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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 more abundant Euphausiidae contains 10 different genera with a total of 85 species.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.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.The largest krill species is the bathypelagic Thysanopoda spinicauda.Except for Bentheuphausia amblyops, krill are bioluminescent animals having organs called photophores that can emit light.Most krill species display large daily vertical migrations, thus providing food for predators near the surface at night and in deeper waters during the day.
Of this, over half is eaten by whales, seals, penguins, squid, and fish each year, and is replaced by growth and reproduction.
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.
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), No extant fossil can be unequivocally assigned to Euphausiacea.