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Thysanoessa inspinata Nemoto, 1963

Etymology: Thysanoessa - small tassel; inspinata - without spines

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. inspinata eye & rostrum).

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

Carapace: There is a denticle on the lateral margin (T. inspinata carapace denticle). The distance from the anterior-lateral angle of the carapace to the dentical averages about 75% of the length of the lateral edge of the carapace (range: 65-85%).

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

Abdomen: The 3rd to 5th abdominal segments have dorsal keels, and the sixth usually has a small spine at the dorsal end (T. inspinata).

Length: Adults are 12-18 mm.

Petasma: The spine shaped process is small and curved. The terminal process is rather broad at the base and about the same length as the proximal process. The space between the terminal and proximal processes is narrow. Both the terminal and proximal processes taper to blunt, curving ends that often bear small acute protrusions (T. inspinata petasma).

Thelycum: Described by Costanzo and Guglielmo, 1977.

Comments: Before it was recognized (Nemoto, 1963), T. inspinata was considered an unspined from 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. inspinata is an important food source for whales.

T. inspinata overlaps T. longipes ' range across the North Pacific in the zone of 40-45°N. It is known to have occurred in the California Current southward to central California (37-40°N) (T. inspinata distribution).

T. inspinata occurs from the surface layer to about 500 m.


Thysanoessa inspinata