Our Species, Dichotomous key, and Matrix key modules rely upon adult euphausiid morphology. Full maturity in males is determined by the fully developed petasmas (the male copulatory organ; E. pacifica male photo 2 , petasma detail), and in females by a conspicuously developed or empty ovary, usually visible through the carapace cuticle (E. pacifica female photo 3 , N. simplex female with eggs). Spermatophores developing within, or extruding from, the male vas deferens (E. pacifica male photo 2), or already attached ventrally to the female (E. pacifica female photo 1) are further confirmations of maturity.

Euphausiids, unlike some other crustaceans such as copepods, continue to grow during adulthood. However, the initial adult morphology, both reproductive and somatic, remains fixed. Adult body-length ranges are included with the species descriptions.

In Dichotomous key, the important characters commonly used are specific modifications of the following body parts:

1. Peduncle of 1st antenna (lappet, spines, keels) (peduncle of A1 , lappet shape)
2. Eye (shape, size, enlarged ommatidia) (eye shape , eye size , S. longicorne eye photo)
3. Carapace (rostrum, frontal plate, keels, spines) (rostrums & frontal plates)
4. Abdomen (dorsal spines, keels, lengths and shapes of segments) (abdomen , abdominal spines & keels)

Here we avoid a basic generic character, the thoracic legs (number, elongation modifications). In zooplankton net samples thoracic legs are rarely intact. In Dichotomous key , we rarely refer to carapace lateral denticles, the petasma, or thelycum (in the female, the external pocket on the ventral surface of the thorax used to hold the spermatophore), as they are considered difficult characters. In the Species module we provide illustrations for the petasma and citations to description of the thelycum.