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Thysanoessa inermis (Krøyer, 1846)

Etymology: Thysanoessa - small tassel; inermis - unarmed

Eye: The eye is large, almost circular, sometimes slightly higher than broad (T. inermis eye & rostrum). The ratio of eye height : carapace length is about 0.23.

Peduncle of 1st Antenna: The 1st segment of the male has on its inner side, a rounded lobe directed forward and upward and bearing two rows of slightly curved setae. This is lacking in the female. The 2nd segment of the male has its outer, anterior margin produced dorsally as a fingerlike process; the inner margin is produced as an upward-pointing lobe with its upper margin armed with hook-shaped setae. These characteristics of the 2nd segment are lacking in the female. The 3rd segment of the male has a high dorsal keel, this is lacking in the female (T. inermis,).

Rostrum: It is narrow and acute, reaching beyond the eyes. It extends somewhat downward, forward of the mid-point of the eyes, so that the tip is lower than the frontal plate (T. inermis eye & rostrum) (T. inermis dorsal head).

Carapace: There are no denticles on the lateral margin.

Thoracic legs: The 6 legs are usually similar in length and uniformly setose, but the 2nd leg is sometimes slightly elongate and thickened with partly naked 4th and 5th segments (T. inermis thoracic leg).

Abdomen: There is a dorsal spine on the 6th segment (one-spined form) (T. inermis abdominal spine). In addition, there is sometimes a spine on the 5th segment as well (two-spined form), and rarely a spine on 4th to 6th segments (three-spined form) (T. inermis ) (See HORIZONTAL DISTRIBUTION).

Length: Adults are 25-32 mm.

Petasma: The spine-shaped process is slender and curved. The terminal process has, on its inner side, two long, thin, membranous expansions between which runs a canal from the distal end almost to the base. The distal end is blunt. The proximal and lateral processes are cylindrical, straight, and tapered to acute ends. There is a slender, straight, acute additional process (T. inermis petasma).

Thelycum: Described by Einarsson, 1945.

Comments: Because of apparent distributional significance it is useful to distinguish between specimens having an abdominal spine on only the 6th segment and specimens with spines on both the 5th and 6th segments.

ECOLOGY
T. inermis is a food source for whales, fish, seals and birds.

HORIZONTAL DISTRIBUTION
T. inermis is a coastal species, occurring in the cool-temperate North Atlantic southward to the Gulf of Maine in the west and the English channel in the east. In the North Pacific it occurs north of about 50°N. It is known from the Arctic Ocean. Along the western side of the North Pacific it extends southward through the Okhotsk Sea and Sea of Japan. The two forms usually co-occur, but the one-spined form has been found to be dominant near Japan, while the two-spined form is most strongly dominant in the Gulf of Alaska (T. inermis distribution).

VERTICAL DISTRIBUTION
T. inermis occurs through the 200 m water column in shelf waters, and to about 300 m in more offshore waters.

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

Metanauplius - (1 stage) (T. inermis A)

Carapace:
Shape - The carapace is rounded dorsally without a crest and slender.
Marginal spines - There are short spines along the anterior and lateral margins.

Calyptopis - (3 stages) (T. inermis B)

Carapace:
Shape - The posterior margin of the carapace is not pointed distally, it is shorter than the postero-lateral margins. In C3 the eyes are not wholly covered by the frontal hood. The carapace is short in relation to abdomen length.
Marginal spines - absent
Postero-dorsal spine - absent
Lateral denticles - absent

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

Furcilia - (9 stages) (T. inermis C), (T. inermis D)

Eye: The pigmented area of the eye is pear-shaped with a distinct division into upper and lower portions in the young furcilia. The eye becomes bilobed in F8.

Carapace:
Frontal plate / rostrum - The frontal plate is anteriorly rounded and leaf-like. By F4 it is narrower and somewhat truncate.
Marginal spines - absent
Postero-dorsal spines - absent
Lateral denticles - absent

Thoracic legs: Leg 2, the elongate leg of the adult, begins to lengthen in F4 and is quite long by F9.

Abdomen:
Mid-dorsal spines - absent

Pleopods: There is a variety of forms with different levels of pleopod development and a variety of pathways of development, for example 0 - 4' - 4"1' - 5" and 0 - 5' -5".

Telson:
Postero-lateral spines - PL3 and PL2 are similar in length.

Comments: The larvae are relatively slender and the carapace is short in relation to abdomen length in calyptopis and early furcilia stages. In life the calyptopes are perfectly transparent except for conspicuous crimson chromatophores on the telson, 1 pair in C1 and 2 pairs in C2-C3. The furcilia are also transparent except for the chromatophores on the telson, there may be a diffuse pinkish color near the mouthparts.


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

(T. inermis Graph 1), a histogram of length freqency by stage, from the northern North Sea.

Pictures
T. inermis, selected stages
T. inermis A [egg, nauplius, metanauplius]
T. inermis B [calyptopis 1-3]
T. inermis C [furcilia 1-2]
T. inermis D [furcilia 4-5, 7-9]
key to larval illustrations

Thysanoessa inermis