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(Krøyer, 1846)

Eye bilobed with a transverse constriction; lower lobe wider than upper lobe [T.longicaudata eye & rostrum].
Peduncle of first antenna without lappet, but the first segment ends dorsally as an sub-acute angle [T.longicaudata lappet lacking ].
Rostrum prominent and acute, extending beyond the middle of the first segment of the of the antennula (first antenna) [T.longicaudata eye & rostrum].
Carapace without denticles on the lateral margin.
Thoracic leg 8 rudimentary, seventh leg with only two segments. The second pair is elongate and thickened. Setae are present on both margins of the last two segments [T.longicaudata thoracic legs ].
Abdomen without keels or spines. The sixth abdominal segment is approximately equal to the length of the fourth and fifth segments combined [T.longicaudata-drawing ].
Petasma with long and curved spine-shaped process; terminal process with 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 ].
Reproduction: matures sexually at a length of ca. 10 mm and breeds at an age of one year in spring and summer. Probably most individuals only live for one year in the North Sea: (after Mauchline, 1984).

Adults are 10-16 mm.

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

Depth range:
Lives between 0-400 m during the day and migrates vertically to 0-100 m at night.

Distribution in the North Sea:
N, Central, W and SW North Sea.

World distribution:
Restricted to the N Atlantic north of about 40°N in both the East and West; occasionally in the Arctic.

The length of the sixth abdominal segment is an important character.
Recorded as host of Anisakis (Nematoda) and larvae of Palaeoacanthocephala. Cysts of apostome Ciliata have been found (Mauchline, 1984).

Thysanoessa longicaudata