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Euphausia fallax Hansen, 1916

Etymology: Euphausia - brightly shining; fallax - deceptive, fraudulent

Eye: The eye is round and large (E. fallax eye & rostrum). The eye diameter : carapace length is about 0.24.

Peduncle of 1st Antenna: The 1st segment has a simple, strong, pointed lappet (E. fallax lappet), somewhat more forward-directed than in E. sanzoi, The 2nd segment has a rounded distal cover over part of 3rd segment; the 3rd segment has a dorsal distal keel, the margin of which is rounded and somewhat indented anteriorly, but not notched as in E. sanzoi .

Rostrum: This is acute, curving slightly upward over the eyes, extending about to the front of the eye (E. fallax eye & rostrum).

Carapace: The gastric region is domed, with the vestige of a dorsal bump (more prominent in the juvenile, as also in E. sanzoi) situated on or posterior to the crest of the gastric dome. There is one pair of lateral denticles (E. fallax carapace denticle).

Abdomen: The 3rd segment is without a spine but the mid-dorsal part of the posterior margin is slightly extended posteriorly as a covering over a small portion of the 4th segment (E. fallax).

Length: Adults are 15-17 mm.

Petasma: The terminal process is cleft into 3 points distally. The middle point is longest and the innermost is shortest. The median lobe has a single subacute triangular process below the lateral process, extending towards the inner lobe. The outer lobe has 7 terminal setae and one or two on the outer basal edge (E. fallax petasma).

Thelycum: Described by Guglielmo and Costanzo, 1978.

Comments: The terminal process of male petasma is trifid, a critical character, and the median lobe possesses one triangular process. There is no dorsal spine on the third abdominal segment.

E. fallax, together with E. gibboides and E. sanzoi forms an "E. gibboides group" of closely related species. The most useful character for separating E. fallax and co-occuring E. sanzoi is the shape of the dorsal keel on the 3rd segment of the antennular peduncle. In E. fallax it is more or less rounded distally, not notched as in E. sanzoi.

E. fallax is a food of fish.

E. fallax inhabits the Indo-Australian Archipelago from the Philippines to the Coral Sea; in the Indian Ocean it has been found in the Andman Sea, south of Ceylon and north of Madagascar (E. fallax distribution).

Adults have been caught in large numbers at the surface at night in the Philippines. There is no information on daytime depths.

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

Metanauplius - undescribed

Calyptopis - (3 stages) (E. fallax A)

Shape - The frontal hood is expanded and wide.
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.

Mandible: There is a lateral tubercle on each mandible, proximal to the antero-lateral process. (E. fallax E)

Shape - The telson is wide.
Postero-lateral spines - PL2 is longer than PL3.

Furcilia - (6 stages) (E. fallax B), (E. fallax C), (E. fallax D)

Eye: The faceted area is rectangular with the pigment developing in 3 zones.

Frontal plate / rostrum - The frontal plate is rectangular in F1 and becomes broadly triangular by F6. A tiny rostral spine appears in F3 and lengthens by F6.
Marginal spines - The frontal plate is fringed with small spines in F1-F2, there may be a few tiny spines in F3 and F4.
Postero-dorsal spines - The postero-dorsal spine is present in F1 only.
Lateral denticles - A pair of denticles is present.
Dorsal keel - A small keel is present.

Mandible: There is a lateral tubercle on each mandible, proximal to the antero-lateral process, in F1-F2. It may be small or absent in F3, and it disappears in F4. (E. fallax E)

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

Mid-dorsal spines - absent

Pleopods: The common developmental pathway is 1' - 1"3' - 4"1' - 5".

Postero-lateral spines - PL2 is longer than, or equal to, PL3 in F1-F3.
Lateral spines - One pair of lateral spines is present.

Comments: E. fallax , with E. sanzoi and E. gibboides , forms the "Euphausia gibboides group" of related species. The larvae of E. fallax and E. sanzoi may be very similar, especially in furcilia stages. Some features which may be useful to separate the larvae of E. fallax and E. sanzoi are:

Carapace -
The frontal hood is wider in E. fallax .
The postero-dorsal spine is larger and tilted dorsally in E. fallax .

Eye - The eye is rectangular in E. fallax and rounder in E. sanzoi .
Carapace -
The postero-dorsal spine of F1 is longer and stronger in E. fallax .
The dorsal keel is larger and more distinct in E. sanzoi , especially in late furcilia.
The lateral spines are situated more distally in E. fallax.

Photophore on abdomen segment 3-
The photophore is pigmented in E. fallax F4-F6 and may or may not be pigmented in E. sanzoi F6.

E. fallax and E. sanzoi may be very difficult to identify in F2-F5 and may be referrable only to the species pair.

E. fallax and E. sanzoi may also be very similar to the E. brevis -E. diomedeae -E. mutica -E. recurva group of related species. They may be separated by the following features: 1) mandible with lateral tubercle in calyptopis and early furcilia stages, and 2) postero-lateral spine 3 of telson with inner margin bare except for a few distal spinules in F3 and following stages.

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

E. fallax, selected stages
E. fallax A [calyptopis 1-3]
E. fallax B [furcilia 1-2]
E. fallax C [furcilia 3-4]
E. fallax D [furcilia 5-6]
E. fallax E [mandible]
key to larval illustrations

Euphausia fallax