Euphausia similis G.O. Sars, 1883
Euphausia similis variety armata
Etymology: Euphausia - brightly shining; similis - like or similar; armata - armed
Eye: The eye is round and medium in size (E. similis eye & rostrum). The eye diameter : carpace length is about 0.19.
Peduncle of 1st Antenna: The 1st segment bears a strong dorsal lappet, projecting forward over the proximal part of the 2nd segment at an angle of 30° with the horizontal. The tip of the lappet is bifurcate. The anterior part of the 2nd segment bears short keel-like teeth at the inner and outer dorsal-lateral angles, the inner one being the larger. The 3rd segment bears a smoothly rounded dorsal keel (E. similis,).
Rostrum: It is acute, reaching about to the anterior limit of the eye, and is strongly keeled along the frontal plate (E. similis eye & rostrum) (E. similis dorsal head). In rare specimens (variety crassirostris) the keel on the frontal plate is elevated and convex (E. similis, ).
Carapace: There is one pair of lateral denticles (E. similis carpace denticle).
Abdomen: The mid-dorsal part of the 3rd segment is expanded posteriorly as a convex expansion over a short portion of the 4th segment (as in E. tenera) (E. similis ). E. similis variety armata possesses a mid-dorsal spine of variable length on the third segment, rather than the convex expansion (E. similis variety armata).
Length: Adults are 22-26 mm.
Petasma: The terminal process is nearly straight and ends in a short subacute hook. At the base of the hook, the trunk of the terminal process is expanded as a small rounded plate, projecting distally toward the curved tip of the distal hook. The proximal process bends at nearly a 90° angle at its midpoint, and the distal half broadens before terminating acutely (E. similis petasma).
Thelycum: Described by Lomakina, 1972; Guglielmo and Costanzo, 1978.
Comments: The conspicuously keeled rostrum and frontal plate are characteristic. The shape of the processes on the 1st two segments of the peduncle of the 1st antenna distinguish E. similis from other large Euphausia species of tropical and warm temperate seas.
ECOLOGY
It is an important food source for whales and fish.
HORIZONTAL DISTRIBUTION
E. similis is circumglobal in the south temperate ocean basins, about 25-55°S with the most northern record being near New Caledonia (Casanova, 1996); it is present in the tropical Indian Ocean and western Northern Pacific, northward into the Kuroshio Current, with a center of reproduction in the East China Sea. E. similis variety armata occurs together with E. similis in the southern circumglobal belt of distribution, but not elsewhere (E. similis distribution).
VERTICAL DISTRIBUTION
E. similis inhabits 0-300 m in the cooler, southern hemisphere part of its range. In and near the tropics, adults are found only beneath the thermocline. It is not known whether or not it is a vertical migrator.
LARVAE
See the development summary (E. similis Table) for the stage descriptors and length in stage.
Metanauplius - (1 stage) (E. similis A)
Carapace:
Shape - The frontal hood is wide.
Marginal spines - The frontal hood is fringed with small spines. There are longer pairs interspersed along the anterior margin.
Calyptopis - (3 stages) (E. similis B)
Carapace:
Marginal spines - absent
Postero-dorsal spine - A spine is present; it is quite small in C1.
Lateral denticles - A pair of denticles is present in C3.
Telson:
Shape - The telson is wide distally by C3.
Postero-lateral spines - PL3 is longer than PL2.
Furcilia - (6 stages) (E. similis C), (E. similis D)
Eye: The faceted portion has pigment in 2-3 zones.
Carapace:
Frontal plate / rostrum - The frontal plate is truncate in F1 and becomes triangular in later stages. There is a rudimentary rostral spine in F1 and a small acute spine in later stages.
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 - absent
Pleopods: The common developmental pathway is 1' - 1"3' - 4"1' - 5".
Telson:
Shape - The telson is broadly rounded distally in furciliae 1-4, it is almost pear-shaped in F1.
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. similis in calyptopis and early furcilia stages, 1) carapace with smooth anterior margin and postero-dorsal spine and 2) telson broadly rounded distally, especially in F1-F3.
The larvae of E. similis variety armata , which is found in the southern hemisphere have not been described.
Development
(E. similis Table), development summary for the stage descriptors and length in stage.
Pictures
E. similis, selected stages
E. similis A [egg, nauplius, metanauplius]
E. similis B [calyptopis 1-3]
E. similis C [furcilia 1-3]
E. similis D [furcilia 4-6]
key to larval illustrations