Nematobrachion sexspinosum Hansen, 1911
Etymology: Nematobrachion - thread arms; sexspinosum - with 6 spines
Eye: The upper lobe is somewhat broader than lower. The height of the eye is greatest at the posterior edge. The lobes are equal in height (N. sexspinosum eye & rostrum ).
Peduncle of 1st antenna: The 1st segment has an elevated distal lobe that is truncated at the tip, (in lateral view), that does not overhang the 2nd segment. The forward-directed spine on the distal outer margin of the 1st segment extends to the midpoint of the 2nd segment. The 2nd segment bears a dorsal leaflet-like spine which overhangs the 3rd segment; it is more elevated and spiniform in the male than in the female (N. sexspinosum,).
Rostrum: It extends to the anterior limit of the eye as a slender upright keel, with the base considerably higher than in flexipes. It is straight, angling barely upward above the eyes (N. sexspinosum eye & rostrum ).
Carapace: There is a denticle on the posterior lateral margin (N. sexspinosum, ). A small, post-ocular spine is present behind the eye at each posterior angle of the frontal plate (N. sexspinosum ).
Thoracic legs: The 3rd thoracic leg is greatly elongated (N. sexspinosum).
Abdomen: The 4th and 5th segment each have a short, slender median spine flanked by a pair of shorter spines, one on each sub-dorsal posterior margin(N. sexspinosum abdominal spine). The pleura of abdominal segments 1-5 end posterior-ventrally as acute tooth-like angles. The 6th segment is short, 1.7 times longer than high. In the male there is a ventral photophore on segments 1-4. In the female the 3rd photophore is missing (Mikkelsen, 1987).
Length: Adults are 20-25 mm.
Petasma: The terminal process is short, stocky, and bent just beyond the base, terminating in a broad notch. The proximal process is slender, extending through a semi-circle, and terminating at a point beyond the tip of the terminal process. The lateral process reaches near the tip of the median lobe and is distally hooked. The additional process is half as long as the lateral, the distal half being broadly hooked (N. sexspinosum petasma).
Thelycum: Described by Guglielmo and Costanzo, 1983.
Comments: This species is stockier in appearance than N. flexipes or N. boopis. It has a shorter 6th abdominal segment. It can be further separated from N. flexipes by the 3 dorsal-posterior spines on each of the 4th and 5th segments.
ECOLOGY
N. sexspinosum has been reported in the stomachs of planktivorous fishes.
HORIZONTAL DISTRIBUTION
N. sexspinosum is uncommon, occuring in the western and mid-Pacific Ocean 30°N-30°S, in the Indian Ocean between 5°N and 30°S, and in the Atlantic between about 35°N and 30°S, and the Gulf of Mexico (N. sexspinosum distribution).
VERTICAL DISTRIBUTION
N. sexspinosum appears to inhabit waters between 200 and 100 m.
LARVAE
Undescribed.