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(Krøyer, 1846)

Description
Eye large, almost circular and undivided; sometimes slightly higher than broad [T.inermis eye & rostrum ]. The ratio of the eye height and the carapace length is about 0.23.
Peduncle of first antenna: the first 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 second 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 second segment are lacking in the female. The third segment of the male has a high dorsal keel, this is lacking in the female [T.inermis ped 1st].
Rostrum 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 without denticles on the lateral margin.
Thoracic leg 8 is rudimentary. Leg 7 has fully developed exopodites in both sexes, but without endopodites in males and with a short endopodite in females (one or not more than two segments). Legs 1-6 are usually similar in length and uniformly setose, but the second leg is sometimes slightly elongate and thickened with partly naked fourth and fifth segments [T.inermis thoracic leg ].
Abdomen with a dorsal spine on the sixth segment (one-spined form) [T.inermis abdominal spine ]. In addition, there is sometimes a spine on the fifth segment as well (two-spined form), and rarely a spine on fourth to sixth segments (three-spined form) [T.inermis].
Petasma with slender and curved spine-shaped process. 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 ].
Reproduction: matures at an age of one year, body length >14 mm, from spring to early summer; may survive to breed a second time at the age of two years (after Mauchline, 1984).

Size
Adults are 25-32 mm.

Larval stages
T.inermis-A
T.inermis-B
T.inermis-C
T.inermis-D

Comments
Because of apparent distributional significance it is useful to distinguish between specimens having an abdominal spine on only the sixth segment and specimens with spines on both the fifth and sixth segments. (See below, Distribution in the North Sea).

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

Depth range
T. inermis occurs through the 200 m water column in shelf waters, and to about 300 m in more offshore waters. Diurnal vertical migration from 0-300 m during the day to 0-100 m at night is reported by Mauchline, 1984.

Distribution in the North Sea
T. inermis is a coastal species reported from the N, Central, and W North Sea, occasionally also present in the SW North Sea (Hamond, 1971). The one-spined form occurs all over the N North Sea, the two-spined form occurs mainly off Shetland and hence the two forms co-occur in the NW North Sea (Jones et al., 1963).

World distribution
Cool-temperate N Atlantic, southward to the Gulf of Maine in the west and the English Channel in the east; N Pacific north of about 50°N; Arctic Ocean.

Remarks
T. inermis can be infected with Thalassomyces fagei (Ellobiopsidae). Host to larval nematodes of Anisakis and Ascarophis and of the cestode Nybelinia surmenicola. Cysts of apostone Ciliata have been found in the appendages (after Mauchline, 1984).

[After Brinton et al., 1999]

Thysanoessa inermis