Krill
Krill are small and exclusively marine crustaceans of the order Euphausiacea, found in all of the world's oceans. The name "krill" comes from the Norwegian word krill, meaning "small fry of fish", which is also often attributed to species of fish.
Krill are considered an important trophic level connection near the bottom of the food chain. They feed on phytoplankton and, to a lesser extent, zooplankton, and are also the main source of food for many larger animals. In the Southern Ocean, one species, the Antarctic krill, makes up an estimated biomass of around 379 million tonnes, making it among the species with the largest total biomass. Over half of this biomass is eaten by whales, seals, penguins, seabirds, squid, and fish each year. Most krill species display large daily vertical migrations, providing food for predators near the surface at night and in deeper waters during the day.
Krill are fished commercially in the Southern Ocean and in the waters around Japan. The total global harvest amounts to 150,000–200,000 tonnes annually, mostly from the Scotia Sea. Most krill catch is used for aquaculture and aquarium feeds, as bait in sport fishing, or in the pharmaceutical industry. Krill are also used for human consumption in several countries. They are known as okiami in Japan and as camarones in Spain and the Philippines. In the Philippines, they are also called alamang and are used to make a salty paste called bagoong.
Krill are also the main food for baleen whales, including the blue whale.
Taxonomy
Krill belong to the large arthropod subphylum, the Crustacea. The most familiar and largest group of crustaceans, the class Malacostraca, includes the superorder Eucarida comprising the three orders, Euphausiacea, Decapoda, and the planktonic Amphionidacea.The order Euphausiacea comprises two families. The more abundant Euphausiidae contains 10 different genera with a total of 85 species. Of these, the genus Euphausia is the largest, with 31 species. The lesser-known family, the Bentheuphausiidae, has only one species, Bentheuphausia amblyops, a bathypelagic krill living in deep waters below. It is considered the most primitive extant krill species.
Well-known species of the Euphausiidae of commercial krill fisheries include Antarctic krill, Pacific krill and Northern krill.
Phylogeny
, the order Euphausiacea is believed to be monophyletic due to several unique conserved morphological characteristics such as its naked filamentous gills and thin thoracopods and by molecular studies.There have been many theories of the location of the order Euphausiacea. Since the first description of Thysanopode tricuspide by Henri Milne-Edwards in 1830, the similarity of their biramous thoracopods had led zoologists to group euphausiids and Mysidacea in the order Schizopoda, which was split by Johan Erik Vesti Boas in 1883 into two separate orders. Later, William Thomas Calman ranked the Mysidacea in the superorder Peracarida and euphausiids in the superorder Eucarida, although even up to the 1930s the order Schizopoda was advocated. It was later also proposed that order Euphausiacea should be grouped with the Penaeidae in the Decapoda based on developmental similarities, as noted by Robert Gurney and Isabella Gordon. The reason for this debate is that krill share some morphological features of decapods and others of mysids.
Molecular studies have not unambiguously grouped them, possibly due to the paucity of key rare species such as Bentheuphausia amblyops in krill and Amphionides reynaudii in Eucarida. One study supports the monophyly of Eucarida, another groups Euphausiacea with Mysida, while yet another groups Euphausiacea with Hoplocarida.
Timeline
No extant fossil can be unequivocally assigned to Euphausiacea. Some extinct eumalacostracan taxa have been thought to be euphausiaceans such as Anthracophausia, Crangopsis—now assigned to the Aeschronectida —and Palaeomysis. All dating of speciation events were estimated by molecular clock methods, which placed the last common ancestor of the krill family Euphausiidae to have lived in the Lower Cretaceous about.Distribution
Krill occur worldwide in all oceans, although many individual species have endemic or neritic distributions. Bentheuphausia amblyops, a bathypelagic species, has a cosmopolitan distribution within its deep-sea habitat.Species of the genus Thysanoessa occur in both Atlantic and Pacific oceans. The Pacific is home to Euphausia pacifica. Northern krill occur across the Atlantic from the Mediterranean Sea northward.
Species with neritic distributions include the four species of the genus Nyctiphanes. They are highly abundant along the upwelling regions of the California, Humboldt, Benguela, and Canarias current systems. Another species having only neritic distribution is E. crystallorophias, which is endemic to the Antarctic coastline.
Species with endemic distributions include Nyctiphanes capensis, which occurs only in the Benguela Current, E. mucronata in the Humboldt Current, and the six Euphausia species native to the Southern Ocean.
