Euphausiacea
Euphausiid morphology

Special attention is placed in the following account on the diagnostic features used in the accompanying key to separate species
generalized euphausiid

—Body
The body is composed of a cephalothorax and an abdomen (generalized euphausiid). Unlike the carapace of mysids, that of euphausiids is attached to all thoracic segments. Features of the carapace useful in identifications include the length and shape of the rostrum or frontal plate (which lies between the eyes), as well as the number of various lateral and post-ocular spines. A mid-dorsal keel and accompanying cervical groove may be present in some species.

—Antennule
The antennule is composed of a three segmented peduncle basally, each segment of which is numbered proximal to distal, and a pair flagellae. The shape of projections (first segment), spines or tubercles (second segment), and keels (third segment) on the basal segments are of use in identification. The anterio-dorsal margin of the first segment may project beyond the base of the second segment, and form what is known as the antennular lappet. This can be rounded or have one (simple), two (bifid) or many (pectinate) spines dorsal view of antennular peduncles.

—Antenna
Each antenna is composed of a basal segment bearing an antennal scale and a two segmented antennal peduncle ending in a long flagellum. The relative size of the scale can be a useful character in separating out some groups of species within the genus Stylocheiron (Stylocheiron).

—Limbs
Limbs are composed of two basal segments, the distal one of which bears a two segmented exopodite and a 5 segmented endopodite (thoracic leg (generalised thoracic limb)). The coxa bears obvious gills on its posterior margin and these are characteristic of euphausiids and can be used to immediately separate them from decapods. Legs are numbered from anterior to posterior, and although there are eight in the primitive condition (Bentheuphausia ambylops), most species have essentially lost the last pair. The length and number of segments present on the seventh leg varies with genus, and the position of this limb can be identified by the presence of a photophore on the coxa. For genera which display eloganted second and/or third legs (Stylocheiron, Nematoscelis, Thysanoessa, Nematobrachion) the arrangement of setae on their more distal segments can be diagnostic. It should be noted, however, that all legs are liable to damage during collection, and elongated limbs are frequently lost.

—Eyes
The eyes of euphausiids may be simple and rounded, or constricted along the midline and bilobed (Stylocheiron, Nematobrachion, Nematoscelis, Thysanoessa). Generally, species within a genus have either rounded or bilobed eyes (exception is Thysanoessa, but not South Atlantic species), and in those with the latter type the size and configuration of the two lobes can be diagnostic. Genera with bilobed eyes also tend to have elongated second or third thoracic legs.
Genera with undivided eyes are e.g. Thysanopoda, Nyctiphanes and Euphausia.

—Abdomen
Variations in the morphology of the abdomen are used to separate many euphausiids at the species level. There are six abdomenal segments which are numbered from anterior to posterior. The dorsal surface of segments three to five may be raised to form a longitudinal keel, the posterior edge of which may be extended as a spine of variable length. The exoskeleton plates covering segments one to five are known as pleurae and the sculpturing of their margins varies with species. Although the sixth segment is cylindrical and lacks pleura, its relative dimensions can be used in the identification of some species.

With the exception of Bentheuphausia ambylops, photophores are present on all the species of euphausiids recovered from the South Atlantic. Their distribution on the body is generally conservative: one at the base of each eye, a pair on the coxae of thoracic segments three and seven, and one mid-ventrally on abdomenal segments one to four. The exception to this pattern includes members of the genus Stylocheiron, which have lost photophores on the coxae of the third thoracic segment and on abdominal segments two to four.

Each abdominal segment carries a pair of pleopods ventrally. These are of uniform structure and are composed of a basal segment bearing an exopodite and endopodite. The endopodites of pleopods on segments one and two are modified in males to form petasmae, or (presumed) copulatory organs. Although the petasma of most euphausiids conforms to a basic pattern, the fine structure is of diagnostic value at the species level. The petasma is composed of four main lobes (setiferous, auxiliary, median and inner), some of which may be enlarged or lost (see generalised petasma). For example, in the genus Pseudeuphausia the auxiliary lobe is missing, whilst the inner and median lobes are coalesced in the genera Stylocheiron and Tessarabrachion. The relative size, shape and arrangement of the different processes on the lobes of the petasma are also variable, and it is here that differences should be sought when separating species. For example, the terminal process on the petasma of Euphausia paragibba is long and tapering, bears a small tooth distally and has a very well developed heel; the lateral process has a series of three minute teeth distally. By contrast, the terminal process of E. gibba is markedly reduced (relative to the proximal process) and lacks a terminal tooth and enlarged heel; the lateral process is strongly curved and lacks teeth of any form.

Sexes can be readily distinguished by the presence or absence of petasmae. In short, if the distal segments of all pleopods resemble each other in appearance then the specimen is female, if they do not, and the differences are associated with pleopods on abdominal segments one and two, then the specimen is male. Males can also be identified by the presence of developing spermatophores which can be visible below the carapace in the region of thoracic segments six and seven. Recently mated females can be readily identified by the presence of one or more spermatophores attached to the thelycum, which is located on the ventral surface of thoracic segments six and seven. The thelycum is the external opening to the female reproductive tract, and it consists of various outgrowths from the ventral body wall and the coxal plates (thorax of Euphausia hemigibba). Whilst it has some diagnostic value, we urge against its use because its observation is difficult and adequate descriptions for all species are missing. However, it is used here to differentiate members of the Euphausia gibba group. Points of difference to notice when examining thelyca include the relative shape and sizes of the coxal and sternal plates. The use of the thelycum in identification has been discussed by Costanzo and Guglielmo (1980), Guglielmo and Costanzo (1977, 1978, 1983) and James (1977).

The morphology of the eyes in combination with the number of thoracic limbs can provide valuable clues to the identification of 7 important genera:

Euphausia
Nematobrachion
Nematoscelis
Nyctiphanes
Stylocheiron
Thysanoessa
Thysanopoda