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Nematoscelis gracilis Hansen, 1910

Etymology: Nematoscelis - thread leg; gracilis - slender, thin

Eye: The eye is bilobed and medium in size. The height of eye/carapace length = about 0.2. The upper lobe is almost as broad as the lower lobe, and appears to be tilted somewhat anteriorly in relation to the lower lobe (N. gracilis eye & rostrum).

Peduncle of 1st Antenna: There is no lappet, but the anterior dorsal apex of the 1st segment appears in lateral view, as a short acute projection (N. gracilis, ). The 2nd segment sometimes has a cluster of small but conspicuous internal oil-like globules (N. gracilis globules).

Rostrum: It is short, acute in both sexes, and somewhat upturned, extending forward about to the midpoint of the obliquely tilted eye (N. gracilis eye & rostrum) (N. gracilis rostrum).

Carapace: There is a small ill-defined dorsal keel. There are no lateral denticles in adults (N. gracilis).

Thoracic legs: The 1st leg has the dactylus straight, and without setae on the dorsal surface. The propodus is slender with almost parallel margins, and with setae arranged in two rows, three setae form an inner row and there are four on the dorsal row (N. gracilis thoracic leg). The 2nd leg has a cluster of long apical bristles arising from both the dactylus and propodus.

Abdomen: There are no dorsal keels or distinguishing characteristics (N. gracilis).

Length: Adults are 11-15.5 mm.

Petasma: The spine-shaped and terminal processes are both very short as in N. tenella. Two geographical variants of N. gracilis distinguished by Gopalakrishnan (1975) are based on differences in the relative lengths of the proximal process, the lateral process and the median lobe. In N. gracilis "Old Form" the proximal process is much shorter than the median lobe and in the Pacific population does not reach to the limit of the lateral process, while in the Indian Ocean population the proximal process extends beyond the limit of the lateral process, but not as far as the limit of the median lobe. In N. gracilis "New Form", the long proximal process extends well beyond the end of the median lobe and is twice as long as the lateral process. The distal part of the proximal process bears saw-teeth in "Old Form" but not in "New Form" (N. gracilis petasma). Gopalakrishnan noted petasma morphology which in some specimens was intermediate between the two forms.

Thelycum: Described by Einarsson, 1942; Sebastian, 1966; Costanzo and Guglielmo, 1980.

Comments: The short, slightly upturned rostrum is useful as a distinguishing character in N. gracilis. The eye is a little smaller than in N. microps, and barely larger than in N. atlantica. The upper lobe tilts somewhat forward in a characteristic way in this species.

Throughout the genus, the peduncle of the 1st antenna of the male has the two distal segments thicker and shorter than in the female (N. gracilis dorsal head). A short basal part of the lower flagellum of the 1st antenna is much thickened in the male. Only in N. gracilis does the proximal part of this flagellum bend sharply downward just beyond this enlarged basal part. Within the 2nd segment of the peduncle of the 1st antenna, globules, apparently of glandular origin are often clearly visible (N. gracilis globules). The opaque state of these globules is characteristic of N. gracilis.

The ventral photophore on the 1st abdominal segment of N. gracilis is occasionally extremely enlarged.

ECOLOGY
N. gracilis is a member of a tropical group of species occurring in the Pacific-Indian Ocean equatorial belt including the 02-deficient regions, (cf. E. diomedeae, E. distinguenda-E. sibogae, S. microphthalma).

HORIZONTAL DISTRIBUTION
The combined range of the Old and New Forms is 20°N-20°S in the Pacific and Indian ocean basins, but it extends a little farther north and south of 20° at the eastern and western edges of those regions, and even to 40°N within the Kuroshio, near Japan. In the Indian Ocean the Old Form is replaced south of the equator by the New Form. In the Pacific the Old Form is replaced by the New Form in the zone of about 10°-20°S, except east of 100°W off South America (N. gracilis distribution).

VERTICAL DISTRIBUTION
In the Eastern Tropical PacificN. gracilis Old Form adults and juveniles are centered near 400 m by night. Adults are at about 100-200 m by day (within the thermocline). Larvae are above 100 m.

LARVAE
See the development summary (N. gracilis 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 a truncated anterior end and a small rostal spine.
Marginal spines - absent
Postero-dorsal spines - absent
Lateral denticles - A pair of denticles is present.
Dorsal keel - A small, usually triangular 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 ventral margin of the sixth segment is not 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: Brinton (1975) notes that the larvae of theN. atlantica -N.microps -N . gracilis complex of subtropical-tropical Nematoscelis species have proven difficult to identify.

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. gracilis Table), development summary for the stage descriptors and length in stage.

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

Nematoscelis gracilis