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Euphausia pacifica Hansen, 1911

Etymology: Euphausia - brightly shining; pacifica - of the Pacific

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

Peduncle of 1st Antenna: The 1st segment has a very acute, small, dentate process which projects anteriorly over the proximal part of the 2nd segment; also, there is a row of long, strong, recurved setae on the dorsal side of the 1st segment, behind the dentate process. The 2nd segment is longer than the 3rd and there is a low short spine on the distal margin inside of the midline. The 3rd segment bears a low, dorsal keel (E. pacifica,) (E.pacifica gills, eye, & lappet).

Rostrum: There is no rostrum. The short frontal plate forms an obtuse triangle (E. pacifica eye & rostrum) (E. pacifica dorsal head).

Carapace: There is one pair of lateral denticles (E. pacifica carapace denticle).

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

Length: Adults are 11-25 mm, the largest being in the more northern, cool-water range. In the California Current this is north of about 34°N. Adults as small as 11 mm sometimes occur along Baja California, 28-32°N, but usually in association with larger ones.

Petasma: The terminal process, beyond its basal part, is nearly straight. The distal part first constricts, then ends as a flat blade lying at an angle to the rest of the process. The proximal process is shorter than the terminal and ends distally in a large, oblong blade, usually serrated (not always in large >18 mm males), set at a right angle to the rest of the process; near the base of the blade is a projection which appears to adhere to part of the plate. There is a hooked lateral process on the distally crested median lobe. On large males there may be an additional hooked process near the lateral process (E. pacifica petasma).

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

Comments: E. pacifica is closely related to E. nana . E. pacifica is much larger than E. nana (E. pacifica vs. E. nana photo). There are small differences between the petasma organs of these two species, and their distributions appear to be separated near southern Japan. The small adults of E. pacifica (ca 11 mm) sometimes found in the southern part of the California Current overlap in range with larger E. pacifica , whereas E. pacifica and E. nana appear not to overlap.

In the North Pacific, E. pacifica andThysanoessa raschii co-occur. It can be distinguished fromT. raschii by the elongate rostrum and the trough-like apperance of the frontal plate (E. pacifica vs. T.raschii photo).

ECOLOGY
E. pacifica forms large swarms (Saanich Inlet aggregation) and is an important food for whales, fish and birds. It has been the object of a fishery in the Sea of Japan and in Canadian waters near Vancouver Island. It is part of the subarctic Pacific–California Current association of species.

HORIZONTAL DISTRIBUTION
E. pacifica is restricted to the North Pacific. It occurs in the Bering Sea and southward to about 40° in the North Pacific Drift in mid-ocean. It is present in the Sea of Japan and southward to about 36°N east of Japan. In the California Current it is the principal cool-water species, extending southward to 30° or 25°N, off southern California and Baja California (E. pacifica distribution).

VERTICAL DISTRIBUTION
E. pacifica is a strong vertical migrator; daytime depths for adults may reach 1000 m but are usually near 300 m. At night the range is close to the surface. Daytime surface swarms have been observed off Japan and California.

LARVAE
See the development summaries (E. pacifica Table 1) and (E. pacifica Table 2) for the stage descriptors, length in stage, and for two systems of staging the furcilia larvae, one for an Eastern Pacific population (Boden, 1950) and one for a Yellow Sea population (Suh et al., 1993).

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

Carapace:
Shape - The frontal hood is wide.
Marginal spines - The frontal hood is fringed with small spines, and there are longer spines interspersed.
Dorsal crest - There is a dorsal crest without spines.

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

Carapace:
Shape - The carapace is smoothly rounded.
Marginal spines - absent
Postero-dorsal spine - absent
Lateral denticles - absent

Leg 1 (maxilliped):
There are 4 setae spaced along the inner margin of the basis and 1 submarginal seta. (larval maxilliped basis)

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

Furcilia - (6 or 7 stages) (E. pacifica C), (E. pacifica D), (E. pacifica E)

Eye: The eye is round and becomes darkly pigmented.

Carapace:
Frontal plate / rostrum - The frontal plate is rounded without a rostral spine, the sides taper in later furcilia.
Marginal spines - absent
Postero-dorsal spines - absent
Lateral denticles - A pair of denticles is present.

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

Abdomen:
Mid-dorsal spines - absent

Pleopods: There is a variety of forms in F1 and F2 that differ in level of pleopod development and the developmental pathways vary. Some common developmental pathways are: 2' - 2"3' - 5", 3' - 3"2' - 5 and 0 - 1' - 1"3' - 4"1' - 5".

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

Comments: The medial setation of the basal endite of the maxilliped is characteristic of E. pacifica and E. nana in calyptopis stages, the species lack the short, stout seta present in other species of Euphausia examined. This setation may also be used to separate E. pacifica from calyptopes of some species of other genera, i.e. Nematoscelis difficilis , Nyctiphanes simplex ,Thysanoessa gregaria andT. spinifera . The shape of the carapace frontal plate and the eye are characteristic of the species in furcilia stages.

Although there is a dramatic difference in the body length of the adults of E. nana and E. pacifica , their larvae in the Western Pacific are similar in size until the mid-furcilia phase when E. pacifica larvae become larger than E. nana at the same stage of development.

E. pacifica and N. simplex co-occur along the coast of California; they are both round eye species and sometimes can be difficult to distinguish in the calyptopis stages. A comparison of the telsons of Euphausia pacifica and Nyctiphanes simplex in C3 (E. pacifica vs. N. simplex) shows that: 1) the telson of E. pacifica is more rounded distally and the inner pair of postero-lateral telson spines (PL3) is relatively long, 2) the telson of N. simplex has a more flattened distal margin and spine PL3 is shorter in relation to other telson spines.

Development
(E. pacifica Table 1) and (E. pacifica Table 2), development summaries for the stage descriptors and length in stage, for the California Current and the Yellow Sea, respectively.

Pictures
E. pacifica, selected stages
E. pacifica A [egg, nauplius, metanauplius]
E. pacifica B [calyptopis 1-3]
E. pacifica C [furcilia 1-3]
E. pacifica D [furcilia 4-5]
E. pacifica E [furcilia 6-7]
larval maxilliped basis [species characters]
E. pacifica vs. N. simplex [calyptopis 3 telson photo]
E. pacifica vs. N. simplex [furcilia 1 telson photo]
Ontogeny of swimming
Hindgut peristalsis
key to larval illustrations

Euphausia pacifica