Hamner, 2000; Kawaguchi, et al., 2011; Knox, 1994; Nicol, 2006; Quetin and Ross, 1991)
Mating involves 5 phases: chase, probe, embrace, flex, and push. First, a male (sometimes more than one at a time) pursues a gravid female. Then, one male probes a female with his petasma (specialized structures found on the first pair pleopods). Male and female then embrace each other, abdomen to abdomen. Spermatophores are transferred as the male flexes his body around the female, forming a T-shaped pair. Hooks on his petasma aid in spermatophore transfer. Rapid spinning occurs during flexing, lasting about 5 seconds, which aids in pushing spermatophores into her thelycum. After flexing, the pair continue to swim together as the male pushes his rostrum and antennae against the female's ventral surface. Finally, the pair detach and swim away from each other. (Kawaguchi, et al., 2011)
All adult female Antarctic krill develop a brood during a reproductive season, with eggs produced periodically and released in several spawning events. Up to four oocytes may undergo vitellogenesis (yolk production) per female. Warmer temperatures may increase spawning and molting activities in females. Females lay their eggs in deep waters, between December and March. Eggs begin development on the sea floor, but it is not known where in the water column they are laid. Eggs sink for approximately 10 days before hatching and entering the larval stages described above. (Cuzin-Roudy, 2000; Kawaguchi, et al., 2011; Knox, 1994)
There is no parental investment observed in this species beyond the nutrients involved in gamete production. (Nicol, 2006)
The average lifespan of Antarctic krill is 5-7 years. ("Species Fact Sheet: Euphausia superba (Dana, 1852)", 2013; Everson, 2000; Nicol, 2006)
Antarctic krill are an obligate schooling species, with schools primarily moving horizontally in the water column, along with currents. Schools may be extremely large, with an average length of 100 m, but may extend to 100 km, with an average thickness of 15 me. Densities may measure 1,0000-100,000 krill per cubic meter, with lower density schools measuring 1 to 100 krill per cubic meter. Schooling in groups of similar body size enables these animals to avoid any one individual being singled out by a predator. (Hamner and Hamner, 2000; Hamner, et al., 1983; Sahrhage, 1988)
These organisms are not known to maintain specific home ranges or defend territories.
Krill form dense schools, within which all individuals swim in the same direction, evenly spaced from each other. All individuals in a given school are approximately the same size. An individual will gauge its size in relation to the rest of the school and join or leave as appropriate. Individuals at the front of a school use rheotactic cues, such as turning to face oncoming currents, to communicate while swimming. Vision helps individuals to maintain schools and during feeding. Mechanoreception and olfaction may also play a role in schooling behavior. Chemoreceptors are used to detect amino acids (even at very low levels), which indicate the presence of food sources, and pheromones likely play a role in mating. (Hamner, et al., 1983; Knox, 1994; Strand and Hamner, 1990)
Usually, Antarctic krill feed by using their thoracic endopodites to create a watertight feeding basket, which encloses a pocket of food and water. Water is then filtered out laterally by compression-filtration through setae. Phytoplankton remains caught in the feeding basket as water is filtered out, and is brushed forward by the setae into the mouth for ingestion. Antarctic krill are primarily planktivores, but occasionally eat other krill or molted exoskeletons. They are considered the dominant herbivore of the Southern Ocean. During the winter, Antarctic krill rely heavily on ice algae as a food source. They are filter feeders, but do not feed continually, relying on chemical cues to indicate the presence of food particles. (El-Sayed, 1994; Hamner and Hamner, 2000; Hamner, et al., 1983)
Antarctic krill serve as prey to many marine mammals, invertebrates, fishes, and birds. The only anti-predatory adaptation of these krill is their schooling behavior. A disruption to the school may cause mass molting, which can act as a distraction to predators. Krill may also avoid predators by remaining in deep, cold water below the surface. (Hamner and Hamner, 2000; Knox, 1994; Quetin and Ross, 1991)
Antarctic krill play an important role as a primary food source for many animals in the Southern Ocean. They may be parasitized by several organisms, in particular by protozoans in the genus Ephelota. Infected krill become more opaque and whitish in color and are affected with tumors and molting problems, in which parts of their exoskeletons remain attached. (El-Sayed, 1994; Hamner and Hamner, 2000; Knox, 1994; Stankovic and Rakusa-Susczewski, 1996; Takahashi, et al., 2003)
There have been attempts to use Antarctic krill for human consumption, but this species is used mainly for domestic animal and aquaculture feed. Krill products have pharmaceutical and industrial uses. In particular, chitin may have potential uses in lowering cholesterol levels, and the lipid composition of Antarctic krill may be useful as a nutritional source of fatty acids. The lipids of Antarctic krill are more stable than those of some fishes consumed by humans. Krill digestive proteases can also be injected into humans to reduce pressure on nerve roots between vertebral discs. (El-Sayed, 1994; Everson, 2000)
There are no known adverse effects of Antarctic krill on humans.
