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Euphausia triacantha Holt and Tattersall, 1906

Etymology: Euphausia - brightly shining; triacantha - 3 spined

Eye: The eye is round and medium in size (E. triacantha eye & rostrum). The eye diameter : carapace length is about 0.16.

Peduncle of 1st Antenna: The lappet on the 1st segment is bifid, the inner process is longer than the outer, with its distal end turned outwards. There is a tooth on the distal end of the 2nd segment. The 3rd segment has a low dorsal keel (E. triacantha,) (E. triacantha lappet).

Rostrum: It is long, terminating in an acute triangle beyond the front of the eyes (E. triacantha eye & rostrum) (E. triacantha dorsal head).

Carapace: The carapace is expanded somewhat laterally over each eye-stalk. There are no post-ocular or hepatic spines. On each side there is a low mound-like projection at the position where hepatic spines occur on certain species. There is one pair of lateral denticles near the lower edge of the carapace (E. triacantha carapace spine).

Abdomen: The 3rd-5th segments have dorsal spines (E. triacantha ), the same as in E. spinifera and E. longirostris.

Length: Adults are 24-41 mm.

Petasma: The terminal process has a wide foot-like base with a high heel. The whole process tapers inwards from heel to the distal end. About half-way up from the heel there is a short finger-like structure. The proximal process curves inwards about halfway and outwards again near the end. Its proximal portion is moderately thick. The lateral process is hook-shaped and has no secondary spines or projections on it. There is a small additional spine on the median lobe (E. triacantha petasma).

Thelycum: Described by Lomakina, 1972; Guglielmo and Costanzo, 1978.

Comments: E. triacantha, E. hanseni, E. spinifera, and E. longirostris form John's species group d. They all have mid-dorsal spines on the 4th-5th abdominal segments and are similar in body size. E. triacantha can be separated from the other three species because it does not have a post-ocular spine, or a hepatic spine and there are no incisions on the pleura of the 3rd-5th abdominal segments. Also, the eye of E. triacantha is the smallest in this species group.

HORIZONTAL DISTRIBUTION
E. triacantha is a circumpolar species. It occurs between 50°S and 65°S, living in subantarctic waters southwards to the northern limits of the East Wind and Weddell Drifts (E. triacantha distribution). Occasional individuals are found in coastal waters of the Antarctic Continent.

VERTICAL DISTRIBUTION
E. tricacantha lives between 250 and 750 m depth during the day, and above 250 m depth at night.

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

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

Carapace:
Shape - The body is domed, the frontal hood is wide and the lateral lobes are large.
Marginal spines - There are spines of various lengths on the frontal plate, the lateral lobes, and the postero-dorsal extension which has 2 dorsal setae and a cluster of marginal setae.
Dorsal crest - There is a high, round prominence without dorsal spines.

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

Carapace:
Shape - The carapace is domed in early stages, the anterior margin is slightly pointed in C3.
Marginal spines - absent
Postero-dorsal spine - A spine is present.
Lateral denticles - A pair of denticles is present in C3.
Dorsal crest - There is a small rounded prominence.

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

Furcilia - (7 stages) (E. triacantha C), (E. triacantha D)

Eye: The pigmented area is rounded.

Carapace:
Frontal plate / rostrum - The frontal plate is broad in early furciliae and broadly triangular in later stages. The rostral spine, tiny in F1, becomes long and acute.
Marginal spines - absent
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 - A mid-dorsal spine is present on segments 3, 4, and 5 from F2. Some or all of the spines may be present in F1. The spine on segment 5 is the strongest.
Postero-lateral spines - The spines on segment 6 are quite long in the furcilia phase.

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

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

Comments: The following features are characteristic of E. tricantha :
1) carapace with smooth anterior margin and postero-dorsal spine in C1-C3 and F1, and
2) presence of a mid-dorsal spine on abdomen segments 3, 4, and 5 in F2-F7, some or all the mid-dorsal spines may be present in F1.

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

(Euphausia Table), developmental form frequency in 5 species of Euphausia furcilia larvae from the Southern Ocean.

Pictures
E. triacantha, selected stages
E. triacantha A [metanauplius]
E. triacantha B [calyptopis 1-3]
E. triacantha C [furcilia 1, 3]
E. triacantha D [furcilia 4-5, 7]
key to larval illustrations

Euphausia triacantha