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Euphausia diomedeae Ortmann, 1894

Etymology: Euphausia - brightly shining; diomedeae - after Diomedes, a warrior in the Trojan War; Diomedea- a genus of Albatrosses. The type specimens of this euphausiid were collected by the steamer "Albatross".

Eye: The eye is round and large in size (E. diomedeae eye & rostrum). The eye diameter : carpace length is 0.20.

Peduncle of 1st Antenna: The short lappet on the 1st segment extends more upward than forward, at an angle of about 50° with the horizontal. Tip of the lappet is bifid and bends anteriorly. The 2nd segment bears a dorso-lateral spine-like process on both inner and outer margins. These arise from a distal depression on the dorsal surface of the segment. The process on the outer margin is sub-acute and appears as the more sturdy of the two, curving upward and forward. The 3rd segment bears a low rounded keel (E. diomedeae,).

Rostrum: It is acute and almost straight, extending about to the anterior limit of the eyes (E. diomedeae eye & rostrum).

Carapace: There are two pairs of lateral carapace denticles (E. diomedeae carapace denticle).

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

Length: Adults are 10-16 mm.

Petasma: The terminal process is straight but with a curved hook-like end, more slender than the main stem of the process. At the base of the curved part is a small, straight, distally-directed spine. The proximal process ends in a flattened plate. This plate extends distally beyond an obliquely- directed spine (E. distinguenda petasma).

Thelycum: Described by Sebastian, 1966; Guglielmo and Costanzo, 1978.

Comments: The angle (ca. 50° with horizontal) of the lappet (E. diomedeae lappet) on the 1st segment of the peduncle of the 1st antenna is characteristic. The short, stout, slightly curved, tusk-like shape of the outer spine on the second segment also stands out as a specific character. This spine is longer than in E. recurva , more curved than the acute straight spine in the same place on E. brevis , while E. mutica lacks this spine (though sometimes there is a nodule at that position). The eye is larger than in E. brevis . Distal portions of the terminal and proximal processes of the petasma are useful in specific determination of males of these four species when the antennular peduncle is damaged.

Rare specimens of this species have a "remarkable variation" (Hansen, 1911) in which the frontal plate extends extremely convexly and laterally over each of the eyes, having also the effect of shortening the rostrum (E. diomedeae dorsal head).

ECOLOGY
E. diomedeae is an important food source for whales and fish.

HORIZONTAL DISTRIBUTION
E. diomedeae is a tropical species, extending across the Indian and Pacific ocean basins, including the 02-deficient regions. It is carried northward in the western Pacific in the Kuroshio Current to Japan. At least one male specimen has been confirmed from the mid South Atlantic (Tsetlin, 1981a) possibly carried in the Agulhas Current from the Indian Ocean (E. diomedeae distribution).

VERTICAL DISTRIBUTION
Most adults and juveniles are at 250-350 m in the daytime and above 75 m at night.

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

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

Eye: If present, the striated body of the eyestalk photophore is relatively small.

Carapace:
Shape - The frontal hood is wide.
Marginal spines - The frontal hood is fringed with spines of varying length. There are three relatively long pairs on the anterior margin and three smaller spines between the inner pair of long spines. There may be tiny spines on the posterior margin.
Dorsal crest - The crest is high with a pair of relatively long dorsal spines.

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

Carapace:
Shape - The frontal hood curves in medially. The lateral margins are oblique in C1, in dorsal view, and curve ventrally around the eyes in C2 and C3.
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.

Leg 1 (maxilliped) - In C1, as in eximia , the stout marginal seta on the 1st segment of the endopod is smaller than the stout seta on the basis. (Euphausia larval maxilliped)

Telson:
Shape - The lateral margins are indented below the lateral spines and rounded postero-laterally.
Postero-lateral spines - PL2 is longer than PL3.

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

Eye: The eye is large. The faceted area is somewhat rectangular and the eyestalk photophore is relatively large; proportionally smaller than the photophore in E. brevis and larger than the photophore of E. eximia . (Euphausia larval eye)

Carapace:
Frontal plate / rostrum - The frontal plate is rectangular in F1. It becomes broadly triangular in late furcilia. There is a small rostral spine by F2.
Marginal spines - The frontal plate is fringed with small spines through F3. There may be a few tiny spines in F4.
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"4' - 5".

Telson:
Shape - The telson is rounded postero-laterally.
Postero-lateral spines - PL2 is longer than PL3 through F2.
Lateral spines - One pair of lateral spines is present. They are situated relatively distally. The distance between the lateral spines and PL1 is less than in equivalent stages of related species, E. eximia, E. mutica, E. recurva , and E. brevis .

Comments: Larvae of Euphausia species group 1A (Brinton, 1975), E. brevis, E. diomedeae, E. mutica, E. recurva, E. eximia and E. krohni are very similar. E. americana is undescribed.

See the closely related species E. eximia for a lateral view of the development of the second antenna, thoracic legs, and pleopods.

In calyptopis stages, E. diomedeae may be distinguished by small body size, relatively distal position of telson lateral spines, the shape of the telson, and the setation of the maxilliped in C1.

In the furcilia phase, the relatively large eye, small body size, and distal position of the lateral spines of the telson may be useful in recognizing the species.

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

(E. diomedeae Graph 1) , a histogram of developmental form frequency, from the southwest coast of India.

Pictures
E. diomedeae, selected stages
E. diomedeae A [metanauplius]
E. diomedeae B [calyptopis 1-3]
E. diomedeae C [furcilia 1-2]
E. diomedeae D [furcilia 3-4]
E. diomedeae E [furcilia 5-6]
Euphausia larval eye
Euphausia larval maxilliped [species characters]
key to larval illustrations

Euphausia diomedeae