Krill Ultimate

[Shari Alvine]

KrillEuphausiids),[1] (sg.: krill) are small and exclusively marine crustaceans of the order Euphausiacea, found in all the world's oceans.[2] The name "krill" comes from the Norwegian word krill, meaning "small fry of fish",[3] which is also often attributed to species of fish.

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 (krill), Decapoda (shrimp, prawns, lobsters, crabs), 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.[5] The lesser-known family, the Bentheuphausiidae, has only one species, Bentheuphausia amblyops, a bathypelagic krill living in deep waters below 1,000 m (3,300 ft). It is considered the most primitive extant krill species.[6]

As of 2013[update], the order Euphausiacea is believed to be monophyletic due to several unique conserved morphological characteristics (autapomorphy) such as its naked filamentous gills and thin thoracopods[10] and by molecular studies.[11][12][13]

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.[14] Later, William Thomas Calman (1904) ranked the Mysidacea in the superorder Peracarida and euphausiids in the superorder Eucarida, although even up to the 1930s the order Schizopoda was advocated.[10] It was later also proposed that order Euphausiacea should be grouped with the Penaeidae (family of prawns) in the Decapoda based on developmental similarities, as noted by Robert Gurney and Isabella Gordon.[15][16] The reason for this debate is that krill share some morphological features of decapods and others of mysids.[10]

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 (with basal Mysida),[17] another groups Euphausiacea with Mysida (the Schizopoda),[12] while yet another groups Euphausiacea with Hoplocarida.[18]

Krill occur worldwide in all oceans, although many individual species have endemic or neritic (i.e., coastal) distributions. Bentheuphausia amblyops, a bathypelagic species, has a cosmopolitan distribution within its deep-sea habitat.[20]

Species of the genus Thysanoessa occur in both Atlantic and Pacific oceans.[21] 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.[22] They are highly abundant along the upwelling regions of the California, Humboldt, Benguela, and Canarias current systems.[23][24][25] Another species having only neritic distribution is E. crystallorophias, which is endemic to the Antarctic coastline.[26]

Species with endemic distributions include Nyctiphanes capensis, which occurs only in the Benguela current,[22] E. mucronata in the Humboldt current,[27] and the six Euphausia species native to the Southern Ocean.

Krill are crustaceans and, as do 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.

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.

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 (a kind of pigment) 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[37] and that the krill probably do not produce this substance themselves but acquire it as part of their diet, which contains dinoflagellates.[38] Krill photophores are complex organs with lenses and focusing abilities, and can be rotated by muscles.[39] 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.[40][41]

Many krill are filter feeders:[24] 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 (such as Euphausia spp.) that feed primarily on phytoplankton, in particular on diatoms, which are unicellular algae. Krill are mostly omnivorous,[43] although a few species are carnivorous, preying on small zooplankton and fish larvae.[44]

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.[44]

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,[46] for instance, the diatom concentration dropped in the affected area. Krill cannot feed on the smaller coccolithophores, and consequently the krill population (mainly E. pacifica) 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.[47]

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.[48][49] Some ectoparasites of the family Dajidae (epicaridean isopods) afflict krill (and also shrimp and mysids); 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.[50]

By the calyptopsis stages differentiation has progressed far enough for them to develop a mouth and a digestive tract, and they begin to eat phytoplankton. By that time their yolk reserves are exhausted and the larvae must have reached the photic zone, the upper layers of the ocean where algae flourish. During the furcilia stages, segments with pairs of swimmerets are added, beginning at the frontmost segments. Each new pair becomes functional only at the next moult. The number of segments added during any one of the furcilia stages may vary even within one species depending on environmental conditions.[53] After the final furcilia stage, an immature juvenile emerges in a shape similar to an adult, and subsequently develops gonads and matures sexually.[54]

During the mating season, which varies by species and climate, the male deposits a sperm sack at the female's genital opening (named thelycum). The females can carry several thousand eggs in their ovary, which may then account for as much as one third of the animal's body mass.[55] Krill can have multiple broods in one season, with interbrood intervals lasting on the order of days.[25][56]

Krill employ two types of spawning mechanism.[25] The 57 species of the genera Bentheuphausia, Euphausia, Meganyctiphanes, Thysanoessa, and Thysanopoda are "broadcast spawners": the female releases the fertilised eggs into the water, where they usually sink, disperse, and are on their own. These species generally hatch in the nauplius 1 stage, but have recently been discovered to hatch sometimes as metanauplius or even as calyptopis stages.[57] The remaining 29 species of the other genera are "sac spawners", where the female carries the eggs with her, attached to the rearmost pairs of thoracopods until they hatch as metanauplii, although some species like Nematoscelis difficilis may hatch as nauplius or pseudometanauplius.[58]

Some high-latitude species of krill can live for more than six years (e.g., Euphausia superba); others, such as the mid-latitude species Euphausia pacifica, live for only two years.[7] Subtropical or tropical species' longevity is still shorter, e.g., Nyctiphanes simplex, which usually lives for only six to eight months.[62]

3a8082e126