Dana, 1852
Euphausiacea are small shrimp-like crustaceans. Like other members of the Superorder Eucarida, they have a carapace fused to the entire thorax and have stalked, compound eyes. Their basic body plan includes five cephalic, eight thoracic, and six abdominal segments. The first two body regions are fused as a cephalothorax. Their cephalic appendages consist of paired first antennae with peduncles and two flagella, second antennae with one flagellum and scale (squame), and the following mouthparts: labrum; mandibles usually with palps; labia; and first and second maxillae. The relatively small size of the exopod on the second maxilla is thought to be characteristic of Euphausiacea.
Euphausiaceas bear eight pairs of thoracic legs, which are similar in shape and function in the genus Bentheuphausia, but variously modified in other genera. The thoracic limbs bear large, feathery gills on the coxae. The first five abdominal segments bear pleopods and the 6th segment bears uropods and a telson. Most species have photophores, typically arranged in pairs on the eyestalks and bases of thoracic limbs 2 and 7, and as single photophores located mid-ventrally on abdominal segments 1-4. Euphausiids lack statocysts.
The term "euphausiid" is commonly used for all members of the crustacean order Euphausiacea, which contains the two families Euphausiidae and the mono-specific Bentheuphausiidae. Although the ending -id , as in euphausiid, correctly applies only to members of a given family, e.g. Euphausiidae, in common usage it also includes the one species of the second family.
The word Euphausia derives from Greek eu for good or true, combined with -phausia for shining or light emitting. Early naturalists were impressed by the brightness of the photophores of these small animals. Another term, krill, has become synonymous with euphausiid. Krill was first used in this sense by Norwegian whalers who applied it to the swarming little fish (krill), which signalled whale-feeding grounds. A whale may consume many tons of North Atlantic or Antarctic krill in a day.
Larval euphausiids to 4 or 5 mm body length are considered true components of the zooplankton, being dispersed primarily by water circulation. Maturing euphausiids become increasingly capable of directional migration and are sometimes designated micro-nekton. However, this distinction is frequently ignored in analysis of zooplankton net-samples, which, indeed, retain substantial proportions of adults, particularly if collected at night.
Systematics
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.
¥ Family Bentheuphausiacea Colosi, 1917
In the male, the endopod of the 1st pair of pleopods is not modified as a complex petasma. Basipod of 1st pair of pleopods carries 1-6 spines. Endopod of 1st maxilla is two jointed. Endopod of 2nd maxilla is three jointed. Outer uropod has a sub-distal transverse suture. Eyes are poorly developed. All eight thoracic legs are well developed.
¥ Family Euphausiacea Holt and Tattersall, 1905
The 8th thoracic leg in all instances, and the 7th in all genera except Thysanopoda, are reduced in the number of segments or are rudimentary. The 1st to the 8th thoracic legs carry gills, which are retained on the last pair irrespective of the amount of reduction undergone by these appendages. The gills on the anterior thoracic legs have but one main branch, whereas the posterior ones have several. The inner rami of the 1st and 2nd pairs of pleopods in the males are modified, respectively, as a petasma and as an appendix masculina, believed to function during copulation. These structures are of great systematic importance. Bell-shaped luminescent organs are found, usually one on each eyestalk, a pair on the base of the 2nd and 7th thoracic appendages and one ventrally on the 1st four segments of the abdomen between the pleopods. There are some exceptions, notably in the genus Stylocheiron.
Identification
In the present key, the important characters commonly used are specific modifications of the following body parts:
— Peduncle of 1st antenna (lappet, spines, keels)
— Eye (shape, size, enlarged ommatidia)
— Carapace (rostrum, frontal plate, keels, spines)
— Abdomen (dorsal spines, keels, lengths and shapes of segments)
The basic generic characters, involved with the thoracic legs (number, elongation modifications) are avoided, as in zooplankton net samples thoracic legs are rarely intact. Also, carapace lateral denticles, the petasma, or thelycum are avoided as characters in the key, as they are considered difficult.
[After Brinton et al., 1999]
The key to North Sea euphausiids starts at Page 316: Euphausiacea. The following species are included:
Order Euphausiacea
Family Bentheuphausiidae
Bentheuphausia amblyops
Family Euphausiidae
Thysanopoda acutifrons
Thysanopoda microphtalma
Meganyctiphanes norvegica
Nyctiphanes couchi
Euphausia krohni
Thysanoessa gregaria
Thysanoessa inermis
Thysanoessa longicaudata
Thysanoessa raschi
Nematoscelis megalops
Nematobrachion boopis
Stylocheiron abbreviatum
Stylocheiron elongatum
Stylocheiron longicorne
Stylocheiron maximum