Although this species is currently in no particular danger, it is a concern to some conservation groups. Antarctic krill are essential to the diet of many animals in the Antarctic and the Southern Oceans. A main concern is that krill fisheries may overdevelop in order to feed farmed fish, decreasing the Antarctic krill biomass in the Southern Ocean and potentially endangering other animal species living in the region. Measures being taken to protect Antarctic krill include preventing krill fisheries from expanding, taking regular biomass surveys, and strengthening and funding programs dedicated to monitoring the Antarctic ecosystem. ("AKCP", 2010; "CCAMLR", 2010; "Krill Conservation", 2012)
Rachel Gierak (author), University of Michigan-Ann Arbor, Alison Gould (editor), University of Michigan-Ann Arbor, Jeremy Wright (editor), University of Michigan-Ann Arbor.
body of water between the southern ocean (above 60 degrees south latitude), Australia, Asia, and the western hemisphere. This is the world's largest ocean, covering about 28% of the world's surface.
having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.
uses smells or other chemicals to communicate
a substance used for the diagnosis, cure, mitigation, treatment, or prevention of disease
animals which must use heat acquired from the environment and behavioral adaptations to regulate body temperature
union of egg and spermatozoan
a method of feeding where small food particles are filtered from the surrounding water by various mechanisms. Used mainly by aquatic invertebrates, especially plankton, but also by baleen whales.
A substance that provides both nutrients and energy to a living thing.
An animal that eats mainly plants or parts of plants.
fertilization takes place within the female's body
offspring are produced in more than one group (litters, clutches, etc.) and across multiple seasons (or other periods hospitable to reproduction). Iteroparous animals must, by definition, survive over multiple seasons (or periodic condition changes).
a species whose presence or absence strongly affects populations of other species in that area such that the extirpation of the keystone species in an area will result in the ultimate extirpation of many more species in that area (Example: sea otter).
A large change in the shape or structure of an animal that happens as the animal grows. In insects, "incomplete metamorphosis" is when young animals are similar to adults and change gradually into the adult form, and "complete metamorphosis" is when there is a profound change between larval and adult forms. Butterflies have complete metamorphosis, grasshoppers have incomplete metamorphosis.
having the capacity to move from one place to another.
specialized for swimming
the area in which the animal is naturally found, the region in which it is endemic.
reproduction in which eggs are released by the female; development of offspring occurs outside the mother's body.
An aquatic biome consisting of the open ocean, far from land, does not include sea bottom (benthic zone).
chemicals released into air or water that are detected by and responded to by other animals of the same species
photosynthetic or plant constituent of plankton; mainly unicellular algae. (Compare to zooplankton.)
an animal that mainly eats plankton
the regions of the earth that surround the north and south poles, from the north pole to 60 degrees north and from the south pole to 60 degrees south.
the kind of polygamy in which a female pairs with several males, each of which also pairs with several different females.
mainly lives in oceans, seas, or other bodies of salt water.
breeding is confined to a particular season
reproduction that includes combining the genetic contribution of two individuals, a male and a female
associates with others of its species; forms social groups.
uses touch to communicate
movements of a hard surface that are produced by animals as signals to others
uses sight to communicate
animal constituent of plankton; mainly small crustaceans and fish larvae. (Compare to phytoplankton.)
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