In the Antarctic, seven species are known, one in genus Thysanoessa and six in Euphausia. The Antarctic krill commonly lives at depths reaching, whereas ice krill reach depth of, though they commonly inhabit depths of at most. Krill perform Diel Vertical Migrations in large swarms, and acoustic data has shown these migrations to go up to 400 metres in depth. Both are found at latitudes south of 55° S, with E. crystallorophias dominating south of 74° S and in regions of pack ice. Other species known in the Southern Ocean are E. frigida, E. longirostris, E. triacantha and E. vallentini.
Anatomy and morphology
Krill are crustaceans and, like all crustaceans, they have a chitinous exoskeleton. They have anatomy similar to a standard decapod with their bodies made up of three parts: the cephalothorax is composed of the head and the thorax, which are fused, and the abdomen, which bears the ten swimming appendages, and the tail fan. This outer shell of krill is transparent in most species.Krill feature intricate compound eyes. Some species adapt to different lighting conditions through the use of screening pigments.
They have two antennae and several pairs of thoracic legs called pereiopods or thoracopods, so named because they are attached to the thorax. Their number varies among genera and species. These thoracic legs include feeding legs and grooming legs.
Krill are probably the sister clade of decapods because all species have five pairs of swimming legs called "swimmerets" in common with the latter, very similar to those of a lobster or freshwater crayfish.
In spite of having ten swimmerets, otherwise known as pleopods, krill cannot be considered decapods. They lack any true ground-based legs due to all their pereiopods having been converted into grooming and auxiliary feeding legs. In Decapoda, there are ten functioning pereiopods, giving them their name; whereas here there are no remaining locomotive pereiopods. Nor are there consistently ten pereiopods at all.
Most krill are about long as adults. A few species grow to sizes on the order of. The largest krill species, Thysanopoda cornuta, lives deep in the open ocean. Krill can be easily distinguished from other crustaceans such as true shrimp by their externally visible gills.
Except for Bentheuphausia amblyops, krill are bioluminescent animals having organs called photophores that can emit light. The light is generated by an enzyme-catalysed chemiluminescence reaction, wherein a luciferin is activated by a luciferase enzyme. Studies indicate that the luciferin of many krill species is a fluorescent tetrapyrrole similar but not identical to dinoflagellate luciferin and that the krill probably do not produce this substance themselves but acquire it as part of their diet, which contains dinoflagellates. Krill photophores are complex organs with lenses and focusing abilities, and can be rotated by muscles. The precise function of these organs is as yet unknown; possibilities include mating, social interaction or orientation and as a form of counter-illumination camouflage to compensate their shadow against overhead ambient light.
Ecology
Feeding
Many krill are filter feeders: their frontmost appendages, the thoracopods, form very fine combs with which they can filter out their food from the water. These filters can be very fine in species that feed primarily on phytoplankton, in particular on diatoms, which are unicellular algae. Krill are mostly omnivorous, although a few species are carnivorous, preying on small zooplankton and fish larvae.Krill are an important element of the aquatic food chain. Krill convert the primary production of their prey into a form suitable for consumption by larger animals that cannot feed directly on the minuscule algae. Northern krill and some other species have a relatively small filtering basket and actively hunt copepods and larger zooplankton.
Predation
Many animals feed on krill, ranging from smaller animals like fish or penguins to larger ones like seals and baleen whales.Disturbances of an ecosystem resulting in a decline in the krill population can have far-reaching effects. During a coccolithophore bloom in the Bering Sea in 1998, for instance, the diatom concentration dropped in the affected area. Krill cannot feed on the smaller coccolithophores, and consequently the krill population in that region declined sharply. This in turn affected other species: the shearwater population dropped. The incident was thought to have been one reason salmon did not spawn that season.
Several single-celled endoparasitoidic ciliates of the genus Collinia can infect species of krill and devastate affected populations. Such diseases were reported for Thysanoessa inermis in the Bering Sea and also for E. pacifica, Thysanoessa spinifera, and T. gregaria off the North American Pacific coast. Some ectoparasites of the family Dajidae afflict krill ; one such parasite is Oculophryxus bicaulis, which was found on the krill Stylocheiron affine and S. longicorne. It attaches itself to the animal's eyestalk and sucks blood from its head; it apparently inhibits the host's reproduction, as none of the afflicted animals reached maturity.
Climate change poses another threat to krill populations.