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The order Euphausiacea is divided into the families Bentheuphausiidae, with the genus Bentheuphausia , and Euphausiidae, with all the other 10 genera. Characteristics of the Euphausiacea are as follows: gills are exposed specialized podobranches, 1st maxilla with pseudexopod in most genera (true larval exopod persists only in Pseudeuphausia sinica ), 2nd maxilla with relatively small exopod, complex moveable photophores are present on the abdomen (except in certain bathypelagic species), mode of larval development is distinctive.

Identity of the Order Euphausiacea
Arrangement of Genera
Species groups within Genera
Thysanopoda species groups
Euphausia species groups
Thysanoessa species groups
Nematoscelis species groups
Stylocheiron species groups

Identity of the Order Euphausiacea

Historical evolution of the systematic position of the Euphausiacea and rationale for various orderly arrangements of families and genera is reviewed by Mauchline and Fisher (1969). Most 19th Century carcinologists, including G.O. Sars (1885) who, in his "Challenger" Report, introduced 20 new euphausiid species, grouped mysids and euphausiids together in the Schizopoda, shrimp-like forms with "split," biramous legs. About the same time, Boas (1883) demonstrated fundamental differences in thoracic anatomy between the two groups and defined the orders Euphausiacea and Mysidacea. Calman (1904) then proposed that not only euphausiids and mysids, but also Natantia (swimming) Decapoda, all shared basic shrimp-like characters, comprising a common caridoid facies : carapace, moveable eyes, scale-like exopod on second antenna , elongate abdomen, and tail fan. Calman also determined that both euphausiids and natantian decapods, unlike mysids, have the carapace dorsally fused with all 8 thoracic segments.

H.J. Hansen continued to use "Schizopoda" to encompass mysids and euphausiids throughout a career of four decades in which he described 32 new euphausiid species and greatly enhanced our knowledge of most others. In his final expedition report (1927) entitled "Sergestides et Schizopodes," in which he included sergestid natantia for the first time, he explained that "Schizopoda" had become a common, purely practical expression; he had long recognized euphausiids and mysids as distinct.

There are similarities in larval development of euphausiids and decapod Sergestidae, which also indicate affinities. Both groups have, predominantly, free nauplius larvae (Gurney, 1942; Gordon 1955). Supported by earlier letters from Stanley Kemp and Calman, Isabella Gordon considered uniting the Euphausiacea and the sergestid Penaeidea as a suborder coordinate with the rest of the Decapoda. Brinton (1966) suggested that coincidence in the two groups of 1) free nauplii, 2) metamorphic post-naupliar development, 3) reduction of posterior thoracic legs, and 4) presence of petasma and thelycum might be parallel adaptations to pelagic life at similar euphotic and mesopelagic depths. Unclear relationships of this sort might now be addressed by means of molecular genetics.

Arrangement of Genera

The order in which genera and species are treated in taxonomic and distributional reports on euphausiids varies, but has tended to follow phylogenetic patterns inferred by the specialists of the early 1900's, most notably H.J. Hansen. Recognizing that Bentheuphausia possesses all 8 thoracic legs (endopods and exopods), has a transverse suture on the outer branch of uropod (as in certain mysids), and lacks photophores and complex petasma, we presume that this distinctive species most closely represents ancestral stock and is consistently listed first as a monospecific family. The remaining 10 genera are arranged in various nominal sequences considered by different specialists to represent their affinities with each other, especially with their degrees of departure from Bentheuphausia in reduction and/or specialization of thoracic legs, e.g.: see Table (sequence of genera).

Species groups within Genera

These groupings derive from observations by several authors who have recognized affinities of unclear taxonomic integrity within genera. These relationships are more subjective than those at the generic level but seem to be useful in further relating phylogeny to ecology. They supplement the important generic distinctions, above, and the more complete descriptions under Genera.

Thysanopoda species groups
T. cornuta, T. spinicaudata, T. minyops, T. egregia
-Abdominal segments 4, 5 each with 3 short rows of integumental pits in depressed
grooves which have elevated keel-like borders.
-Carapace without lateral denticles, but with prominent laterally-branching cervical
-Abdominal segment 6 short, length = or slightly > segment 5.
-Very large species, crimson color, bathypelagic.

