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Euphausia mutica Hansen, 1905

Etymology: Euphausia - brightly shining; mutica - curtailed, cut off

Eye: The eye is round and large in size (E. mutica eye & rostrum). The eye diameter : carapace length is 0.20.

Peduncle of 1st Antenna: The lappet on the 1st segment is bifid, low, and forward-directed. Viewed from the side, the dorsal surface of the lappet appears straight, but posteriorly, just anterior to the eye, it angles downward (E. mutica lappet). (This characteristic angle is not conspicuous. It may be clearly seen in lateral view if the specimen is slightly rotated so that the lateral surface of the peduncle is tilted or rotated slightly upward.) The 2nd segment bears no dorsal spine on the outer distal edge, only a slight elevation. The 3rd segment bears a low dorsal keel (E. mutica,).

Rostrum: It is acute, almost straight, extending to the forward limit of the eyes (E. mutica eye & rostrum).

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

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

Length: Adults are 7-12 mm.

Petasma: The distal portion of the terminal process is curved with a tapering acute ending. A small distally-directed spine is situated on the concave margin of the process, near the tip. The proximal process is curved through 90°, terminating in a rounded plate which has a heel-like process on the inner margin directed toward the base of the lateral process. The shape of the median lobe is particularly characteristic: distal to the small, curved lateral process, the median lobe has an acute tip, which curves back toward the origin of the lobe (E. mutica petasma).

Thelycum: Described by Guglielmo and Costanzo, 1978.

Comments: The lappet on the 1st segment of the peduncle of the 1st antenna is lower and more anteriorly directed than in E. brevis, E. diomedeae and E. recurva. The obtuse angle at the posterior upper limit of the lappet, just in front of the eye, may be clearly seen with practice.

HORIZONTAL DISTRIBUTION
E. mutica is subtropical in all ocean basins, but extends into the tropics in the Caribbean Sea, the eastern Atlantic, the Indian Ocean, and the western Pacific. In the mid-South Pacific, 90-170°W, it is restricted to the zone of 10-20°S. It is not known from the western South Atlantic (E. mutica distribution).

VERTICAL DISTRIBUTION
Adults and juveniles are concentrated near 300-400 m in the daytime and above 50 m at night.

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

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

Carapace:
Shape - The frontal hood is rounded and moderately wide.
Marginal spines - The frontal hood is fringed with small spines. There are three pairs of longer spines interspersed on the anterior margin and more than 3 spines between the inner pair of long spines. The spines are tiny in the lateral margins and gradually increase in length progressing anteriorly.
Dorsal crest - The crest is high with a pair of dorsal spines.

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

Eye: The striated body of the developing eyestalk photophore is relatively large.

Carapace:
Shape - The frontal hood curves in medially. The lateral margins are oblique in C1 and curve ventrally around the eyes in C2 and C3.
Marginal spines - The frontal hood is fringed with small spines.
Postero-dorsal spine - A postero-dorsal spine is present.
Lateral denticles - A pair of denticles is present in C3.

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, unlike E. diomedeae and E. eximia . (Euphausia larval maxilliped)

Telson:
Shape - The lateral margins are distinctly indented below the lateral spines and rounded postero-laterally.
Postero-lateral spines - PL2 is longer than PL3.

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

Eye: The eyestalk photophore is relatively large. The faceted area is rectangular and the pigment developes in 3 zones. (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. There is variation in the degree of development.

Carapace:
Frontal plate / rostrum - The frontal plate is rectangular in F1 and becomes broadly triangular in late furcilia. A rostral spine is present from F2, it is small in early furcilia.
Marginal spines - The frontal plate is fringed with spines through F3, the number of spines is reduced in F4 and there may be a few small spines in F5.
Postero-dorsal spines - A spine is present in F1 only.
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:
Shape - In early furcilia stages, the postero-lateral margins are rounded.
Postero-lateral spines - PL2 is longer than, or equal to, PL3 in early to mid- furcilia. 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. mutica may be distinguished by the relatively large striated body in the developing eyestalk photophore and by the shape of the telson, especially the rounded postero-lateral margins. The setation of the maxilliped in C1 may be used to separate E. mutica from E. eximia and E. diomedeae .

In the furcilia phase, the shape of the telson, particularly in early stages, and the proportions of the eye and eyestalk photophore are the most useful features for distinguishing the species. It is more difficult to separate the larvae of E. mutica from E. recurva 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 between and within populations, is proportionally larger than the photophore in E. recurva and is helpful in comparable stages when the species co-occur. The postero-lateral margins of the telson are more rounded in E. mutica. In E. recurva , they tend to be angular, and PL2 is relatively 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. mutica Table), development summary for the stage descriptors and length in stage.

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

Euphausia mutica