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Thysanoessa macrura G.O. Sars, 1883

Etymology: Thysanoessa - small tassel; macrura - long tailed

Eye: The eye is bilobed with a transverse constriction. The width of the upper lobe is shorter and somewhat narrower than the width of the lower lobe (T. macrura eye & rostrum).

Peduncle of 1st Antenna: There is no elevated lappet on the 1st segment, but the 1st segment ends dorsally in an acute angle. In the females the 2nd and 3rd segments are slender, the 3rd segment is longer and more slender than the second. In the males the 2nd and 3rd segments are shorter and thicker than in the female (T. macrura,). In the females the upper flagellum is longer than the 3rd segment and is 10-jointed but is shorter than the sum of 2nd and 3rd segments. The lower flagellum is 13-jointed.

Rostrum: It may be shorter in the male than the female and is an acute triangle (T. macrura rostrum photo).

Carapace: There is a well-developed denticle on the lateral margin (T. macrura carapace denticle). At its post-ocular edge, the base of the frontal plate bends slightly upward and outward as a narrow flange, appearing to shield slightly the upper part of the eye.

Thoracic legs: The 2nd pair of legs is elongate (T. macrura thoracic legs).

Abdomen: There are no keels or spines. The 6th segment is less than or equal to the length of the sum of 4th and 5th segments (T. macrura).

Length: Adults are up to 30 mm.

Petasma: The spine-shaped process is thin and curved. The terminal process is very broad at the base, with the distal part flattened and expanded both inwards and outwards. The terminal margin is long, somewhat oblique and noticeably emarginate. The proximal process is almost as long as the terminal process but is much narrower at the distal end. The lateral process is nearly as long as the terminal process. The median lobe is somewhat obtuse (T. macrura petasma).

Thelycum: Described by Costanzo and Guglielmo, 1977.

Comments: T. macrura and T. vicina are difficult to distinguish. T. macrura reaches a much larger size. The flagella of 1st antenna (T. macrura,), the shape of the rostrum (T. macrura vs. T.vicina rostrum) and the terminal process of the male petasma are useful characters to separate the two closely related species.

ECOLOGY
T. macrura is a food for whales.

HORIZONTAL DISTRIBUTION
T. macrura is circumpolar in the Antarctic, south of about 55°S, extending to the Antarctic coastal waters. The similar, smaller species T. vicina occurs along a zone bordering and overlapping the northern limit of T. macrura. This overlap is not indicated on the map of the distribution of the two species (T. macrura distribution).

VERTICAL DISTRIBUTION
T. macrura lives in the 0-400 m layer, concentrating above 200 m at night.

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

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

Carapace:
Shape - The lateral margins of the carapace are without clear lateral lobes.
Marginal spines - absent

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

Carapace:
Shape - The carapace is short relative to abdomen length. The posterior margin is pointed and, in C3, somewhat angled dorsally.
Marginal spines - absent
Postero-dorsal spine - absent
Lateral denticles - absent

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

Furcilia - (7 stages) (T. macrura C), (T. macrura D), (T. macrura E)

Eye: The pigmented area of the eye is pear-shaped in young furcilia larvae. The eye becomes bilobed in late stages.

Carapace:
Frontal plate / rostrum - The rostrum is triangular tapering to a point.
Marginal spines - absent
Postero-dorsal spines - absent
Lateral denticles - A pair of denticles is present from F2.

Thoracic legs: Leg 2, the elongate leg of the adult, appears to begin to lengthen in F5.

Abdomen:
Mid-dorsal spines - absent

Pleopods: The common developmental pathway is 0 - 5' - 5".

Telson:
Postero-lateral spines - PL3 is longer than PL2.
Lateral spines - The pair of lateral spines is situated proximal to the midpoint of the telson.

Comments: The larvae are slender and their carapace is short in relation to the abdomen length. The relatively proximal position of the pair of lateral spines on the telson appears to be characteristic of T. macrura .

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

Pictures
T. macrura, selected stages
T. macrura A [nauplius and metanauplius]
T. macrura B [calyptopis 1-3]
T. macrura C [furcilia 1-2]
T. macrura D [furcilia 3-5]
T. macrura E [furcilia 6-7]
key to larval illustrations

Thysanoessa macrura