Brandt, 1851
Generic diagnosis:
In most species the eyes are higher than broad and constricted transversely; in 3 of the species the eye is almost circular, with only a vestige of an upper lobe (eye shape).
The rostrum is always well developed, being long to very long. The peduncles of the 1st antenna usually show sexual differences. The dorsal limit of the 1st segment is not distinctly elevated as a lobe or lappet, but ends as a short blunt angle not overreaching the 2nd segment (except in male T. inermis). The flagella are short in both sexes, but the two distal joints of the peduncle are more slender in the female.
The first six pairs of thoracic legs are normally developed, as in Euphausia . When the 2nd pair is elongate and thick, as it is in most Thysanoessa species, the two terminal segments have spinelike bristles along both edges (Thysanoessa-thoracic leg). The endopod of the 7th thoracic appendage is lacking in the males, but it is present in the females, consisting of either one or two segments. The endopod of the 8th thoracic appendage is absent in both sexes; the exopod is represented by a small styliform process.
The male petasma has the spine-shaped process of the inner lobe thin and curved; proximal, terminal, and lateral processes are well developed. The additional process is usually absent, or, if present, very poorly developed.
The eggs of Thysanoessa are shed before hatching.