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Euphausia superba Dana, 1850

Etymology: Euphausia - brightly shining; superba -magnificent

Eye: The eye is round, the male's eye is medium in size and the female's is small (E. superba eye & rostrum). The eye diameter : carapace length in the male is about 0.19 and the female, 0.15.

Peduncle of 1st antenna: It is somewhat stouter in the male. The 1st segment is wide and with a strong, low-lying, distal lappet which extends over 0.25 to 0.5 of the 2nd segment. Its distal margin is oblique and usually emarginate. A wide, long lobe projects horizontally from the upper surface of the 2nd segment over the 3rd. The 3rd segment has a keel (E. superba,) (E. superba lappet).

Rostrum: It is short, extending to about the mid-point of the eye, rounded at the tip, and may be a little shorter in the male than in the female (E. superba eye & rostrum) (E. superba dorsal head).

Carapace: There is a distinct cervical groove. The front part is produced at the anterior-lateral corners into strong, oblique projections behind each eye. There is one pair of lateral denticles, but in older males and sometimes females the lateral denticles are reduced or absent (E. superba carapace denticle).

Thoracic legs: They are much longer than in any other species of the genus, with the merus of legs 1-5 extending forward to the end of the 1st segment of peduncle of 1st antenna. The legs are feathered with setae which have rows of both primary and secondary setules, forming a filtering basket capable of retaining nano-planktonic particles (E. superba filtering basket).

Mandibular pap: The distal segment of the mandibular palp is at least seven times as long as broad, differing from all other Antarctic species.

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

Length: Adults are 42-65 mm. In most years, post-larval E. superba are concentrated in two modal body-length groups: immatures, of about 25-40 mm (believed to be 1-2 yrs. old) and adults of >42 mm (3-5+ yrs. old).

Petasma: The terminal process is slender, curved and distally hooked. The curved proximal process extends to the midpoint of the terminal and ends as an irregularly contoured leaflike plate. The lateral process is a large hook (E. superba petasma).

Thelycum: Described by Lomakina, 1972; Guglielmo and Costanzo, 1978; Thomas and Nash, 1987.

Comments: E. superba , commonly called "Antarctic krill," is the largest species in the Antarctic and the most numerous. The length of the distal segment of the mandibular palp and the length of the thoracic legs are species specific and provide exceptional means for grasping, scraping ice algae, filtering, and ingesting (Krill feeding on ice algae).

ECOLOGY
E. superba swarms and schools (Dense school of krill) (Krill schools), particularly in slope waters, and is an important food source for whales, seals, fish, squid, decapods and birds.

HORIZONTAL DISTRIBUTION
E. superba has a circumpolar distribution. The northern limit is usually at the Antarctic Convergence, between 50°S and 60°S (E. superba distribution). On 24 December 1975, Antezana et al. (1976) discovered an isolated population in a fjord by Isla Guarello in southern Chile at about 50°S consisting of immature specimens 24-37 mm in length. A 1984 sampling of that region proved fruitless. Fortuitous eddies with entrapped larvae may sometimes meander far northward of the species' usual range.

VERTICAL DISTRIBUTION
E. superba lives above 250 m during the day, often swarming at the surface.

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

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

Carapace:
Shape - The frontal hood is rounded and the body is domed.
Marginal spines - absent

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

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

Thoracic legs: Buds of legs 2-6 are visible in C3.

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

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

Eye: The eye is rounded.

Carapace:
Frontal plate / rostrum - The frontal plate is rounded in early furcilia and broadly triangular by F5-F6. There is a small rostral spine from F3.
Marginal spines - absent
Postero-dorsal spines - absent
Lateral denticles - A pair of denticles is present.

Thoracic legs: There is precocious and nearly synchronous development of legs 2-6.

Abdomen:
Mid-dorsal spines - absent

Pleopods: There is a variety of forms in F1 and F2 with different levels of pleopod development. Common developmental pathways are: 4' - 4"1' - 5" and 5' - 5".

Telson:
Postero-lateral spines - PL3 is longer than PL2. PL3 widens basally in F3.
Lateral spines - One pair of lateral spines is present.

Comments: Calyptopis 3 and the furcilia stages are characterized by the early and nearly synchronous development of thoracic legs 2-6.

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

(E. superba Graph 1) , a histogram of developmental form frequency, from the east and north of Elephant Island.

Pictures
E. superba, selected stages
E. superba A [egg, nauplius, metanauplius]
E. superba B [calyptopis 1-3]
E. superba C [furcilia 1-2]
E. superba D [furcilia 3-4]
E. superba E [furcilia 5-6]
E. superba F [egg development]
key to larval illustrations

Euphausia superba