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Thysanoessa longipes Brandt, 1851

Etymology: Thysanoessa - small tassel; longipes - with a long foot

Eye: The eye is bilobed with a transverse constriction. The width of the upper lobe is smaller than the width of the lower lobe (T. longipes eye & rostrum).

Peduncle of 1st Antenna: There is no lappet (T. longipes lappet lacking), but the first segment ends dorsally as a sub-acute angle. In the female, the 2nd and 3rd segments are rather slender. The 3rd segment is slightly longer than the 2nd. In the male, these segments are thicker and the 3rd segment is only slightly thinner than the 2nd.

Rostrum: It is long, narrow, keeled, and reaches to the anterior margin of the eyes (T. longipes dorsal head).

Carapace: There is a denticle present near the mid-point of the lateral margin (T. longipes carapace denticle). The distance from the anterior-lateral angle of the carapace to the denticle averages about 56% of the length of the lateral edge of the carapace (range: 49-64%).

Thoracic legs: The 2nd pair is distinctly elongated (T. longipes thoracic leg).

Abdomen: The posterior-lateral angles of the abdominal segments are acute. The 3rd through 5th segments have dorsal spines (T. longipes abdominal spines). In the size range of 12-16 mm, abdominal segments 3-5 of T. longipes may not yet have developed spines.

Length: Adults are 18 to 30 mm.

Petasma: The spine-shaped process is fairly long and very curved. The terminal process is long, slender, and curved, proximal to the middle. The proximal process is longer than the terminal and has a thinner base. The ends of both the terminal and proximal processes are blunt. The long median lobe is truncate distally and has an oblique emargination on the distal margin. The lateral process is only slightly curved and tapers (T. longipes petasma).

Thelycum: Described by Costanzo and Guglielmo, 1977.

Comments: Before it was recognized (Nemoto, 1963), T. inspinata was considered an "unspined form" of T. longipes . However, the only unspined T. longipes are the immature specimens. T. inspinata can be separated from T. longipes by the absence of abdominal spines on segments 3-5 and the location of the lateral carapace denticle.

T. longipes is an important food source for birds and whales.

T. longipes occurs from the Bering Sea southward to 40°N near Japan, to 50° in mid-ocean, and to about 50°N near the North American coast. T. inspinata occurs along the southern edge of the distribution of T. longipes and southward to 35°N or 40°N in mid-ocean (T. longipes distribution).

T. longipes occurs above 280 m, day and night, with maxima above 140 m.

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

Metanauplius - undescribed

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

Shape - The posterior margin becomes slightly pointed by C3. The carapace is short in relation to the abdomen length.
Marginal spines - absent
Postero-dorsal spine - absent
Lateral denticles - usually absent

Postero-lateral spines - PL3 is very long.
Terminal spines - There are 6 terminal spines in C1-C3.

Furcilia - (6 stages) (T. longipes B), (T. longipes C)

Eye: The pigmented area of the eye is pear-shaped in intermediate stages and the eye is bilobed in F6.

Frontal plate / rostrum - The rostrum is triangular, tapering to a point.
Marginal spines - absent
Postero-dorsal spines - absent
Lateral denticles - A pair of denticles is present from F2.

Thoracic legs: Leg 2, the elongate leg of the adult, appears to be lengthening in F5-F6.

Mid-dorsal spines - absent

Pleopods: The common developmental pathway is 0'- 5'- 5".

Postero-lateral spines - PL3 is longer than PL2.
Terminal spines - There is an even number of terminal telson spines during the reduction to 1 spine, e.g. 6 - 4 - 2- 1.

Comments: The larvae are slender. The even number of terminal telson spines in calyptopis 1-3 and furcilia 1-5 is characteristic of the species within the genus Thysanoessa .

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

T. longipes, selected stages
T. longipes A [calyptopis 1-3]
T. longipes B [furcilia 1-3]
T. longipes C [furcilia 4-6]
key to larval illustrations

Thysanoessa longipes