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Thysanoessa raschii (M. Sars, 1864)

Etymology: Thysanoessa - small tassel; raschii - after Professor Raschi

Eye: The eye is large and nearly round and without a transverse constriction (T. raschii eye & rostrum). The vertical eye diameter : carapace length is about 0.21.

Peduncle of 1st Antenna: There is no elevated lappet (T. raschii lappet lacking), but the 1st and 2nd segments end dorsally as short narrow covers over 1/4 or less of the 2nd and 3rd segments.

Rostrum: It is 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: There is one small denticle situated anterior to the middle of the lateral margin (T. raschii carapace denticle).

Thoracic legs: The 2nd leg is only slightly elongate or equal to the 5 others (T. raschii thoracic leg). (Nemoto, 1966 states that they are equal.)

Abdomen: There are no spines or keels (T. raschii).

Length: Adults are 20-25 mm.

Petasma: The spine-shaped process is curved and robust. 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).

Thelycum: Described by Einarsson, 1945.

Comments: The lack of abdominal armature, elevated lappet, and elongate legs, together with presence of an elongate rostrum, distinguish T. raschii in subarctic coastal waters. In the Pacific, it co-occurs withEuphausia pacifica . It can be distinguished fromE. pacifica by the elongate rostrum and the trough-like apperance of the frontal plate (E. pacifica vs. T.raschii photo).

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

HORIZONTAL DISTRIBUTION
T. raschii inhabits neritic waters of the northern Pacific and Atlantic ocean basins. It is known from the Arctic Ocean. In the western Pacific it has been found in the Okhotsk Sea and southward to 40°N on the east coast of Japan. In the eastern Pacific it occurs along the inland waterway of Alaska and British Columbia southward to Puget Sound and the coast of Oregon. In the western Atlantic it is sometimes an immigrant southward to the Gulf of Maine and in the eastern Atlantic, to the northern British Isles (T. raschii distribution).

VERTICAL DISTRIBUTION
T. raschii is usually above 200 m and is often common in shallower embayments.

LARVAE
See the development summaries (T. raschii Table 1) and (T. raschii Table 2), for stage descriptors, length in stage, and for comparison of the development of the species in the Clyde Sea (McDonald 1928; Mauchline 1965, 1971) and Iceland (Einarsson 1945).

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

Carapace:
Shape - The carapace is rounded dorsally and slender.
Marginal spines - There are small spines spaced along the entire margin of the carapace.

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

Carapace:
Shape - The carapace is short relative to abdomen length.
Marginal spines - absent
Postero-dorsal spine - absent
Lateral denticles - absent

Telson:
Postero-lateral spines - PL2 and PL3 are both long. PL3 is slightly longer than or equal to PL2.

Furcilia - (10-11 stages) (T. raschii C), (T. raschii D)

Eye: The pigmented area of the eye is somewhat pear-shaped in young furcilia larvae. The eye is nearly round in F9.

Carapace:
Frontal plate / rostrum - The frontal plate is rounded distally in young furcilia and tapers to a point in later stages.
Marginal spines - absent
Postero-dorsal spines - absent
Lateral denticles - A pair of denticles is present from F2.

Thoracic legs: Leg 2 may be slightly elongate.

Abdomen:
Mid-dorsal spines - absent

Pleopods: There is a variety of forms with different levels of pleopod development. The common developmental pathways are 0 - 5' - 5" and 0 - 4 - 4"1'- 5".

Telson:
Postero-lateral spines - PL2 and PL3 are relatively long and 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 there are four red chromatophores on the telson in two pairs, 1 proximal and 1 distal. After F1, the larvae of T. raschii can be distinguished from those of T. inermis and T. lonigicaudata by the presence of a lateral denticle on the carapace. The development of the species has been examined in detail by Einarsson (1945) in Iceland and by Mauchline (1965, 1971) in the Clyde Sea area. The number of furcilia stages and details of development were found to be different. (See T. raschii Table 1 and T. raschii Table 2).


Development
(T. raschii Table 1) and (T. raschii Table 2), development summaries for the stage descriptors and length in stage, for the Clyde Sea and Iceland, respectively.

Pictures
T. raschii, selected stages
T. raschii A [egg, nauplius, metanauplius]
T. raschii B [calyptopis 1-3]
T. raschii C [furcilia 1-3]
T. raschii D [furcilia 6-8]
key to larval illustrations

Thysanoessa raschii