Nematobrachion flexipes (Ortmann, 1893)
Etymology: Nematobrachion - thread arms; flexipes - with bent foot
Eye: The eye is large, with the width of the upper lobe greater than the width of the lower. The lobes are of equal height, with a constriction between them (N. flexipes eye & rostrum).
Peduncle of 1st antenna: The 1st segment is elevated distally forming a short, almost vertical lobe which appears abruptly truncated (in lateral view), and does not overhang the 2nd segment. There is a forward-directed spine on the distal outer margin of the 1st segment extending to at least half the length of the 2nd segment. The proximal parts of the 2nd and 3rd segments are slender, as compared with the adjacent distal parts of the 1st and 2nd segments. The 2nd segment bears a long, strong, forward-and slightly upward-directed spine at the outer dorsal angle (N. flexipes,).
Rostrum: It extends to the anterior limit of the eye as a slender keeled extension of the frontal plate (N. flexipes eye & rostrum) (N. flexipes dorsal head).
Carapace: There is a small lateral denticle near the lower margin of the carapace (N. flexipes carapace denticle). The anterior part of the carapace has a well-developed keel.
Thoracic legs: The 3rd thoracic leg is greatly elongated, terminating in a cluster of 5 long straight spines (N. flexipes thoracic leg).
Abdomen: The 3rd to the 6th segments are armed mid-dorsally with spines. The spine on the 3rd segment is longer than the others (N. flexipes abdominal spine). The 6th segment is twice as long as high. The male lacks a ventral photophore on the 3rd abdominal segment, and the 4th photophore is reduced in size. The female lacks the 2nd and 3rd abdominal photophores.
Length: Adults are 20-23 mm.
Petasma: The terminal process is broad, but strongly constricted medially; the distal part is irregularly ovoid and is distally cleft. The proximal process is longer than the terminal, slender, curving 180°, and terminating in a small rounded plate. The lateral process is long, reaching to the tip of the median lobe, and distally hooked. The additional process is half as long as the lateral process, and the distal one-third is sharply bent, but extends beyond the tip of the median lobe (N. flexipes petasma).
Thelycum: Described by Sebastian, 1966; Guglielmo and Costanzo, 1983.
Comments: N. flexipes can easily be separated from N. sexspinosum on the basis of the following: it has a spine rather than a leaflet-like spine on the 2nd segment of the peduncle of the 1st antenna, the 3rd to 6th abdominal segments are each with one median spine, while in N. sexspinosum there is an additional spine on each side of the median spine on the posterior margins of each of the 4th and 5th segments.
ECOLOGY
N. flexipes has been reported in the stomachs of planktivorous fishes.
HORIZONTAL DISTRIBUTION
N. flexipes occurs patchily across the Pacific 40°N-35°S, most consistently in the equatorial eastern-Pacific 10°N-10°S, the California Current, and the Peru-Chile Current. In the eastern Gulf of Alaska it has been caught to 56°N. It is present in the tropical and subtropical waters of the Indian Ocean 10°N-40°S, and in the Atlantic between 45°N and 40°S (N. flexipes distribution).
VERTICAL DISTRIBUTION
Adults have been found between 100 m and 600 m.
LARVAE
Undescribed.
Because little is known of this species, information is presented in a format different from that for other species.
N. flexipes larvae may be recognized by the shape and pigmentation of the eye (N. flexipes eye detail photo) and by a large oil globule in the thorax which is frequently present.
In the photograph of a furcilia with 3"2' pleopods (N. flexipes photo ), the larva is slender with eyes that resemble those of some Stylocheiron species, especially S. longicorne . The facted area is cylindrical and higher than wide. The eyes of N. flexipes and S. longicorne differ in the distribution of pigment in the eye (S. longicorne vs. N. flexipes). In N. flexipes there is pigment in the upper and lower portions with a small band near the lower pigmented area. In S. longicorne , the whole lower half of the eye is pigmented.
If present the oil globule may be used to separate Nematobrachion flexipes from species of Stylocheiron . N. boopis also has an oil globule in the thorax (Mauchline 1985), but the eyes differ in the two species of Nematobrachion .
Pictures
N. flexipes, furcilia
N. flexipes photo [furcilia]
N. flexipes eye detail photo [furcilia]
N. flexipes pleopods photo [furcilia]
S. longicorne vs. N. flexipes [furcilia eye photo]
key to larval illustrations