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Nematoscelis tenella G.O. Sars, 1883

Etymology: Nematoscelis - thread leg; tenella - quite delicate

Eye: The eye is bilobed and medium in size. The upper lobe is much larger than the lower, 1 1/2 times wider (N. tenella eye detail photo ).

Peduncle of 1st Antenna: The 1st segment ends dorsally as a simple angle, somewhat higher than the 2nd segment (N. tenella,).

Rostrum: It is obliquely triangular, reaching about to the mid-point of the eye (N. tenella dorsal head). It is horizontal in lateral view, and is the same shape in both sexes (N. tenella rostrum).

Carapace: The gastric region is elevated but with a short, low keel or no keel at all. There are no lateral denticles in adults (N. tenella).

Thoracic legs: The 1st leg has one long seta on the dorsal surface of the dactylus; the propodus has two rows of setae, of 3 and 5 setae respectively. The elongate 2nd thoracic leg has a cluster of long straight bristles arising from both the dactylus and propodus (N. tenella thoracic leg).

Abdomen: The abdomen is slender, contributing to an elongate appearance of the body and there are no dorsal spines or keels (N. tenella).

Length: Adults are 13-20 mm, females attaining the larger sizes.

Petasma: The spine-shaped and terminal processes are both very small; the spine-shaped is the longer, but, owing to differences in place of origin, the tips extend equi-distantly. The proximal process is long, and straight, tapering to a round end which bears saw-teeth on the outer margin. The lateral process is similar in shape to the proximal, extending a little farther and with rounded rather than saw-like teeth on the distal margin (N. tenella petasma).

Thelycum: Described by Sebastian, 1966; Costanzo and Guglielmo, 1980.

Comments: The relatively very large upper lobe of the eye (N. tenella eye photo) is characteristic of N. tenella , within the genus. The body is distinctly slender. The small spine-shaped and terminal processes of the petasma resemble only N. gracilis.

ECOLOGY
N. tenella is a food source for fishes.

HORIZONTAL DISTRIBUTION
N. tenella is distributed throughout the tropical-subtropical oceanic areas of all 3 ocean basins, where depths exceed ca. 300 m, except areas with extreme 02-deficiency at depths of 200-500 m (the eastern Tropical Pacific ca. 10-20°N and 10-20°S, and the northern Indian Ocean ca. 10-20°N). It is also lacking in the Mediterranean Sea (N. tenella distribution).

VERTICAL DISTRIBUTION
Juveniles and adults occur throughout about 100-450 m by night (maximum at 300-400 m, below the thermocline), and at about 200-450 m by day. Larvae are found above the thermocline, ca. 100-150 m.

LARVAE
See the development summary (N. tenella Table) for the stage descriptors and length in stage.

Metanauplius - undescribed

Calyptopis - undescribed

Furcilia - (3 stages)

Carapace:
Frontal plate / rostrum - The frontal plate is rectangular with truncated anterior end and small rostral spine. It is usually curved downward.
Marginal spines - absent
Postero-dorsal spines - absent
Lateral denticles - A pair of denticles is present.
Dorsal keel - A relatively large, hump-shaped keel is present.

Thoracic legs: There is precocious growth of Leg 2, the elongate leg of the adult. During the furcilia phase the dactylus develops spines.

Abdomen:
Mid-dorsal spines - absent
Segment 6 - The sixth segment is relatively short and its ventral margin is largely convex.

Pleopods: The developmental pathway is 1'- 1"3'- 4"1' - 5".

Telson:
Postero-lateral spines - PL3 is longer than PL2.
Lateral spines - One pair of lateral spines is present.
Terminal spines - The terminal spines each have a pair of conspicuous sub-spines and have setules proximal to the subspines.

Comments: The furcilia larvae of N . tenella are characterized by the particularly high and long carapace dorsal keel, the downward curving rostrum, and a short sixth abdominal segment.

The characters used to identify the furcilia larvae of Nematoscelis species are carapace frontal plate and rostrum, carapace dorsal keel, pleopod developmental pathway, and armature of terminal telson spines. See Nematoscelis larval characters (Nematoscelis A) and (Nematoscelis B).

Development
(N. tenella Table), development summary for the stage descriptors and length in stage.

Pictures
N. tenella, selected stages
Nematoscelis A [species characters]
Nematoscelis B [species characters]
key to larval illustrations

Nematoscelis tenella