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Euphausia nana Brinton, 1962

Etymology: Euphausia - brightly shining; nana - dwarf, small

Eye: The eye is round and large (E. nana eye & rostrum). The eye diameter : carapace length is about 0.27.

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; at its inner part the dorsal margin of the 2nd segment extends forward as a triangle, rather than as a low tooth (cf. E. pacifica). The 3rd segment bears a low, dorsal keel (E. nana,).

Rostrum: There is no rostrum. The anterior margin of the short frontal plate forms an obtuse triangle (E. nana eye & rostrum).

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

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

Length: Adults are 7-8.5 mm.

Petasma: The description closely parallels that of E. pacifica . However, in E. nana the terminal and proximal processes are relatively shorter and appear more robust. The blade at the end of the proximal process is distally broadened and bears a minute terminal tooth, possibly corresponding to the serrated distal margin in E. pacifica. There is no additional process beside the lateral process on the median lobe (E. nana petasma).

Thelycum: Not yet described.

Comments: E. nana is very closely related to E. pacifica but the two distributions appear separated. Adults of E. nana are much smaller than E. pacifica (E. pacifica vs. E. nana photo), while larvae of comparable stages are only slightly smaller. There are small differences , described above, between the petasma organs of these two species. Gravid females of E. nana bear 10-12 eggs similar in size to the 50-200 eggs observed in gravid E. pacifica.

The reproductive population appears to be centered in the eastern basin of the East China Sea, west of Okinawa and the Ryuku Island chain. This, and a similarly local population of E. similis , appear to inhabit what Uda (1959) has called the locally formed Tusima Water Mass.

E. nana lives off southern Japan in the East China Sea-Kuroshio region. To the west of Japan, its population appears to be separated from that of E. pacifica in the Sea of Japan by the shallow Korea Strait. East of Japan the warm Kuroshio may be the barrier between the two populations (E. nana distribution).

Adults have been caught in tows sampling to about 260 m, but, at night, and occasionally in the day, it has been caught above 110 m.

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

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

Shape - The frontal hood is rounded laterally.
Marginal spines - The frontal plate is fringed with small spines, with longer pairs interspersed on the anterior portion.

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

Shape - It 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)

Postero-lateral spines - PL3 is longer than PL2.

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

Eye: The eye is round and grows darkly pigmented.

Frontal plate / rostrum - The frontal plate is rounded without a rostral spine.
Marginal spines - absent
Postero-dorsal spines - absent
Lateral denticles - absent

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

Mid-dorsal spines - absent

Pleopods: There is a variety of forms in F1 and F2 with different levels of pleopod development, and the developmental pathways vary. Common developmental pathways may be 1' - 1"4' - 5", 2' - 2"3' - 5", and 3' - 3"2' - 5.

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. nana and E. pacifica in calyptopis stages, the species lack the short, stout seta present in other species of Euphausia examined.

The forms of the carapace frontal plate and of the eye characterize 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. nana Table), development summary for the stage descriptors and length in stage.

E. nana, selected stages
E. nana A [egg, nauplius, metanauplius]
E. nana B [calyptopis 1-3]
E. nana C [furcilia 1-3]
E. nana D [furcilia 4-6]
larval maxilliped basis [species characters]
key to larval illustrations

Euphausia nana