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Euphausia sibogae Hansen, 1908 and E. distinguenda Hansen, 1911

Etymology: Euphausia - brightly shining; sibogae - after a ship

Eye: The eye is round and medium in size (E. sibogae eye & rostrum). The eye diameter : carapace length is about 0.19 for the smaller species E. sibogae and 0.16 for E. distinguenda .

Peduncle of 1st Antenna: The 1st segment bears a low rounded lobe, barely extending over the base of the second segment. The 2nd segment has the upper distal outer angle produced as a very low rounded process; in large specimens of E. sibogae from the northern Indian Ocean this process is more ridge-like, directed obliquely upward and forward. The 3rd segment has a very low simple keel (E. sibogae,) (E. sibogae dorsal head).

Rostrum: This is short, and obliquely triangular, extending to the midpoint of the eye (E. sibogae eye & rostrum).

Carapace: The gastric area is angularly elevated as a hump. There is one pair of lateral denticles (E. sibogae carapace denticle).

Abdomen: The third segment bears a stout mid-dorsal tooth (E. sibogae abdominal spine).

Length: Adults of E. distinguenda are 10-14.5 mm and of E. sibogae , 8-11.5.

Petasma: The terminal process is heavy, tapering to a narrow, curved tip which appears acute in lateral view. The proximal process does not reach as far as the end of the terminal process; the basal half is thick and curved; the middle portion narrows, the distal half broadens, and the terminal part tapers to an acute, inward-directed end. The median lobe tapers to a rounded end, and the lateral process is strongly hooked, with an acute secondary tooth on the convex margin at or beyond the top of the curvature. The setiferous lobe has six distal setae and one sub-distal with 2-3 smaller setae on the outer margin, the larger specimens having the more setae on the outer margin (E. sibogae petasma).

Thelycum: Described by Guglielmo and Costanzo, 1978.

Comments: E. sibogae and E. distinguenda are distinguishable from the "E. gibba group" by the somewhat angular shape of the gastric keel, the more heavily pigmented (blacker), somewhat larger eyes and by the rounded shape of the anterior-dorsal lappet on the 1st segment of the antenna peduncle. They are easily separated from E. tenera because they possesse a spine on the third abdominal segment.

Sebastian (1966) pointed out that the petasma of his specimens from the Indian Ocean resembled E. sibogae (described by Hansen, 1908, from north of Java) particularly with respect to the shortness of the proximal process relative to that in E. distinguenda from the eastern Pacific. The northern Indian Ocean form is, indeed, more like the East Indian Archipelago form than like the eastern Pacific form. It is intermediate between the two in all characteristics, including size (eastern Pacific E. distinguenda is 10-14.5 mm; the northern Indian Ocean form is 9-11.5 mm; Archipelago specimens are 8-9 mm). We apply the name E. sibogae to the northern Indian Ocean form because the population appears to be at least in tenuous contact with the population of E. sibogae from the region of Java which provided Hansen's type specimens.

ECOLOGY
E. sibogae and E. distinguenda occupy separate tropical regions characterized by high productivity and 02-deficiency below the thermocline.

HORIZONTAL DISTRIBUTION
E. sibogae and E. distinguenda are distinguishable on the basis of distribution. E. distinguenda occurs throughout the Eastern Tropical Pacific, to mid-ocean (about 170°W) in the North Equatorial Current, and to near 130°W in the South Equatorial Current. E. sibogae occurs in the Red Sea, Arabian Sea, Bay of Bengal and around the East Indian Archipelago (E. sibogae distribution).

VERTICAL DISTRIBUTION
Most adults of E. distinguenda live in the upper 50 m at night and near 300-400 m in the daytime. The vertical range of E. sibogae has not been defined.

LARVAE
See the development summary (E. sibogae Table) for the stage descriptors and length in stage.

Metanauplius - undescribed

Calyptopis - (3 stages) (E. sibogae A)

Carapace:
Shape - The posterior margin is pointed.
Marginal spines - absent
Postero-dorsal spine - absent
Lateral denticles - A pair of denticles is present in C3.

Telson:
Postero-lateral spines - PL3 is longer than PL2.

Furcilia - (4 stages) (E. sibogae B), (E. sibogae C)

Eye: The eye is long and nearly cylindrical with rounded faceted area.

Carapace:
Frontal plate / rostrum - The frontal plate is rounded in F1 and becomes triangular in later stages. There is a rostral spine from F2.
Marginal spines - absent
Postero-dorsal spines - absent
Lateral denticles - A pair of denticles is present.
Dorsal keel - A keel is present.

Thoracic legs: There is sequential development without elongate leg (s).

Abdomen:
Mid-dorsal spines - absent

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

Telson:
Shape - The telson is wide and rounded distally in F1.
Postero-lateral spines - PL3 is longer than PL2.
Lateral spines - One pair of lateral spines is present.

Comments: The long cylindrical eyes are characteristic of the species. See figures of E. distinguenda for a view of the eyes and carapace dorsal keel in a closely related species.

Development
(E. sibogae Table), development summary for the stage descriptors and length in stage.

(E. sibogae Graph 1) , a histogram of developmental form frequency, from the southwest coast of India.

Pictures
E. sibogae, selected stages
E. sibogae A [calyptopis 1-3]
E. sibogae B [furcilia 1-2]
E. sibogae C [furcilia 3-4]
key to larval illustrations

Euphausia sibogae