Marine Species Identification Portal : Thysanoessa raschi

(M. Sars 1864)

Description
Eye large and nearly round and without a transverse constriction [T.raschii eye & rostrum ]. The ratio of the vertical eye diameter and carapace length is about 0.21.
Peduncle of first antenna without elevated lappet [T.raschii lappet lacking ], but the first and second segments end dorsally as short narrow covers over 1/4 or less of the second and third segments.
Rostrum broad, with a rounded tip (dorsal view) in males [T.raschii dorsal head ]. In females it is more narrow, tapering to a point. (Nemoto, 1966 did not find a difference between sexes).
Carapace with one small denticle situated anterior to the middle of the lateral margin [T.raschii carapace denticle ].
Thoracic leg 8 rudimentary, seventh leg with only two segments. The second leg is only slightly elongate or equal to legs 1 and 3-6 [T.raschii thoracic leg ].
Abdomen without spines or keels [T.raschii-drawing ].
Petasma with curved and robust spine-shaped process. The terminal process has on its inner side two membranous wings, one of which protrudes freely; the other, together with a thin keel from the outer side, forms a groove running from the base to the end. The process is distally hooked and is quite acute. The proximal process is very robust proximally, tapers to an acute point, and is nearly straight. The lateral process is heavy, bent sharply at its proximal one-third and again at its distal one-third to form a sharp hook. There is no additional process [T.raschii petasma ].
Reproduction: sexually mature at a length of ca. 14 mm, breeding in spring and early summer at an age of one year. Some individuals survive to breed a second time at two years of age (Mauchline, 1984).

Size
Adults are 20-25 mm (up to 30 mm, according to Mauchline, 1984).

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

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

Depth range
Usually above 200 m; often common in shallower embayments. Diurnal vertical migration from 50-300 m daytime to 0-100 m at night (Mauchline, 1984).

Distribution in the North Sea
N North Sea from Scotland (Mauchline, 1984) to Norway (Glover, 1952) and Central North Sea.

World distribution
Neritic waters of N Pacific and N Atlantic Oceans; known from the Arctic Ocean.

[After Brinton et al., 1999]

Thysanoessa raschi