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Thysanoessa longicaudata (Krøyer, 1846)

Etymology: Thysanoessa - small tassel; longicaudata - long tailed

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. longicaudata eye & rostrum).

Peduncle of 1st Antenna: There is no lappet, but the 1st segment ends dorsally as an sub-acute angle (T. longicaudata lappet lacking).

Rostrum: It is acute and extends beyond the middle of the 1st segment of the of the 1st antenna (T. longicaudata eye & rostrum).

Carapace: There are no denticles on the lateral margin.

Thoracic legs: The 2nd pair is elongate and thickened. Setae are present on both margins of the last two segments (T. longicaudata thoracic legs).

Abdomen: There are no keels or spines. The 6th abdominal segment is approximately equal to the length of the 4th and 5th segments combined (T. longicaudata).

Length: Adults are 10-16 mm.

Petasma: The spine-shaped process is long and curved. The terminal process has a thin keel on the outer side. The keel begins at the middle and continues to almost the end. The proximal process is cylindrical at the base, but flattens out above the middle and forms a spoon-shaped plate with an uneven upper edge. There are no additional processes (T. longicaudata petasma).

Thelycum: Described by Einarsson, 1945.

Comments: The length of the 6th abdominal segment is an important character.

T. longicaudata aggregates and is a food for fishes and birds.

T. longicaudata is restricted to the North Atlantic north of about 40°N in both the east and west (T. longicaudata distribution). It also sometimes can be found in the Arctic.

T. longicaudata lives between 0-400 m during the day but migrates vertically to 0-100 m at night.

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

Metanauplius - undescribed

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

Shape - The carapace is hood-like and completely covers the eyes. The posterior margin covers the anterior part of the abdomen and becomes increasingly pointed.
Marginal spines - absent
Postero-dorsal spine - absent
Lateral denticles - absent

Postero-lateral spines - PL3 is longer than PL2.

Furcilia - (7 stages) (T. longicaudata B), (T. longicaudata C)

Eye: In F1-F2 there is only a small spot of pigment but the ommatidia are discernible along the whole outer margin of the eye. The pigmented area of the eye is pear-shaped in F3 with a division between the upper and lower portions. The eye is bilobed by F5.

Frontal plate / rostrum - In F1-F2 the frontal plate is rounded and leaf-like but considerably broader than in other species of the genus. It covers the base of the eyestalks and more than half of segment one of the antenna. In F3 the frontal plate becomes triangular and pointed with slightly convex sides. The sides are straight in later stages.
Marginal spines - absent
Postero-dorsal spines - absent
Lateral denticles - absent
Dorsal keel - By F5 there is a small, well defined keel.

Thoracic legs: Leg 2, the elongate leg of the adult, begins to lengthen by F5 and becomes increasingly elongate in late stages.

Mid-dorsal spines - absent

Pleopods: There is a variety of forms with different levels of pleopod development. The common pathway of development is 0 - 5' -5".

Postero-lateral spines - PL3 is longer than PL2.

Comments: The calyptopis stages of T. longicaudata may be distinguished from those of T. inermis and T. raschii by the more pointed posterior margin of the carapace. The broad leaf-like frontal plate is characteristic of the early furcilia larvae of T. longicaudata and in later stages the smaller size and the slender limbs distinguish T. longicaudata from T. inermis .

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

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

Thysanoessa longicaudata