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Euphausia tenera Hansen, 1905

Etymology: Euphausia - brightly shining; tenera - delicate

Eye: The eye is round and medium in size (E. tenera eye & rostrum). The eye diameter : carapace length is about 0.18.

Peduncle of 1st Antenna: In the male, the anterior dorsal edge of the 1st segment is raised and rounded, but not extending forward over the 2nd segment as a lappet; in the female, the raised dorsal part is slightly higher than in the male, terminating anteriorly at an acute, rather than blunted angle, but still not extending forward over the 2nd segment. In the male, the 2nd segment ends dorsally as a thin, narrow lobe which overlaps 1/4 of the 3rd segment. In the female, this lobe is shorter and more acute distally. The 3rd segment bears no keel (E. tenera,) (E. tenera dorsal head).

Rostrum: This is short, with the frontal plate obliquely triangular, rarely reaching to the anterior limit of the eye (E. tenera eye & rostrum).

Carapace: The gastric dome is low, not angularly humped. There is a single lateral denticle (E. tenera carapace denticle).

Abdomen: The dorsal posterior edge of the 3rd segment extends as a rounded cover over a short portion of the 4th segment (E. tenera).

Length: Adults are 7-9 mm.

Petasma: The trunk of the terminal process is nearly straight, with the curved distal part ending acutely. The proximal process has a thick curved base and a slender distal half, ending in a four-tooth comb. Opposite this comb is a longer curved process or hook, at the base of which, on the concave margin, is a small process or spine. The lateral process bears an acute secondary tooth on the convex margin of the curving hook-like end (E. tenera petasma).

Thelycum: Described by Guglielmo and Costanzo, 1978.

Comments: The short rostrum, low gastric dome, and rounded or slightly angular dorsal-distal end to the 1st segment of the antennular peduncle are characteristic, distinguishing E. tenera from other Euphausia species which have only medium-sized eyes (i.e. the "E. gibba group"). Also, there is no dorsal spine on the 3rd abdominal segment.

ECOLOGY
E. tenera is an important food source for whales and fish.

HORIZONTAL DISTRIBUTION
E. tenera is a tropical species occurring in all the ocean basins, but it also extends into subtropical parts of the western North Pacific, western North Atlantic, and eastern South Atlantic (E. tenera distribution).

VERTICAL DISTRIBUTION
Most adults are below 150 m in the daytime, down to at least 300 m. At night they are above 140 m, mostly near 50-75 m.

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

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

Carapace:
Shape - The frontal hood is relatively narrow with flatened anterior margin.
Marginal spines - There are two pairs of spines on the anterior margin.
Dorsal crest - There is a pointed crest without spines.

Calyptopis 2-3 (3 stages) (E. tenera B)

Carapace:
Shape - The frontal hood curves down around the eyes. The posterior margin is pointed and there is an angular projection below the eye.
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 - (5 stages) (E. tenera C), (E. tenera D), (E. tenera E)

Eye: The eye is distinctive. The faceted area is nearly rectangular. The lower portion is larger than the upper and has a larger, darker pigmented area.

Carapace:
Frontal plate / rostrum - The frontal plate is truncate in early furcilia and becomes triangular by F5, there is a small rostral spine.
Marginal spines - absent
Postero-dorsal spines - absent
Lateral denticles - A pair of denticles is present.
Dorsal keel - A small 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"4' - 5".

Telson:
Postero-lateral spines - PL3 is longer than PL2.
Lateral spines - One pair of lateral spines is present.

Comments: The distinctive eye is characteristic of the species in the furcilia phase. It is almost the opposite of the eye of E. hemigibba in which the upper portion is both larger and more heavily pigmented.

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

Pictures
E. tenera, selected stages
E. tenera A [metanauplius]
E. tenera B [calyptopis 2-3]
E. tenera C [furcilia 1-2]
E. tenera D [furcilia 3-4]
E. tenera E [furcilia 5]
key to larval illustrations

Euphausia tenera