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

Etymology: Euphausia - brightly shining; distinguenda - separating

Eye: The eye is round and medium in size (E. distinguenda 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 margin produced as a low, rounded transverse elevation; in large specimens of E. sibogae from the northern Indian Ocean this is more ridge-like, directed obliquely upward. The 3rd segment has a low rounded keel (E. distinguenda, ).

Rostrum: The short obliquely triangular frontal plate bears a short acute rostrum extending near to the midpoint of the eye (E. distinguenda eye & rostrum).

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

Abdomen: The third segment bears a stout mid-dorsal tooth (E. distinguenda 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. distinguenda petasma).

Thelycum: Described by Sebastian, 1966; Guglielmo and Costanzo, 1978.

Comments: The geographically separated sibling species E. sibogae and E. distinguenda (each with a spine on the 3rd abdominal segment) are distinguishable from the superficially similar 4 species of "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 first antennular segment. E. distinguenda and E. sibogae are easily separated from E. tenera which has no spine on the 3rd abdominal segment.

Sebastian (1966) pointed out that the petasma of his specimens from the Indian Ocean resembles 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). Here, we will also apply the name E. sibogae to the Indian Ocean form because that 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. distinguenda distribution). Records of E. distinguenda from the Indian Ocean are now believed to be of E. sibogae or of a geographical varient of that species (e.g. Sebastian 1966, Brinton 1975).

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. distinguenda Table) for the stage descriptors and length in stage.

Metanauplius - (1 stage) (E. distinguenda A)

Carapace:
Shape - The frontal hood is evenly rounded.
Marginal spines - There are strong spines spaced around the margin of the frontal hood, with smaller spines interspersed.
Dorsal crest - There is a pointed crest without spines.

Calyptopis - (3 stages) (E. distinguenda B)

Carapace:
Shape - The posterior margin is pointed in C2-C3.
Marginal spines - absent
Postero-dorsal spine - absent
Lateral denticles - A pair of denticles is present in C3.
Dorsal crest - There is a small crest in C1 and a very distinct high keel in C2-C3.

Telson:
Postero-lateral spines - PL3 is longer than PL2. The spinules on the postero-lateral and terminal spines are relatively strong and evenly spaced.

Furcilia - (4 stages) (E. distinguenda C)

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

Carapace:
Frontal plate / rostrum - The frontal plate is rounded in F1 and becomes triangular by F4. There is a small rostral spine.
Marginal spines - absent
Postero-dorsal spines - absent
Lateral denticles - A pair of denticles is present.
Dorsal keel - The keel is high and distinctive with an abrupt drop anteriorly.

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

Abdomen:
Mid-dorsal spines - absent

Pleopods: The common developmental pathways 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 high, distinctive carapace dorsal keel in calyptopis and furcilia stages, and the cylindrical eye of the furcilia larvae, are characteristic of the species. It should be noted that the terminal telson spines begin reduction in number before the larva acquires 5 setose pleopods. See the closely related species E. sibogae for a lateral view of the pleopod development typical of E. distinguenda , and an estimate of variability in furcilia forms.

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

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

Euphausia distinguenda