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Euphausia recurva Hansen, 1905

Etymology: Euphausia - brightly shining; recurva - bending backward

Eye: The eye is round and large (E. recurva eye & rostrum) (E. recurva head photo). The eye diameter : carapace length is about 0.21.

Peduncle of 1st Antenna: The lappet on the 1st segment is sexually dimorphic (E. recurva,). In males it is a vertical or slightly reflexed leaf-like process, the base of which is half as wide as the segment, and it narrows distally. It is long, hollowed anteriorly and reaches to the upper margin of the eye. In females, this lappet is a slightly shorter vertical process, also half as wide as the segment, and is forked distally into two acute parts, the inner part being the longer (E. recurva female lappet photo) It is less conspicuously hollowed anteriorly than in the male. The 2nd segment in both sexes has a short, forward-directed conical process on the outer distal margin and a longer, more acute process on the inner distal margin. The 3rd segment bears a high keel, notched anteriorly so as to form an anteriorly-directed dorsal tooth.

Rostrum: It is acute and straight, extending to the forward limit of the eyes (E. recurva eye & rostrum) (E. recurva dorsal head).

Carapace: There are two pairs of lateral carapace denticles (E. recurva carapace denticle).

Abdomen: There are no dorsal spines or specific features (E. recurva).

Length: Adults are 10-14 mm.

Petasma: The trunk of the terminal process is straight; the distal part is thick, but the short tip is curved beyond the origin of a slender straight spine that extends distally to the underside of the curving end of the process. The proximal process is only slightly curved throughout its length, broadening distally and terminating as a truncated plate that is indented (cleft) in large specimens (E. recurva petasma).

Thelycum: Described by Guglielmo and Costanzo, 1978.

Comments: The lappet on the 1st segment of the peduncle of the 1st antenna is broader and higher than in E. brevis; in E. recurva (female) the outer branch of the forked tip of this lappet is shorter than the inner branch, but the outer branch is the longer of the two branches in E. diomedeae. In E. brevis the two branches are of equal length. The eye is somewhat larger in E. recurva than in E. brevis . The shape of the keel on the 3rd segment also distinguishes E. recurva from E. diomedeae, E. brevis and E. mutica.

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

HORIZONTAL DISTRIBUTION
E. recurva is a subtropical species in all ocean basins, but has been believed lacking in the North Atlantic (Brinton, 1975). Its presence at 3 localities 36-51°N in the North Atlantic, reported by Karpinskji (1991) is of much interest and needs to be confirmed (E. recurva distribution).

VERTICAL DISTRIBUTION
Adults are above the thermocline at night, up to the surface. In the daytime they are at 200-500 m.

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

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

Carapace:
Shape - The frontal hood is rounded and moderately wide.
Marginal spines - The frontal hood is fringed with small marginal spines with 3 pairs of longer spines interspersed on the anterior margin. There are more than 3 spines medially between the inner pair of long spines. There are small spines around the posterior margin which may be visible in dorsal view.
Dorsal crest - There is a high dorsal crest with a pair of dorsal spines.

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

Carapace:
Shape - The frontal hood curves in medially. It is rounded and expanded laterally in C1 and curves around the eyes in C2 and C3. It appears wider than in calyptopes of related species.
Marginal spines - The frontal hood is fringed with small spines.
Postero-dorsal spine - A strong, relatively long postero-dorsal spine is present.
Lateral denticles - A pair of denticles is present.

Leg 1 (maxilliped) - The stout seta on the medial margin of endopod segment 1 and the stout seta on the basis are similar in length in C1. (Euphausia larval maxilliped)

Telson:
Shape - The lateral margins curve in below the lateral spines. This is most marked in C1-C2. The postero-lateral margins tend to be more angular than those of E. mutica .
Postero-lateral spines - PL2 is longer than PL3.

Furcilia - (6 stages) (E. recurva C), (E. recurva D), (E. recurva E)

Eye: The ocular photophore is large but smaller than in the related species E. mutica in comparable stages. The facted area is rectangular with the developing pigment in 3 zones; it tends to be higher than in E. mutica . (Euphausia larval eye)

A1: In F6, the angle of the developing lappet on the first segment of the peduncle may be useful in identifying the species.

Carapace:
Frontal plate / rostrum - The frontal plate is rectangular in F1-F2 and becomes broadly triangular in late furcilia, a tiny medial spine is present in F2 and the rostral spine lengthens in F5-F6.
Marginal spines - The frontal plate is fringed with spines through F4. There may be a few small spines in F5.
Postero-dorsal spines - A spine is present in F1 only, it is relatively strong and long.
Lateral denticles - A pair of denticles 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 - PL2 is longer than or equal to, PL3 in F1-F4. PL3 is longest in late furcilia larvae.
Lateral spines - One pair of lateral spines is present.

Comments: Larvae of Euphausia species group 1A (Brinton, 1975), E. brevis, E. diomedeae, E. mutica, E. recurva, E. eximia and E. krohni are very similar. E. americana is undescribed.

See the closely related species E. eximia for a lateral view of the development of the second antenna, thoracic legs, and pleopods.

The calyptopis stages of E. recurva may be distinguished by 1) large body size 2) a carapace with flared frontal hood, especially in C1, and a long postero-dorsal spine with dorsal tilt, and 3) telson with postero-lateral spine 2 proportionally long in C2 and C3. The setation of the maxilliped may be useful in C1.

In the furcilia phase the most useful features for distinguishing E. recurva are 1) proportions of eye and eyestalk photophore, and 2) relative length of PL2 in early furcilia and the shape of the telson. It is more difficult to separate the larvae of E. recurva from E. mutica than from other closely related species until F6 when the shape of the developing lappet on the A1 peduncle may identify the species. The size of the eyestalk photophore, although variable within a population is proportionally smaller, less high, than in E. mutica in comparable stages and may be helpful when the species co-occur. The posterior margins of the telson are more angular in E. recurva and PL2 is longer in C1-C3 and F1-F2. Often it may not be possible to separate the species in F3-F5 when the species "look" is not well developed.

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

Pictures
E. recurva, selected stages
E. recurva A [metanauplius]
E. recurva B [calyptopis 1-3]
E. recurva C [furcilia 1-2]
E. recurva D [furcilia 3-4]
E. recurva E [furcilia 5-6]
Euphausia larval eye
Euphausia larval maxilliped [species characters]
key to larval illustrations

Euphausia recurva