T. acutifrons, T. orientalis, T. microphthalma.
-Lappet thick with forward-directed apical tooth.
-Frontal plate with upward directed spine.
-Carapace without lateral denticles.
-Petasmas very similar.
-Large species, mesopelagic.

T. aequalis, T. astylata, T. obtusifrons.
-Lappet thin, scale-like where applied to dorsal side of A1 segment 2.
-Frontal plate rounded, sometimes bears a minute vertical tooth.
-Carapace with one pair of lateral denticles.
-Medium-sized species, epipelagic.

Thysanopoda species not grouped: T. tricuspidata, T. cristata, T. monacantha,
T. pectinata.

Euphausia species groups
-Rostrum long, acute; calyptopis and early furcilia larvae with carapace having anterior marginal spines.

E. brevis, E. diomedeae, E. mutica, E. recurva
-lappet distally bifurcate (simple in male E. recurva only).
-abdomen without mid-dorsal spines

E. eximia, E. americana, E. krohni
-lappet pectinate
-abdomen without mid-dorsal spines

Euphausia gibboides group”: E. gibboides, E. fallax, E. sanzoi.
-lappet a simple rounded projection
-eye and body large (TL, 16-27 mm)

E. longirostris, E. spinifera, E. hanseni
-lappet with 2 or more distal teeth
-abdomen with mid-dorsal spines on segments 3-5

-Rostrum medium-short, acute; calyptopis and furcilia larvae with carapace having smooth anterior margin.

Euphausia gibba group”: E. gibba, E. hemigibba, E. paragibba, E. pseudogibba.
-lappet a simple tooth
-abdomen with mid-dorsal spine on segment 3
-body shape uniform among species

-Rostrum lacking, frontal plate angularly obtuse; calyptopis and furcilia larvae with carapace having smooth anterior margin.

E. mucronata, E. lamelligera, E. sibogae, E. distinguenda
-abdominal segment 3 with mid-dorsal spine

E. pacifica, E. nana, E. lucens, E. frigida.
-abdominal segments without mid-dorsal spines

Euphausia species not grouped: E. similis, E. tricantha, E. superba, E.
crystallorophias, E. vallentini.

Thysanoessa species groups
T. inermis, T. raschii, T. spinifera
-Eye round or nearly so
-2nd thoracic leg not, or only slightly, elongate
-Neritic habitats and subarctic oceanic.

T. gregaria, T. parva, T. longicaudata, T. vicina, T. macrura, T. longipes,
T. inspinata.
-eye distinctly bilobed, upper lobe smaller
-2nd thoracic leg distinctly elongate
-temperate oceanic

Nematoscelis species groups
N. difficilis, N. megalops.
-2nd thoracic leg with apical cluster of long spines arising from both dactylus and
-larval telson without conspicuous subspines on terminal telson spine
-temperate oceanic

N. tenella, N. gracilis, N. microps, N. atlantica, N. lobata
-2nd thoracic leg with apical cluster of long spines arising only from dactylus
-larval telson with conspicuous subspines on terminal telson spines
-subtropical-tropical oceanic

Stylocheiron species groups
Stylocheiron carinatum group”: S. carinatum, S. armatum.
-3rd thoracic leg with false chela (grasping organ) formed of simple, curving
bristles from propodus opposing similar bristles on dactylus.
-upper lobe of eye with only slightly enlarged distal crystalline cones.

Stylocheiron longicorne group”: S. longicorne, S. affine, S. suhmi,
S. microphthalma, S. elongatum, S. indicum, S. insulare
-3rd thoracic leg with false chela on 3rd thoracic leg formed of finely serrate, curved
bristles from propodus extending distally so as to oppose similar bristles on
-upper eye with enlarged distal crystalline cones

Stylocheiron maximum group”: S. maximum, S. abbreviatum, S. robustum
-3rd thoracic leg with true chela
-eye without enlarged crystalline cones

Monospecific genera: Bentheuphausia, Meganyctiphanes, Tessarabrachion

Genera each consisting of a single group of species: Pseudeuphausia, Nyctiphanes, Nematobrachion

Krill (Order Euphausiacea)