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Tessarabrachion oculatum Hansen, 1911

Etymology: Tessarabrachion - four arms; oculatum - with eyes

Eye: The eye is (very) large (height is about 1/2.3x carpace length) with a constriction dividing the upper and lower lobes; the two lobes are nearly equal in size in adults, but the lower lobe is larger in immature specimens (T. oculatum eye & rostrum).

Peduncle of 1st Antenna: The 1st segment has no lappet but the anterior dorsal limit is acute; it is more broad than the 2nd and 3rd. The 2nd and 3rd segments are near equal in length and are more slender in the female than in the male (T. oculatum,).

Rostrum: There is no rostrum; the frontal plate is a short oblique triangle, sharply and angularly upturned as a flange along its oblique anterior margins. There is a low keel cresting over the domed gastric area behind the frontal plate (T. oculatum dorsal head).

Carapace: There are no denticles on the lateral margin.

Thoracic legs: The 2nd and 3rd thoracic legs are similar and both are extremely elongate with the merus reaching to or beyond the end of the peduncle of 1st antenna (T. oculatum thoracic leg). The 4th-6th thoracic legs are similarly developed, and decrease in size posteriorly. The 7th leg scarcely extends to the limit of the gills.

Abdomen: There are no dorsal spines or specific features.

Length: Adult males are up to 20 mm; females up to 26 mm.

Petasma: The petasma is very distinctive, appearing to be rudimentary. The inner and median lobes are very short, partially joined, and have no processes. There are two or three distal spines on the median lobe. The setiferous lobe and the auxiliary lobes are both well developed (T. oculatum petasma).

Thelycum: Described by Guglielmo and Costanzo, 1983.

Comments: T. oculatum is the only species in the genus Tessarabrachion . The upturned anterior edges of the short frontal plate are distinguishing if the elongate legs are broken off.

ECOLOGY
T. oculatum is an important food source for fish.

HORIZONTAL DISTRIBUTION
T. oculatum is restricted to the North Pacific, usually north of 35°N in the western half and 40°N in the east. It rarely occurs in the California Current, but has been recorded as far south as 33°N (T. oculatum distribution). High abundances in plankton samples have been only from 45-50°N in mid-ocean.

VERTICAL DISTRIBUTION
This species has been caught between the surface and about 1000 m. Most adults appear to migrate from ca. 150-400 m in the daytime to ca. 0-150 m at night. They remain beneath the upper layers (100-200 m) in the southern part of their range in the North Pacific Drift, south of 45°N.

LARVAE
See the development summary (T. oculatum Table) for the stage descriptors and length in stage.

Metanauplius - undescribed


Calyptopis - (3 stages) (T. oculatum A)

Carapace:
Shape - The carapace is rounded and short in relation to abdomen length.
Marginal spines - absent
Postero-dorsal spine - absent
Lateral denticles - absent

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

Furcilia - (5 stages) (T. oculatum B), (T. oculatum C)

Eye: The eye is large. In F5, the pigment is pear-shaped with with a division between upper and lower portions. The eye is bilobed in the juvenile illustrated (T. oculatum D).

Carapace:
Frontal plate / rostrum - The frontal plate is broad with a small rostral spine in early furcilia stages. In later stages the rostrum tapers to a point, becomes proportionally shorter, and the lateral margins curve inward dorsally.
Marginal spines - absent
Postero-dorsal spines - absent
Lateral denticles - A pair of denticles is present.
Dorsal keel - A keel is present.

Thoracic legs: There is precocious growth of legs 2 and 3, the elongate legs of the adult. Leg 2 is longer than leg 3 in F4-5, and they are equal in length in the Juvenile illustrated.

Abdomen:
Mid-dorsal spines - absent

Pleopods: The common developmental pathway is 1' - 1"4' - 5".

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

Comments: The lateral carapace denticles are present only in the furcilia phase, they disappear in the juvenile.

Endo (1980) notes that E. pacifica , Thysanoessa longipes and T. inspinata are the dominant euphausiids in the Sanriku area of northeastern Japan. In comparing larvae of these species Endo states that, unlike T. longipes , the carapace of T. oculatum lacks a pointed posterior margin in calyptopis stages and has a dorsal keel in furcilia stages. The calyptopis stages of E. pacifica are smaller than those of T. oculatum and, in furcilia stages, the frontal plate of E. pacifica is rounded, not pointed as in T. oculatum . The larvae of T. inspinata have not been identified but may be similar to those of T. longipes .


Development
(T. oculatum Table), development summary for the stage descriptors and length in stage.

Pictures
T. oculatum, selected stages
T. oculatum A [calyptopis 1-3]
T. oculatum B [furcilia 1-3]
T. oculatum C [furcilia 4-5]
T. oculatum D [juvenile]
key to larval illustrations

Tessarabrachion oculatum